Solomon Islands Country Mission and Technical Advisory Report

EU EDF 8/9 – SOPAC Project Report 90

Reducing Vulnerability of Pacific ACP States

Solomon Islands Country Mission and Technical Advisory Report:

Geologic impacts of the 2nd April 2007 earthquake and tsunami on the islands and marine environment of the Western Province, Solomon Islands

Fieldwork undertaken from 6 to 24 May 2007

Sediment chute on forereef, Makuti Island.

EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 2

[EU-SOPAC Project Report 90 – McAdoo, Kruger & others]

Prepared by:

Brian G. McAdoo Department of Earth Science and Geography, Vassar College

Poughkeepsie, NY 12604, USA

Jens C. Krüger Pacific Islands Applied Geoscience Commission

SOPAC Secretariat

Kelly L. Jackson Rosenstiel School of Marine and Atmospheric Science

Miami, FL, USA

Andrew L. Moore Department of Geosciences, Earlham College

Richmond, IN, USA

Wilson B. Rafiau Department of Mines and Energy

PO Box G37, Honiara, Solomon Islands

Braddley Tiano Dive GizoGizo

Solomon Islands

July 2008

PACIFIC ISLANDS APPLIED GEOSCIENCE COMMISSION c/o SOPAC Secretariat

Private Mail Bag GPO, Suva

FIJI ISLANDS http://www.sopac.org

Phone: +679 338 1377 Fax: +679 337 0040

E-mail: [email protected]

Important Notice

This report has been produced with the financial assistance of the European Community, UNESCO and NSF; however, the views expressed herein must never be taken to reflect the official opinion of the UNESCO, NSF,

or European Community.

mailto:[email protected]



TABLE OF CONTENTS

ACKNOWLEDGEMENTS ....................................................................................................................... 4 EXECUTIVE SUMMARY......................................................................................................................... 5 1 INTRODUCTION.............................................................................................................................. 6 2 METHODS........................................................................................................................................ 8 2.1 Offshore..................................................................................................................................... 8 2.2 Onshore...................................................................................................................................10 3 RESULTS .......................................................................................................................................11 3.1 Nusa Aghana...........................................................................................................................12 3.2 Tapurai ....................................................................................................................................14 3.3 Pailongge.................................................................................................................................16 3.4 Titiana......................................................................................................................................17 3.5 Njari Island...............................................................................................................................18 3.6 Makuti Island ...........................................................................................................................20 3.7 Ranongga................................................................................................................................21 3.8 Landslides ...............................................................................................................................22 4 INTERPRETATIONS......................................................................................................................23 4.1 Coral Transport .......................................................................................................................23 4.2 The Tsunami ...........................................................................................................................24 4.3 Offshore-Onshore Sediment Transport ..................................................................................25 5 LOCAL POPULATIONS AND POST-DISASTER AID..................................................................27 5.1 Water .......................................................................................................................................27 5.2 Reef Livelihoods......................................................................................................................27 5.3 Return to Normalcy .................................................................................................................27 5.4 Tourism....................................................................................................................................28 5.5 Education.................................................................................................................................28 6 CONCLUSIONS .............................................................................................................................31 7 RECOMMENDATIONS..................................................................................................................32 8 BIBLIOGRAPHY.............................................................................................................................33 APPENDICES 1 Tsunami Safe – Public Awareness Leaflet.............................................................................34 2 Toktaem news article in Solomon Star, Thursday, 24 May 2007 ..........................................35 3 Draft Project Scoping Document ............................................................................................36



ACKNOWLEDGEMENTS Prior to our arrival in the Solomon Islands our team received valuable preliminary information on the situation in the Western Province from many sources, especially Kerrie & Danny Kennedy and Hermann Fritz. The survey logistics were facilitated by the staff of Dive Gizo and the National Disaster Management Office. Whilst in the field we relied on the people of Gizo, Ranonga and Simbo, who often went out of their way to assist us whilst facing conditions of hardship and tragedy. We also benefited from the many frank discussions we had with the staff of aid organisations and Non-governmental organisations based in Gizo. Special thanks go to Tingo Leve of WWF for sharing his marine monitoring data with us.



EXECUTIVE SUMMARY On 2nd April 2007 at 7:39 AM local time, a magnitude 8.1 earthquake generated a tsunami that killed 52 people in the Solomon Islands Western and Choiseul Provinces. Jens Kruger, Physical Oceanographer at SOPAC, accompanied a team of geologists sponsored by the US National Science Foundation, UNESCO, and the Solomon Islands National Disaster Center, which was charged with documenting the changes in the coastal geology of the region, focusing on sediment movements and how they might be recorded in the long-term geologic record. On Ghizo Island, the villages of Titiana, New Manda, Nusa Baruka, and Gizo town, the capital of the Western Province, all reported deaths, while infrastructure was severely damaged in villages along the southwest coast. The island of Ranongga was uplifted over three metres in the south, affecting coastal access and groundwater, and causing fear amongst the local population that the sea would return at any time as a violent tsunami. Simbo Island, just 8 km from Ranongga, subsided over 1 m in places causing local populations to fear that the island would drop into the ocean following another earthquake. Many affected villages are relocating further inland. Sediment and topography data were collected from offshore and onshore at the villages of Pailongge, Titiana, and New Manra (onshore only) on Ghizo Island, Tapurai on Simbo, and the uninhabited Nusa Aghana to learn where the sediment was coming from and where it was deposited. It is believed that coral, both massive colonies on the barrier reef front and delicate colonies in the low-energy lagoon, were damaged primarily by the earthquake rather than the tsunami as extensive reef damage in places with little or no tsunami evidence was frequently observed. In places where there was a large tsunami, it appeared as if coral fragments broken by the earthquake, along with sand from the lagoon and beaches, were deposited onshore with the tsunami, and a significant quantity washed offshore into the lagoon. Immediately following deposition in the lagoon, littoral processes (including strong tidal currents) moved the sediment into deeper water by way of tidal channels and grooves in the barrier reef. The earthquake triggered numerous landslides, both inland and on the coast. These were investigated by a separate survey team consisting of Michael Bonte-Grapentin, SOPAC; and Douglas Billy, Department of Mines and Energy, Government of the Solomon Islands. This study, however, included a three-day reconnaissance of Tetepare Island, which included the mapping of a large landslide spilling over the reef flat, impacting on the marine environment. The local populations face numerous challenges in the recovery phase of this event. Debris removal and the fear of future earthquakes and tsunami are slowing down the process of people returning to their villages and returning to normal activities including livelihoods, food acquisition (fishing, farming, etc.), health care and schooling. Continuing artisanal fishing practices will be difficult for the people of Ranongga however, as most of the islands fringing reef has been uplifted, killing this productive ecosystem. It is important for scientists to work with local populations, local governmental officials, non-governmental organisations and media outlets to allay the fears of another imminent disaster while encouraging them to be prepared for the possibility of future events. There is a clear need to continue investigation and geodetic monitoring of the area affected by the 2007 earthquake and of the stressed region of the subduction zone beneath southern New Georgia adjacent to the 2007 rupture. There’s also need to understand the relationships of past earthquakes in these parts of the subduction zone to earthquakes further south, especially whether or not giant earthquakes have occurred in the past. In order to determine with confidence if giant earthquakes are likely in the Solomons – and if so, how often they might occur over long time periods – hence, we need to investigate the past history of major earthquakes and tsunami in the Solomon Islands for periods of thousands of years. Only with knowledge of this history, and thus with estimates of the severity of future earthquake hazards and of the probabilities or return periods of such events, will it be possible to make decisions on the basis of sound scientific evidence regarding public education for earthquake and tsunami hazard awareness and self-warning; land-use zonations and siting of infrastructure and development projects in coastal areas; and the installation of hazard warning and mitigation systems. A scoping document in support of a study that will help to address the above recommendations is included in APPENDIX 3.

EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 6 1 INTRODUCTION On 2nd April 2007 a Mw = 8.1 earthquake occurred at 7:39 AM local time (20:39 on 1st April UTC) in the western Solomon Islands. This earthquake occurred on the subduction zone at the complicated intersection of the Pacific with the Australian, Woodlark and Solomon Sea plates (Figure 1). The epicenter was 45 km south-southeast from Gizo, the capital of the Western Province. The rupture zone extended west for 100 km, stopping south of the Shortland Islands. This was the first earthquake larger than magnitude 7 to hit this area since the early 20th Century (USGS, 2007). The event was analogous to, but at least 30 times smaller than the giant Mw = 9.3 earthquake of December 2004 that affected Sumatra and produced tsunami that struck distant coastlines around the Indian Ocean, and has highlighted the importance of earthquake and earthquake-generated uplifts and tsunami as hazards in the Solomon Islands.

Figure 1. Geographic and tectonic setting of the Solomon Islands region. The Solomon Islands form an island archipelago and are part of the Greater Melanesian arc system. The Rennell Islands lie within the Australian Plate, just off the area of the map due south of Guadalcanal. The map shows the April 2 epicenter (red star), approximate rupture area (blue hachure pattern), and an approximate boundary (red dashed line) delineating coseismic uplift to the south and subsidence to the north (F. Taylor, pers. comm. 2007). Major plates and boundaries are annotated and their relative directions of motion indicated by arrows. Seafloor uplift triggered a tsunami that caused significant damage on Ghizo Island, and further damage in the Shortlands, Simbo, Ranongga, Vella Lavella, Choiseul, and Rendova islands. The earthquake and tsunami killed 52 people; – on Gizo (33), Simbo (9), Ranongga (2), Vella Lavella (2), and Choiseul (6). The earthquake also caused coseismic uplift and subsidence throughout the affected area, exposing the coral reef at Ranongga, and causing up to 1 m subsidence at Simbo and Ghizo islands. Significant landsliding was also reported throughout the region, but was largely focused on Ranongga where two people were killed at Mondo. The Intergovernmental Oceanographic Commission (IOC) of the United Nations Educational, Scientific, and Cultural Organization (UNESCO), and the ITIC is a clearinghouse for tsunami information (http://ioc3.unesco.org/itic/). International Tsunami Survey Teams (ITST) have been


EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 7 working with UNESCO to document the tsunami wave data in order to improve predictive models that can be used as mitigation devices. In the weeks following a tsunami, the ITST travels to the affected areas to measure wave heights, flow velocities, inundation distances and to interview witnesses to assess number of waves, along with other pertinent data (Figure 2). In recent years, geologists have joined the ITST in order to document those features created by the earthquake and/or tsunami that would be recorded in the geologic (permanent) record.

Figure 2. Cartoon of tsunami measurements. The inundation distance or limit is the maximum distance the wave penetrated inland from the post-tsunami coastline (which can change based on either coseismic uplift/subsidence or erosion from the tsunami). Wave height is measured from sea level to the height of the wave at any given point. The flow depth is the height of the wave above ground. The inundation or run-up elevation is the height above sea level at the wave’s inland-most point.

The goals in the survey were to document the coastal changes associated with this earthquake and tsunami by sampling the sediment moved by the tsunami both onshore and offshore, assessing damage to the coral reefs, and measuring uplift and subsidence at the coastline. If we are able to adequately describe the sedimentary evidence left by this present-day tsunami, we should be able to use sediment data from ancient deposits to determine paleotsunami wave properties. This earthquake also generated numerous landslides both on the coast and in the interior; however, studying these in detail are beyond the scope of this project. Another important goal of our team was to inform the local populace as to what had happened during the earthquake and tsunami, why it had happened, and what was likely to happen in the future. People who watched their coastal villages first violently shaken, and then washed away are understandably hesitant to return home. Aftershocks add to the fear. Informing the local population as to what happened and the reasons behind it (and what to expect in the future) goes a long way toward allaying those fears.




2 METHODS The team consisted of scientists from the academia in the United States, the Solomon Islands Government (Department of Mines and Energy), and SOPAC in the second of a multi-phase survey plan. The first team did a rapid-reconnaissance of the damage starting ~2 weeks after the event, noting places worthy of follow up. This team came in after 1 month and collected samples in locations suggested by the first group. Future teams can do even more detailed work following up on our initial findings. Following the Indian Ocean tsunami, civil engineers and geologists undertook rapid-reconnaissance surveys of the affected areas. We quickly found that the two groups work at different paces. Engineers need to cover large areas quickly, documenting topography and water heights along coastal transects. Each transect, depending on the size, takes around an hour. Geologists on the other hand, require time to take detailed samples of the sediment along the transect, as well as measure water heights and topography. A detailed geological survey in a tsunami inundation zone could take as long as one day. Following the 2006 Central Java earthquake and tsunami (Fritz et al., in press), we established a protocol where engineers and geologists were in the field at the same time; the engineers would rapidly survey the affected area before the transient water level data would disappear, locating areas for the geologists to sample. This cooperation allowed the engineers to complete their rapid, large-area surveys without having to worry about documenting the geology, and the geologists would know where the best sampling locations were likely to be based on the information passed on by the engineers. The Solomon Islands’ event was markedly different from the Central Java tsunami because unlike the long, continuous coast of Java, the affected area of the Solomon Islands consists of difficult to access small islands with a dispersed population and poor communications infrastructure. This made the initial relief phase and assessment of the affected area very challenging. A two-person engineering team, H. Fritz (Georgia Tech University) and N. Kalligeris (Tech. University of Crete), visited the affected area two weeks after the event to collect the ephemeral water level data. They visited Ghizo, Rendova, Simbo, Ranongga, Vella Lavella, and Choiseul Islands over the course of 10 days, reporting to us upon their return the best opportunities for studying sediment deposit and reef damage. We began our survey 2 weeks after they returned when the relief phase was over and the long-term recovery phase had begun. Our team consisted of three groups – one was an offshore group that focused on the source of sediment that was to be deposited onland along with the fate of sediment removed during the backwash; an on-land group to complete transects that document areas of erosion and deposition, with detailed vertical sampling in sand deposits along a transect; and two geologists who mapped the landslides. 2.1 Offshore This team characterised the offshore littoral region using a combination of snorkelling and SCUBA-based sampling along with a side-scan sonar and bathymetry mapping system. The sediment sampling transects began in the lagoon at the landward side of the barrier reef and continued towards land in line with the onshore team’s transect. We used a habitat mapping system where a buoyant Global Positioning System (GPS) unit was attached to a diver who would take photos every ~5 m (on a camera with the time stamp synchronized with the GPS) and take photos at the sample location sites (method after Roelfsema et al. 2007). The water depth data along the transect was collected by a sonar unit described below. The Humminbird 997c SI Combo depth-finding sonar system records bathymetry along with coral reef morphology using an integrated GPS and side-scan sonar system (Figure 3). This relatively

EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 9 affordable, easy to use, off-the-shelf technology proved very valuable in the mapping of the nearshore environment (Trond-Inge et al., 2002), and should prove useful in quantifying roughness for tsunami inundation models. The lagoon was too shallow to bring our boat in places (less than ~0.5 m), however we managed to collect shore-parallel and perpendicular lines at each location (Figure 4). The side-scan imagery, combined with groundtruthing, provides a swath of seafloor data that shows different depositional environments including areas of sediment deposition and coral reef colonies. Seaward of the barrier reef, data collection is more difficult as high wave energy causes the data to become noisy.

Figure 3. Humminbird 997c SI Combo depth sounder and side scan sonar system. Top: The data logging system (a) consists of a tablet PC, water resistant case and data transfer cable. The sonar system (b) has an external GPS antenna, CPU and data viewing screen, and transducer head. The power supply consists of a sealed 12V battery unit or any 12V car or marine battery. Bottom: a) The CPU/viewing screen and power supply is attached to the boat at b) and the sonar and GPS antenna are at c).



Figure 4. Offshore bathymetry and sonar tracks, Titiana Lagoon, Ghizo Island. 2.2 Onshore Onshore sampling involves measuring the thickness of the sand deposited by the tsunami (‘tsunami sediment’), sediment grain size and composition along a transect with wave height and topographic data. We measured topography using a laser rangefinder that calculates distance and elevations in tandem with a handheld global positioning system (GPS) unit. Depending on the length of the transect, samples were collected every ~10 m to assess lateral grain size variations. At each sample location, we dug a hole, then vertically sub-sampled the deposit every ~5 cm (again, depending on the thickness of the deposit) to determine vertical changes in the grain size. We then used the lateral and vertical variations in grain size, tied in with the wave heights and topography, to generate a model that shows how a wave must behave to transport sediment in the observed fashion.


EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 11 3 RESULTS We collected samples in the villages of Tapurai on Simbo Island, and Pailongge and Titiana on Ghizo Island, and the undeveloped Nusa Aghana (Figure 5). By sampling the offshore, we can attempt to constrain the ultimate source and deposition of the sediment in the system. The sampling in the three villages started in the offshore and continued onland until the sedimentation ceased. On Nusa Aghana where the tsunami washed over the island, we began in the lagoon on the south side, sampled the island itself, and continued sampling on the fringing reef on the north side of the island. At Njari and Makuti islands there was not enough sediment deposited onland to justify a transect, however, we noted the effects of the earthquake and tsunami on the reef organisms.

Figure 5. Location of Solomon Islands, Western Province survey sites. In addition to our survey, data points include surveys from two Japanese teams (Nishimura et al. pers. comm. 2007; and PARI [Tomita et al. 2007).


EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 12 3.1 Nusa Aghana We completed a sediment and topography transect from the Vonavona Lagoon on the south side of the island, over the island, and into the shallows of the fringing reef to the north of Nusa Aghana (Figure 6). Flow direction indicators show that the wave came from the southwest, overwashed the island to a flow depth of less than 0.5 m, and continued over the island into the shallows. The frontal lagoon was characterised by patches of branching coral (Acropora sp.), coral heads (Porites sp.), Halimeda and shell fragments. The branching coral colonies were 95% destroyed, and again showed no evidence of alignment, suggesting they had been deposited in a strong current. The topography onland (Figure 8) showed a high berm on the south side of the island, a minor swale in the middle, and a steep scarp (~1 m high) on the north side, presumably cut by the exiting tsunami wave. The sediment was deposited mostly in the swale, with shadows of coarse material in the lee of large trees and shrubs. Roots in the scarp were all facing towards the northeast, indicating that they had been oriented in the direction of the tsunami flow.

Figure 6. Locations of Nusa Aghana and Makuti Island, Western Province, Solomon islands. The force of the tsunami wave was directed toward the northeast along the study transect. Backdrop is a 2001 LandSAT image.



Figure 7. Living and fractured Acropora sp. branching coral in the lagoon behind the barrier reef on the southwest side of Nusa Aghana. Over 90% of the Acropora sp. colonies were found to be damaged by the earthquake, as most of the fractured colonies found were located close to their bases.

Figure 8. Nusa Aghana transect. In the cross-section, blue is water and brown is the land. Blue triangles indicate the water level at that point. Photographs: a) Subaerially exposed seagrass, which suggests some degree of uplift associated with the earthquake. b) Palm fronds and pandanus tree leaves indicate flow direction. c) Scarp on back side of island created by the water’s acceleration as it flowed downhill towards the lagoon. d) Living soft coral and anemone in the lagoon behind the island, which were partially covered by a veneer of tsunami sediment.. The delicate branching and tabular forms of hard corals in the lagoons on the southwest and northeast sides of the island were mostly shorn off, lying at the base of the original colony (Figure 7). Further offshore on the edge of the fringing reef on the northweast of the island, large coral heads had been fractured, presumably by the earthquake, and moved down slope into deeper water (> 30 m). Large boulders (>3 m in diameter) with evidence of fresh breaks were present over a 20 m swath (Figure 9). Sediment moved by tidal currents from the beach and lagoon responded to the change in


EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 14 topography by moving toward the newly-formed gap in the reef and eventually into the deeper waters of Blackett Straight.

Figure 9. Examples of reef damage. Following the earthquake, the branching Acropora sp. colonies fell in interlocking nests that would make transport by a tsunami difficult. Large Porites sp. (?) coral heads were sheared off at their narrow bases. The sediment deposited onland by the tsunami was not very thick, around 10 cm at most. It consisted of Halimeda plates, cobble-sized coral fragments consisted of some material long dead and some recently killed by the earthquake, along with broken shell material. There was no internal structure visible to the eye, nor was any evidence of fining away from the ocean present with the exception of the shadow zone behind trees. 3.2 Tapurai Simbo Island, which resides on the Australian/Woodlark plate boundary south of the subduction zone, subsided during the earthquake. Tapurai, on the northern tip, is a small village of 234 people, and was completely destroyed by the tsunami (Figure 10). Flow direction indicators suggest that the wave approached from the north, hit the roughly east-west oriented cliff at the south of the village, and was pushed off to the west. It was unusual in that a thick deposit – 20 cm thick – consisting of a mixture of coral and sand along with basalt boulders of >1 m diameter were moved into the village from a cliff bordering the south side of the village. The sediment deposition was patchy, so rather than taking a transect, we chose to complete a 2-dimensional sampling pattern over the area.



Figure 10. Top. Village of Tapurai on the northeast coast of Simbo Island. This satellite image shows the village prior to the 2nd April, 2007 tsunami. Bottom. Photograph showing the village grounds toward the northeast taken on 17/05/2007.


EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 16 The villagers at Tapurai have decided to relocate their village to higher ground due to fear of another tsunami. Right now, the problem facing them is access to sanitary latrine facilities. Traditionally, they used the beach, but the new location is approximately 100 m further inland and up slope, making it impractical for the very old and very young, especially at night. The water is supplied from the same stream which provided the previous location, and is therefore not a problem. We did notice, however, that there was some planting of crops (watermelon, pandanus trees, bananas) taking place at the old location. As the new village is not very far from the beach, fishermen should have only a minor inconvenience accessing their boats and gear. 3.3 Pailongge Pailongge is a Melanesian village on the south coast of Ghizo Island, west of Gizo town (Figure 11). Of the population of 76 people, none were killed during the tsunami, despite the village having been flattened. Once the ground started shaking, everyone knew to run to the hills. One villager gathered the children together to get them to move to higher ground. Eyewitnesses say the tsunami followed less than 2 minutes after the shaking ceased.

Figure 11. Ghizo Island. Map courtesy of the Solomon Islands National Disaster Management Office. Population estimates (in boxes) are based on 1999 Census data. Like the other locations, the ocean side of Pailongge Lagoon is bounded by a barrier reef consisting largely of massive head corals (Porites sp.) and encrusting algae. The low-profile morphology of these colonies is better adapted to higher wave energy. In the low-energy lagoon landward of the barrier reef, most (>95%) of the branching corals (Acropora sp.) were shattered by the earthquake.


EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 17 Once again, there was no evidence that the branching corals were deposited on the seafloor in the presence of a current. Approximately 20 m seaward of the shoreline, there is a deep (>7 m) tidal channel filled with coarse sand with well-developed ripple marks, indicating strong tidal current (3-5 knots) transporting sand offshore (Figure 12b).

Figure 12. The south coast of Ghizo Island was heavily affected by the tsunami. At Pailongge, boulders were moved from the barrier reef front towards the coastline by the tsunami at a natural channel cut in the reef (a). Inset figure (b) shows a bathymetric profile on the left and a side-scan sonar image of the seafloor (right) in the area of the channel. The well-developed ripple marks on the sandy bottom that butts against the reef suggests active sediment transport. At Titiana (c) as well as Pailongge (a), many houses and vehicles were lifted and floated landward by the tsunami. At New Manda, houses set back ~50 m from the coastline behind a stand of mangroves were destroyed (d). At Pailongge there is a series of large boulders (1.5 m in diameter) of cemented coral that have been thrown into shallow water (intertidal) by the tsunami. Eyewitnesses said that they were not present prior to the tsunami (Figure 12a). Despite the strength of the tsunami required to move these boulders, the houses in the background were floated off their foundations rather than destroyed by the waves. This section is directly onshore of the natural channel in the reef where the sediment from the lagoon exits. Most of the sediment brought offshore during the tsunami is moved rapidly into deeper water via this channel (Figure 12b). The sediment deposited onland was a well-sorted, medium-sized carbonate sand consisting of coral and shell fragments along with Halimeda plates. Occasional coral heads were deposited almost 100 m inland. The sediment deposit continued 115 m inland. 3.4 Titiana Perhaps the most notable things about Titiana, a Gilbertese village west of Gizo, are 1) despite having a flow depth of over 3 m in places, many of the houses and cars floated inland, but were not destroyed, suggesting a quick but not violently rising sea level (like a very rapidly rising and falling tide rather than a massive wave), and 2) despite this quick rise in sea level caused by the tsunami, of a population of 528 people (according to a 2005 study by the Solomon Islands Road Improvement Project), 13 people (2.5%) died, and of those, nearly half (6) were children under 8 years old.


EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 18 According to survivors, the Gilbertese did not know how to react to an earthquake followed by a rapid withdrawal of the sea, and were killed by the tsunami when they went to explore the exposed seafloor. At Titiana, like the other locations, there were sparse boulder-sized coral heads deposited with the sand as far as 100 m inland. Sediment was deposited over 400 m of the 800 m total inundation distance. Flow depths at Titiana were 4.5 m almost 100 m inland, yet several houses floated off their foundations and deposited, relatively intact, at the base of trees. Cars had not been rolled over. The offshore at Titiana was characterised by a barrier reef with low-aspect head corals and encrusting algae, a low-energy lagoon with patches of sheared-off branching coral (Acropora sp.) and coral heads (Porites sp.), sandy sea-bottom with coral and shell fragments and Halimeda plates, and seagrass beds. Shore parallel ripple marks indicate sediment movement via wind waves and shore-normal ripple marks are indicative of tidal currents moving sediment out of the lagoon and into deeper water via the natural channel in the reef (Figure 13).

a b

Figure 13. Side-scan sonar images of sand ripples in the lagoon at Titiana. Field of view is approximately 10 m x 30 m. The shore parallel ripples in (a) are wind-wave generated and the shore normal ripples in (b) are in the base of the tidal channel responsible for removing sediment from the lagoon via a slot in the reef. 3.5 Njari Island On Njari Island, northwest of Ghizo, there was no noticeable evidence for tsunami inundation. The reef offshore, however, was significantly damaged (Figure 14). “Grand Central” is a popular dive spot, noted for the variety of fish species that converge there. In the shallow lagoon on the front, every colony (we counted 12 examples) of table coral (Acropora sp.) was broken at the small <10 cm base of the ~0.5 to 1 m diameter flat, plate like corals. The colonies had simply toppled over and had come to rest on the seafloor adjacent to the base from which it was broken. The colony was dead, and the edges were slightly fractured, suggesting that it had not been battered by a turbulent tsunami bore (Figure 14). On the back (north) side of the island, colonies of branching coral were also entirely destroyed, however the apparently random arrangements of the broken coral suggests that they had


EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 19 not been moved by a current. Coral heads (Porites sp.) with ~50 cm diameter attached to the seafloor with bases with an ~15 cm diameter were also shorn off, but came to rest near their roots.

Figure 14. Broken table and branching coral, Njari Lagoon. Notice how some edges of the table coral are fractured where they landed on hard rock, while other edges are intact, suggesting that the coral was not deposited by a turbulent bore, but rather was broken off by the earthquake. Branching coral that stands high above the seafloor on narrow bases were broken off near the base during the earthquake shaking. Photos courtesy of Tingo Leve, World Wildlife Federation (WWF), Gizo, Solomon Islands.

Figure 15. Njari Island coral head before and after earthquake. Photos courtesy of Tingo Leve, WWF, Gizo. On the north side of the island, there is a drop off from the shallows (< 2 m water depth) along a steep slope, with slope gradients around 60° on the east side of the island, then increases to close to 80° on the northwest side of the island. Here, coral rubble including broken branching corals, Porites sp. coral heads and Halimeda fragments had been moved downslope via sheet- like deposits, with occasional chutes focusing sediment delivery, to depths of 20 to 30 m. The entire reef was draped by a fine layer of sediment in the days following the earthquake, but had been largely removed after one month.


EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 20 3.6 Makuti Island Makuti is east of the deepwater Ferguson Passage (see Figure 6), however it has a similar morphology to Njari. The north side of the island has a narrow, shallow lagoon, then drops off to a vertical ‘wall’ with slope exceeding 90° (overhanging) in places. The tsunami did inundate Makuti, however with flow depths no higher than 30 cm. There is no evidence of on-land sediment deposition on the island with the exception of a 60-m swath on the back (north) side of the island where large, freshly-killed coral boulders (mostly Porites sp. coral heads) had been pushed onland, 2 m above sea level (Figure 16).

Figure 16. Porites sp. coral heads from the lagoon on the north side of Makuti Island were pushed onshore in the opposite direction of the initial tsunami wave (A). The Porites sp. coral heads were broken off by the earthquake, then transported onland by refracted tsunami waves that collided on the back side of the island. Coral species with wider bases (B and C) were not fractured by the earthquake, and were able to withstand the tsunami without breaking. In the shallows on the north side of the island, just offshore from the area where the coral heads had been deposited onland, there were very few in situ coral heads, and the ones that had been broken and recently killed were not obviously near their bases. Seaward (ENE) from the shallows, the seafloor drops off along a wall (“Joe’s Wall”), which is only damaged where chutes delivered fractured branching coral (Acropora sp.), coral heads (Porites sp.) and Halimeda fragments to deeper water (Figure 17). These 2-3 m deep sediment delivery chutes correspond to discontinuities in the reef that were most likely there before the earthquake/tsunami. In the areas between the chutes, delicate branching corals, sea fans and sponges were largely unaffected by the events, save for some minor deposition of sediment particles from suspension.



Figure 17. Sediment chute at Makuti (aka “Joe’s Wall”). These chutes, ~10 m apart, delivered sediment from the lagoon behind the island to deeper water. Notice fragments of recently-killed Acropora sp. branching coral and Porites sp. coral heads in the sandy debris. The tsunami must have been exceptionally strong at Makuti (due to convergence of refracted waves) to have moved these materials. 3.7 Ranongga The reefs surrounding the island of Ranongga sustained the most drastic and remarkable effects of the earthquake. Our ground observations on the southeast coast of Ranongga near the villages of Kara and Kongu confirmed that parts of the island had been raised by 2-3 m during the earthquake. The corals of the fringing reefs were uplifted, exposed to air, and have died. The coral showed none of the damage attributed to shaking that we observed in other locations. Rather the branching and tabular stony corals were preserved in situ and have retained their original structure. Villages that used to be able to launch their fishing canoes from a sandy beach are now separated from the sea by up to 100 m of rocky shoreline. One kilometre south of Kongu, the estuary of Kolokukunde has dried up, with the associated mangrove swamp now disconnected from the sea.



Figure 18. Emerged coral reefs of Ranongga Island. The coseismic uplift has raised the frining reef with devastating long-term impacts on the subsistence fisheries and the livelihoods of local communities. Overall, more than three quarters of the shallow-reef ecosystems and mangrove habitats along the 82-km coastline of Ranongga have been lost due to the coseismic uplift associated with the 2nd April earthquake. While reefs in other locations throughout the Western Province will recover over the coming years the changes on Ranongga are permanent. This has significant effects on the sustainable livelihoods of the local communities, as they rely heavily on inshore fishing as a source of nutrition. Relief efforts will need to pay special attention to the long-term needs of the communities on Ranongga, and assist them to adapt to a new environment of greatly-reduced marine productivity and biodiversity. 3.8 Landslides Numerous landslides were also documented on Tetepare, the largest uninhabited island in the Pacific, an area which has been nominated for World Heritage listing. Tetepare was outside of the main rupture zone and the shaking and tsunami experienced in this area was significantly less than in the areas further west. We estimated that less than 5% of the coral were damaged due to the earthquake, while tsunami flow depths were less than 0.2 m. However, the marine protected area in the west of the island sustained damages caused by landslides delivering large volumes of debris onto the beaches and reefs. The run-out distance of the largest landside was measured at 300 m, blanketing the beach and reef flat. Important Leatherback turtle nesting sites have been impacted, as well as adjacent seagrass beds and coral, which are now covered by a highly mobile veneer of silt.


EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 23 4 INTERPRETATIONS The sediment deposition helps us determine a sequence of events that followed the earthquake and tsunami. By understanding the relative timing of events and being able to separate out the effects of the earthquake from that of the tsunami (i.e. reef damage), we can better predict the far-field effects of tsunami, how such events are preserved in the geologic record, and how this environment might respond in the present day. This has implications for the post-tsunami recovery phase – for example, fish habitat on a reef that has been shorn off by an earthquake is already starting to recover, while sediment moved offshore by the tsunami is quickly being flushed into deeper water by tidal currents. 4.1 Coral Transport The depositional evidence suggests that the delicate coral colonies such as the branching and table corals (Acropora sp.) that grow in low-energy, lagoon environments were shorn off by the earthquake acceleration rather than being battered by a turbulent tsunami bore. Had the tsunami alone been responsible for breaking them, we would have expected to have seen more long-distance transport, broken tips and edges, and/or alignment of elongate coral fragments. The more massive Porites sp. head corals that grow in the lagoons and have the narrow bases also snapped under shaking. The Porites sp. are likely transported (at Makuti, Pailongge, Titiana, and Tapurai) because their morphology makes them more susceptible to rolling than the interlocking nature of the branching coral (Figure 19).

Figure 19. Coral head (Porites sp.) deposited ~20 m inland at Titiana. This colony was killed during the event, as evidenced by its fresh appearance. The round morphology makes it more susceptible to transport via rolling by a tsunami.


EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 24 4.2 The Tsunami This tsunami behaved more like a rapidly rising and falling tide rather than a very fast, turbulent bore. Tabular coral were snapped off their bases but not moved, despite having a flat, sail-like morphology that would lend itself to transport in a strong current (Figure 14). Considering vans and lightly-constructed houses floated inland rather than rolled, it is most likely that the wave was not very energetic (Figure 20). This conclusion, however, raises several questions:–

• If the wave was not very turbulent and energetic, how were the sediment, including coarse-grained to cobble-sized material along with several kilogram Porites sp. coral head transported?

• Why did the wave take on this less-energetic form in this area close to the epicenter where other waves come in as a bore?

• Did the steep offshore barrier reef reflect some of the energy back out to sea? • Were roughness elements in the lagoon (patch reefs, tidal channels, etc.) enough to slow

down the wave without drastically reducing its amplitude? The bathymetry and sonar data we collected will be shared with the civil engineers who can create computer models that can show how these elements might affect inland wave propagation.

Figure 20. House and truck (which belongs to the owner of the house) that were floated ~50 m inland by the tsunami at Titiana. Notice that both house and truck were lightly damaged, suggesting that the tsunami behaved not as a turbulent bore, but rather as a swiftly rising tide.


EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 25 4.3 Offshore-Onshore Sediment Transport As far as we are aware, this is the first study to attempt to track the fate of both sediment deposited onland along with the material removed from the littoral zone and deposited further offshore. Previous work has shown that the sediment deposited onland accounts for only a fraction of the material removed from the beach. At Pailongge for example, there is a very strong tidal current that is very efficient at removing material from the lagoon into deeper water via channels through the barrier reef. It is possible that our data collection is biased as communities would tend to locate where there are good channels in the barrier reef for boat access (which we also needed). Sonar imagery shows large deposits of sand in deeper water near the channels (Figure 21). These sand deposits have sharp contact with rock outcrops, which lead us to believe that they are recent deposits that are outside of the normal depositional regime.

Figure 21. Sonar image from the channel offshore Pailongge. This channel is seaward of the barrier reef, and ripple marks indicate ongoing sediment transport, most likely from the lagoon where coral, bivalves and encrusting algae create sediment, down the channel and into deeper water. Some of sediment from the beach and the lagoon entrained by the tsunami was deposited on shore. The sediment must have been entrained in the lagoon as there were occasional pieces of branching Acropora sp. and more commonly Porites sp. coral in the deposit that had been recently killed. It is possible that in places, the wave came through the lagoon and onshore as a rapidly moving turbulent bore, then slowed significantly (supercritical?) not far from the beach. Patches of the lagoon would have had the evidence of pre-tsunami coral colonies removed by the turbulent wave, whereas other patches would have been largely unaffected. Houses closest to the beach were destroyed while those further back (second row, ~30 m inland from the beach) floated. The road is ~50 m inland, landward of this second row of houses, so cars that would have been parked there were also floated by this part of the tsunami. The sediment deposits in these areas were a thin sheet close to the beach, then dissipated rapidly inshore, supporting this conclusion.




One important remaining question is how coarse-grained (boulder size) material was transported inland if the tsunami was not moving as a highly turbulent bore with sediment suspended throughout the water column? Makuti Island offers one possible explanation for how some of the coarser material may have been transported. The areas on the front and sides of Makuti above the high tide line were largely unaffected by the tsunami. On the back (landward) side of the island where topography is not noticeably different, we found evidence of overland, seaward (SW) directed flow with depths around 30 cm. In the narrow (60-m wide) swath where Porites sp. boulders were deposited in the supratidal zone, the largest northeast propagating tsunami waves must have refracted around the island and converged in a seaward direction in this swath. In this zone, corals were broken but none were deposited near their bases. Outside of this 60-m wide zone, corals were broken but not transported. Nusa Aghana is an interesting laboratory to begin to assess how this tsunami moved and deposited sediment. Our transect follows the unidirectional wave (the wave washed over this narrow island so there was little, if any, backflow) from the lagoon on the seaward (south) side of the island, over land where there was some sediment deposition, and into deeper waters behind (north of) the island. Minimal transport of the coral in the lagoon suggests that the tsunami was not a turbulent bore here, yet it was able to entrain sediment, deposit it onland with a maximum flow depth of only 30 cm, and create a significant, 1+ m high scarp with the outflow on the back side of the island. Results of the grain-size analysis will likely tell us a great deal about the dynamics of this wave. We can speculate as to why this tsunami behaved more like a rapidly rising tide than a violent, turbulent bore. It is possible that the morphology of the barrier reef helped to make this a less violent tsunami. The seaward side of the barrier reef that forms part of the rim encircling Vonavona Lagoon is a very steep slope which would likely reflect a lot of wave energy back out to sea where it can be dispersed. Because of the large wavelength of the tsunami, some water inevitably spilled into the lagoon and on to land, but it had been significantly buffered by the buttressing reef. Individual coral colonies would add roughness to the seabed that might also slow down an incoming wave, but as many of these branching and tabular corals were damaged in the earthquake, they would not have been present to affect the wave. Detailed bathymetry of the region seaward of the barrier reef will be invaluable to help the modelers test the effect of the barrier reef’s morphology on mitigating the effects of this tsunami.



5 LOCAL POPULATIONS AND POST-DISASTER AID As with every major earthquake and tsunami, there is a variety of ways the event affects the local populations. Shaking from the earthquake itself destroys buildings and coral reefs. Uplift and subsidence changes water tables and coastal access. The tsunami that followed the earthquake destroyed many buildings left standing by the earthquake, and instilled a mortal fear of the ocean in a large portion of the coastal populations. All of these factors must be considered when planning the recovery of tsunami-affected regions. 5.1 Water Strong shaking can change water tables by increasing the permeability of aquifers (e.g. Illangasekare et al. 2006) causing wells to go dry. Wells can also be affected by changes in land level by uplift (which can raise the land surface above the height of the water table) and subsidence (which can drop wells in coastal areas into the salt-water wedge, permanently affecting water quality). The uplift in Ranongga also affected ocean access at what should be considered a permanent time scale (10’s to 100’s of years). 5.2 Reef Livelihoods The coral reefs, especially the fringing reefs of Ranongga, were damaged by the earthquake. Vulnerable lagoonal species of coral with branching and tabular morphologies provide habitat for fish, and it is still unclear how those populations will be affected. Both the fisheries and the dive tourism industries rely on healthy fish stocks for their trade, so the livelihoods sector should pay close attention to the results of the habitat studies by the World Wildlife Fund and WorldFish Organization. This is a critical issue for the 6000 inhabitants of Ranongga, as their uplifted reef will not recover, depriving them of inshore fishing grounds, possibly leading to increasing pressure and resultant further decline in the remaining offshore habitats. 5.3 Return to Normalcy There are several factors affecting the local populations’ ability to return to their pre-tsunami occupations. First and foremost is pure fear. Those that are aware of tsunami warning signs know that if the sea retreats, there is likely a tsunami approaching. The uplifted reef at Ranongga makes it appear that the ocean has retreated (rather than the land level going up) so many believe that the ocean will return as another tsunami. On Simbo Island, which subsided during the earthquake, people are afraid that the island will sink once and for all following an aftershock. While these fears seem far-fetched to earth scientists, it is not too difficult a stretch of the imagination to see where they are coming from. The internally-displaced persons (IDP) camps are also affecting the rate of return to normalcy (i.e. permanent housing, jobs, schooling, etc.). The earthquake and tsunami destroyed buildings, leaving a tremendous amount of debris in the coastal villages, most of which must be removed (or recycled) by hand. This will take an extraordinary amount of time. The IDP camps are located on higher ground, acknowledging the affected populations’ fears of returning to the coast, and recognising the massive cleanup effort that must occur there. When considering future livelihoods and food security stability, it is important for people to return to their villages as soon as reasonably possible. There is little incentive to begin planting in or near the villages if there is a steady flow of food coming into the camps from governmental agencies and NGOs. Many villages are considering permanent relocation inland (or upland) which may inconvenience fishermen, and cause marked cultural changes. We noticed that on Simbo Island, further afield from Gizo where NGOs are less active, the locals have started replanting crops in the affected areas, whereas those on Ghizo Island are becoming ever more reliant on food donations, and have not started replanting.

EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 28 5.4 Tourism The SCUBA diving tourism industry in the Solomon Islands around Gizo constitutes a significant portion of the local economy. The shallow, lagoonal reefs were significantly affected by the tsunami, but the “walls” were not as affected. There are chutes on the walls that remove sediment from the lagoon into deeper water. It looks like there was significant scouring in these grooves, but the areas in-between were largely unaffected. According to some dive tourists that had visited 2 years prior, the reef was certainly more damaged in places, but the fish life was equally spectacular. As an aside, the sunken WWII era Japanese cargo ship, the Tora Maru was damaged by the earthquake. The bridge area, which before had been intact, had fallen off, exposing many holds in the ship that were previously inaccessible (Figure 22).

Bridge

Figure 22. Sonar image of the 140 m long Japanese freighter Tora Maru. Prior to the earthquake, the bridge was intact, however shaking caused its collapse, exposing inner holds that had not been previously accessible. 5.5 Education It is important for the scientists to allay the fears of a returning earthquake and/or tsunami. The normal occurrence of aftershocks has the population rattled. As part of our educational efforts, we worked with NGOs (World Vision, Save the Children, UNICEF, and WWF) to provide a short explanation of the science of earthquakes and tsunami, what to expect in the future, and how to respond if a tsunami occurs in the future (Appendix 1).




In summary –

• Earthquakes release “pressure” in the earth – once that pressure is released, it will take a long time to build up enough for another big earthquake.

• Changes in land level (uplift/subsidence) may accompany earthquakes. These changes are long lasting, and should be considered permanent when planning for relocation of villages, coastal access points, etc.

• Aftershocks are merely the earth “returning to normal” after a big change. They are a good thing. Aftershocks are always going to be smaller than the main shock and will decrease in frequency and magnitude over time. It would be very difficult for them to generate another tsunami or cause major structural damage to buildings.

• While another earthquake is highly unlikely, on rare occasions, earthquakes come in pairs, the most recent occurrence was the 28 March 2005 Nias Island earthquake that followed the 26 December 2004 Indian Ocean event. If by chance this should happen, areas immediately adjacent to the primary earthquake should know the warning signs (SAFE):

• Strong Shaking? • Altered Oceans? • FAST! Run to the hills! • Eventually return home.

or perhaps a little less politically correct, but with the potential to appeal to children (SURFR):

• Strong shaking? • Unusual ocean? • RUN to higher ground! • For the next two hours, stay put. • Return home.

We recognise that local populations affected by this tsunami, or possibly in line for the next, are unlikely to be reading this report. “Foreign experts” often come in to affected areas, collect data, leave and write a report that will likely be read only by other foreigners. The most important institutions capable of educating the local populace are local institutions. Our colleagues from the Solomon Islands speak the local dialects, know the culture, and can do a far better job of communicating the issues than scientists from other countries. It is critical that local experts be included in post-disaster assessments.

We used several tools for outreach to a variety of constituents. Again, the most important of these were our colleagues from the Solomons that took the time to explain to curious onlookers (from children to chiefs) what we were up to. One guaranteed method of drawing a crowd was the display of ‘peels’ (Figure 23). Peels are an actual physical display of the sediment deposited by a tsunami. We dig a hole in the sediment, usually a 10-20 cm thick deposit, then use a hydrophyllic grout to preserve a section that can be taken out of the field and used as a display. Something as unusual as this always draws an interested crowd.


Figure 23. Dr Andrew Moore displaying a ‘peel’ from neighboring Simbo Island to people on Ranongga.

We also attended meetings at the Provincial Disaster Center in Gizo where three times a week, the local and international NGOs and government agencies gather to discuss progress. As these are the people interacting with a large percentage of the affected population, this forum proved excellent in getting out our message. It is also important to mention that a lot of aid workers are also rattled by the seemingly constant aftershocks, and are in need of a simple scientific explanation. Local media outlets including radio and newspapers echoed the information we presented to the people we met in the field, and the NGOs that were also looking for information. It is important to distill the critical message into one that is easily digested. We wrote an article for the widely-read Solomon Star that summarised the earthquake and tsunami along with a brief explanation of our activities (Appendix 2). It is also important to recognise the diversity of cultures that exist in a given area, even one as relatively small as the Solomon Islands. The ethnic makeup of the Solomon’s Western Province includes the local Melanesian population, along with immigrants from Polynesia and Micronesia. On Ghizo Island, there is a substantial Gilbertese minority from the island-nation of Kiribati. Of the 52 people killed by the earthquake and tsunami, 31 were Gilbertese, 60% of those killed, and the Gilbertese represent only 14% of the total affected population. This discrepancy can be explained by a lack of cultural knowledge of how to react to tsunami . At Pailongge, nobody was killed because the Melanesian populace knew that if the ground shakes that hard, a tsunami is likely, validated by the initial retreating wave that emptied the lagoon. In Titiana, a Gilbertese community of 366 people, just 3 km east of Pailongge, 13 people were killed as they went to explore the emptied lagoon. The importance of educating the children was underscored by the fact that 46% of the dead were young children (under 10 years old). It is likely that children this young did not have the strength the adults have to swim against a relatively mild tsunami.




6 CONCLUSIONS The 2nd April 2007 earthquake and tsunami in the Western Province of the Solomon Islands killed 52 people. Strong shaking damaged the delicate corals in the lagoons, and in places, moved them on to land in a strong surge that would have resembled a rapidly incoming tide rather than a turbulent bore. As the wave came ashore, it deposited sand, shell, and coral fragments just inland from the beach. The sediment began tapering off landward as the wave lost energy. At Pailongge and Titiana on Ghizo Island, houses closest to the beach were destroyed by the wave, and houses further inland floated off their foundations and deposited tens of meters away. While some sand was deposited onland, much was also taken offshore, where littoral (tidal) currents moved them into deeper water where they are more likely to be preserved in the geologic record. On Nusa Aghana, a smaller, uninhabited island, a unidirectional tsunami came in from the sea, again picked up pieces of sand, shell, and coral, deposited them onland and in the lagoon behind the island as it exited. Much of the sediment moved by the tsunami will be transported into deep water via channels and normal littoral processes in the nearshore lagoons. Many islands in the region experienced uplift and subsidence, killing coral, changing water tables, and limiting coastal access. Those that are living in the affected regions are experiencing aftershocks almost daily, further rattling nerves. The fear is that further changes in land levels will lead to the loss of entire islands and future tsunami . It is critical that scientific teams use all means at their disposal to educate the local populace as to what is going on, what is to be expected, and what has to be done in the case of another event. The best way to maximise exposure is to meet with local officials, NGOs, news media outlets (newspapers, radio, television) and the like. It is best for local collaborating scientists to take the lead on these communications. This knowledge distributed goes a long way to settling nerves. Most that lost their homes are still living in IDP camps a month after the tsunami cleanup continues. Reconstruction is limited as the volume of debris removal is daunting, along with the omnipresent fear of another major earthquake and tsunami, and large amounts of government and NGO aid that are keeping IDPs in the camps. As the likelihood of another major tsunami hitting the same area is very slim, energy should be focused on returning people to their villages as soon as possible. Damage to the reef may have a long-term effect on livelihoods, as fish stocks could begin to decline reflecting the changed habitat, and the SCUBA diving industry, as tourists fear the rumours (unsubstantiated, in our opinion) of severely damaged reef. Assessments of these habitats by scientists at NGOs such as WWF and World Fish are critical to the future economic health of the region.



7 RECOMMENDATIONS The recent 2nd April 2007 earthquake and the resultant uplift, subsidence, and tsunami events in the Solomon Islands have highlighted the possibility of future such events in this region, especially to the east of the 2nd April rupture as far as Guadalcanal. The objective of this project is to contribute to the understanding of geological hazards originating in the Pacific region by utilising palaeogeodesy, palaeoseismology, marine geophysics, and tsunami deposits to identify and characterise historic earthquakes, uplift, and tsunami for the Solomon Islands subduction zone. The goal of such a study would be to produce estimates of the severity and return periods of future earthquake hazards based on the scientific results of field surveys. The results will provide insight for decision makers to better understand these rare but devastating events, make recommendations onland zonation and infrastructure siting, improve public awareness, increase socio-economic resilience, and mitigate the loss of life in future events. There is a clear need to continue investigation and geodetic monitoring of the area affected by the 2007 earthquake and of the stressed region of the subduction zone beneath southern New Georgia adjacent to the 2007 rupture. There is also a need to understand the relationships of past earthquakes in these parts of the subduction zone to earthquakes further south, especially whether or not giant earthquakes have occurred in the past. In order to determine with confidence if giant earthquakes are likely in the Solomons – and if so, how often they might occur over long time periods – hence, we need to investigate the past history of major earthquakes and tsunami in the Solomon Islands for periods of thousands of years. Only with knowledge of this history, and thus with estimates of the severity of future earthquake hazards and of the probabilities or return periods of such events, will it be possible to make decisions on the basis of sound scientific evidence regarding public education for earthquake and tsunami hazard awareness and self-warning; land-use zonations and siting of infrastructure and development projects in coastal areas; and the installation of hazard warning and mitigation systems. A scoping document in support of a study that will help to address the above recommendations is included in APPENDIX 3.



8 BIBLIOGRAPHY Fritz, H. M., W. Kongko, A. Moore, B. McAdoo, J. Goff, C. Harbitz, et al., Extreme run-up from the 17

July 2006 Java tsunami, Geophysical Research Letters 34, in press, 2007. Illangasekare, T., et al., Impacts of the 2004 tsunami on groundwater resources in Sri Lanka, Water

Resour. Res., 42, 2006. Roelfsema, C., Phinn, S., Joyce, K., 2007. A manual for using GPS referenced digital photo transects

to validate benthic cover maps. Centre for remote sensing & spatial information science School of Geography, Planning and Architecture. University of Queensland, Australia. 29p.

Tomita, T., Arikawa, T., Tatsumi, D., Honda, K., Higashino, H., Watanabe, K., and S. Takahashi,

2007. Brief preliminary report on Field Survey of Solomon Islands Earthquake Tsunami in April 2007. (DRAFT. May 2007.) Tsunami Research Center, Port and Airport Research Institute.

Trond-Inge Kvernevik; Mohd Zambri Mohd Akhir; Jill Studholme. A low-cost procedure for automatic

Sea-floor mapping, with particular reference to coral reef conservation in developing nations. Hydrobiologia 474(1): 67-79; 2002.

United States Geologicial Survey, Earthquake Hazards Program Website, Solomon Islands

earthquake page, http://earthquake.usgs.gov/eqcenter/recenteqsww/Quakes/us2007aqbk.php (accessed on 12 June 2007).

http://earthquake.usgs.gov/eqcenter/recenteqsww/Quakes/us2007aqbk.php

EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 34 APPENDIX 1


EU EDF-SOPAC Reducing Vulnerability of Pacific ACP States Solomon Islands – Geologic Impacts of the 2 April 2007 Earthquake and Tsunami – 35 APPENDIX 2




APPENDIX 3

Earthquake and Tsunami Hazards along the Solomon Arc Draft Project Scoping Document

The recent 2 nd April 2007 earthquake and resultant uplift and tsunami events in the Solomon Islands have highlighted the possibility of future events in this region, especially to the east of the 2nd April rupture as far as Guadalcanal. The objective of this project is to contribute to the understanding of geological hazards originating in the Pacific region by utilising palaeogeodesy, palaeoseismology, and tsunami deposits to identify and characterise past earthquakes, earthquake – generated uplifts, and tsunami for the Solomon Islands subduction zone. The goal of this project is to produce estimates of the severity and return periods of future earthquake hazards based on the scientific results of field surveys. The results will provide insights for decision makers to better understand these rare but devastating events, make recommendations onland zonation and infrastructure siting, improve public awareness, increase socio-economic resilience, and mitigate the loss of life of future events. Simon Day, University College London and University of California at Santa Cruz Jens Kruger, Pacific Islands Applied Geoscience Commission, SOPAC June 2007 Introduction The 2nd April 2007 Solomon Islands earthquake, and the associated tsunami waves and uplift of reefs, have highlighted the importance of earthquake and earthquake-generated uplifts and tsunami as hazards in the Solomon Islands. The Magnitude Mw = 8.1 earthquake occurred on the subduction zone where the ocean crust of the Solomon Sea descends eastwards beneath the Solomon Islands themselves (Fig. 1). It was therefore analogous to, but at least 30 times smaller than, the giant Mw = 9.3 earthquake of December 2004 that affected Sumatra and produced tsunami that struck distant coastlines around the Indian Ocean. The December 2004 earthquake was unexpected to many Earth scientists because it was thought that the Sumatra subduction zone could not produce so large an earthquake, and that Mw >9 earthquakes were only produced on a few subduction zones such as those off Alaska and Chile. The Sumatra 2004 earthquake has highlighted the need for direct determination of the histories of earthquake activity on all subduction zones, to find out if they produce giant earthquakes with potential for widespread and severe earthquake damage in the source region and ocean-wide tsunami damage to coastlines. The fact that rupture of only a small segment of the Solomons subduction zone off New Georgia produced a Mw = 8.1 earthquake in April 2007 indicates that rupture of the whole subduction zone to the southeast, at least as far as Guadalcanal, could potentially produce a giant Mw = 8.5 – 9 earthquake, little smaller than the catastrophic Sumatra event. A similar conclusion was reached after the 2nd April earthquake by Cummins (2007) while the United States Geological Survey identified the Solomons subduction zone as a high-priority area for tsunami hazard investigation in a review of all Western Pacific subduction zones (USGS, 2006). An initial geodetic study of the event (Taylor et al. 2007) indicates that stress levels on the subduction zone have risen in the area immediately south of the rupture making an earthquake there more likely over the next decades. Such an earthquake may be relatively small and confined to New Georgia, or it may rupture the whole of the subduction zone to the south, as there is no clear subduction zone segmentation between the Woodlark ridge where this is being subducted beneath New Georgia and the bend in the subduction zone south of Guadalcanal.



There is therefore a clear need to continue investigation and geodetic monitoring of the area affected by the 2007 earthquake and of the stressed region of the subduction zone beneath southern New Georgia, adjacent to the 2007 rupture, and to understand the relationships of past earthquakes on these parts of the subduction zone to earthquakes further south, especially whether or not giant earthquakes have occurred in the past. In order to determine with confidence if giant earthquakes are likely in the Solomons – and if so, how often they might occur over long time periods – we need to investigate the past history of major earthquakes and tsunami in the Solomon Islands for periods of thousands of years. Only with knowledge of this history, and thus with estimates of the severity of future earthquake hazards and of the probabilities or return periods of such events, will it be possible to make decisions on the basis of sound scientific evidence regarding public education for earthquake and tsunami hazard awareness and self - warning; land use zonations and siting of infrastructure and development projects in coastal areas; and the installation of hazard warning and mitigation systems. Background and methodology The short period for which written records of events in the Solomon Islands exist means that investigation of the long-term history of earthquakes can best done by means of geological studies of the different structures and deposits left by such earthquakes and tsunami , although these can be supplemented by collection and interpretation of oral traditions of long-ago events amongst local populations both as a means of research and as a means of enhancing the effectiveness of hazard awareness education. The widespread development of coral reefs in the Solomon Islands, as in other areas of the South Pacific where we have carried out similar investigations, offers particular advantages to this approach, because reefs are distinctively affected by coseismic uplift events as compared to slow uplift between earthquakes (Fig. 2); because blocks of coral ripped off reefs form distinctive tsunami deposits (Fig. 3 and 4); and because it is possible to analyze coral killed by earthquake uplift or by tsunami to determine the ages of these events, using high precision 14C AMS radiocarbon and 230Th/234U disequilibrium radiometric dating techniques. These techniques are most applicable to periods when sea level is at or close to present. They can therefore provide information on the occurrences of earthquakes and earthquake – generated uplift and tsunami in the last eight thousand years or so, since the end of rapid sea level rise at the end of the last ice age. This period is more than fifty times the length of the period of written records of earthquakes in the Solomon Islands, and so studies that extend through it will provide the long – term history of earthquakes and associated events that is required. Palaeogeodesy involves using the present-day position of features that form at known elevation, in this case raised coral microatolls and other raised reefs whose internal structures record sudden and progressive changes in sea level over time (Fig. 2). Cutting through these dead, fossil reefs and sampling the internal structure of annual coral growth rings that is thus revealed allows us to identify both sudden elevation changes associated with earthquakes and the progressive changes that record the buildup of stress in the crust that eventually lead to the earthquakes, whilst radiometric dating of the corals provides a means of determining the ages of these events. On a larger scale, microatoll histories and the histories of development of raised reef terraces can be used to establish sequences of ground movement events over time, with accurate surveying used to establish the amounts of uplift in each event and its variation with distance from the arc and along the arc. Dating subsidence associated with earthquakes is more problematic in coral reef environments, but can be done in associated back reef and swamp environments by dating flooding of shorelines and in particular by 14C dating of fossil trees killed by subsidence and salt water intrusion. Palaeoseismology involves using features developed in earthquakes, such as fissuring of the ground and development of liquifaction in swamp and other soft sediments, to recognize the occurrence of earthquake shaking. The ages of these characteristic structures can be bracketed by dating of over – and underlying sediment units, or by dating felled trees or other organisms killed by the shaking.



A combination of palaeogeodesy and palaeoseismology on coastal sections over wide areas can be used to map out earthquake rupture zones and associated areas of uplift and subsidence. Both the areas and amounts of simultaneous ground movements can be used to define the sizes of the fault ruptures and the magnitudes of the earthquakes that they produced, whilst the dating used to establish simultaneity also provides the basis for estimating the sequence and size / frequency distribution of the earthquakes. Tsunami deposit studies rely on the recognition and dating of the distinctive deposits left on coastlines affected by tsunami , that differ in their distribution and sedimentological characteristics from the deposits left by storms and floods. In tropical settings such as the Solomons and Papua New Guinea, the most distinctive tsunami deposits are sand layers in swamps and coral block deposits. Our previous work in Papua New Guinea has indicated that coral block deposits, consisting of fragments torn off reef fronts and tops, are best preserved on raised reef platforms, especially those forming barrier islands that are isolated from river systems that can erode or mix the tsunami deposits into other sediments (Fig. 3 and 4). These deposits are especially easy to date using the 14C technique applied to well-preserved coral blocks that were living at the time they were broken off the reefs by the tsunami. The distribution of these deposits can be used to identify areas impacted by large tsunami produced by subduction zone earthquakes, whilst their depositional relationships to uplift events at each site can be used to show whether the deposits were produced in the source region or if they were produced by especially large earthquakes or other events, such as landslides, at greater distances. Combining onshore palaeogeodesy, palaeoseismology and tsunami deposit studies is a particularly powerful research strategy as results from the three techniques can be used to cross-check the results from the others, and also to identify the hazards associated with each event. For example, ambiguities that can exist about the number of earthquakes involved in a particular episode of uplift, due to uncertainties in the 14C dating, can be removed by considering whether one or several tsunami occurred at about the same time. Collection and interpretation of oral traditions, for example those identifying particular uplift events as being associated with earthquake shaking and/or tsunami , can provide further cross-checking of the geological data. Data from the 2007 earthquake, which produced extensive fissuring in shallow water, and studies of ancient sequences that indicate that backwash deposits from tsunami form distinctive units in lagoon floor sediments and other settings, indicate that onshore studies may be usefully complemented by subseafloor acoustic profiling and sediment coring to identify and date seabed sediment deformation and deposition associated with earthquakes and tsunami . This is especially so in remote areas where the onshore fieldwork requires a vessel for support that can also be used for inshore survey and coring work concurrently with the onshore work. Proposed research and areas of investigation In order to evaluate earthquake, coseismic uplift and tsunami hazards along the Solomons arc, and in particular to identify and date occurrences of giant earthquakes that rupture the whole length of the subduction zone at once, it will be necessary to carry out fieldwork along the islands facing the Solomon Sea from the Shortland Islands to San Cristobal, as well as in the area of the 2007 rupture near the New Georgia Islands. Further, in the southern part of the island chain, investigation of the raised coral reefs of the Rennell Islands within the Solomons Sea will provide a history of tsunami generated on the Solomons subduction zone and directed southwest towards Australia as well as towards the adjacent coasts of Guadalcanal and San Cristobal. Fig. 5 shows examples of raised reefs produced by uplift events predating the 2007 earthquake, and Fig. 4 shows tsunami deposits from the 2007 earthquake. In order to cover this large area, including many isolated and remote islands, we propose to use a chartered live-aboard dive boat, inshore survey vessel or similar small ship to transport the fieldwork parties and heavy items of field equipment including rock saws, portable cutting frames, generator to power the saw, and surveying equipment. This vessel will also be used to conduct acoustic profiling and coring of nearshore and lagoon seabed sediments, to search for tsunami backwash deposits and for deformation and liquifaction features associated with earthquake shaking, whilst the field parties are ashore. The onshore field parties will carry out surveys of raised reefs and other features, sampling raised reefs and microatolls, and locating, mapping and sampling tsunami deposits and palaeoseismic features as well as collecting oral traditions and carrying out hazard awareness education. We anticipate that in order to do the required fieldwork across the complete length of the island chain, eight to ten weeks of fieldwork will be required. During this time we anticipate that around 200 samples requiring radiometric dating will be collected: this large number of samples reflects the need to correlate earthquake, uplift and tsunami events



across wide areas, particularly between islands and island groups. Such correlation work is critical to the aim of establishing whether or not the subduction zone has a prior history of rupturing along much of its length in single earthquakes, and so producing giant earthquakes and associated tsunami with the potential to affect the whole of the Solomon Islands as well as adjacent island groups and the NE coast of Australia. In addition, the vessel will carry a group of researchers to establish high-quality geodetic monuments and to carry out initial geodetic measurements from these monuments. This network will be designed to facilitate later fieldwork to repeat geodetic measurements of these monuments over the whole length of the arc and so establish the deformation field along and across the subduction zone, including measurement of the overall convergence rate between the Solomon Islands and the Rennell Islands, that are located on the Australian plate whose subduction between the islands is the primary cause of the earthquakes on this subduction zone. Further, it will be integrated with the existing GPS geodetic network established in the New Georgia group (Taylor et al., 2007), in order to understand the relationships between deformation close to and within the area of the 2007 earthquake, and deformation along the arc to the south. Subsequent to the fieldwork, a program of laboratory analysis will be carried out centered upon radiometric dating of the sampled material, both from tsunami deposits and raised reefs and coral microatolls (Fig. 2). This work will be critical both to dating events at particular sites and islands, to establish the frequency and return periods of these events, and also to correlating events between adjacent islands and along the island chain in order to establish which events were relatively small and local in their effects, and which were the major events associated with earthquake ruptures along large parts of the volcanic arc. The large number of highly accurate 14C and 230Th/234U radiometric dates that we propose, in conjunction with the detailed field mapping and surveying, will be therefore critical to defining the occurrence of the largest and most destructive earthquakes and tsunami in the Solomons. Conversely, the work will also define those regions that have a history of past earthquake ruptures and ongoing interseismic deformation but no recent earthquakes that should therefore be regarded as particularly stressed and susceptible to future earthquakes. Results and benefits of the proposed research The proposed research will define the past history of earthquakes, coseismic uplifts and tsunami along the Solomon Islands subduction zone, including both the sequence of events and definitions of the size (earthquake magnitude) and extent of these events, over the period since the last postglacial rise in sea level and the reestablishment of reef systems. This period is some 5,000 to 8,000 years long, over 50 times the length of the period of written and instrumental earthquake records, and is therefore sufficiently long to fully define patterns of earthquakes, uplift and tsunami . The proposed work will therefore provide sound scientific estimates of the severity and return periods of future earthquake, uplift and tsunami hazards as well as defining areas of particular future concern that should be the subject of ongoing geodetic monitoring of strain accumulation. The estimates of severity and return periods will include spatial distributions of the hazards, including definition of seismic shaking distributions in relation to earthquake return periods, and predicted tsunami inundation patterns. The resulting hazard maps and time analyses will provide a sound basis for decision makers to better understand and plan for the effects of these rare but devastating events, and so make recommendations onland zonation and infrastructure siting, improve public awareness, increase socio-economic resilience, and mitigate the loss of life of future events.


Figure 1. Major tectonic elements of the Sololmon Islands region. The Solomon Islands form an island archipelago and are part of the Greater Melanesian arc system. The Rennell Islands lie within the Australian Plate, just off the area of the map due south of Guadalcanal. The map shows the April 2 epicenter (red star), approximate rupture area (blue hachure pattern), and an approximate boundary (red dashed line) delineating coseismic uplift to the south and subsidence to the north (F. Taylor, pers. comm.). Major plates and boundaries are annotated and their relative directions of motion indicated by arrows.

Figure 2. Comparison of actual coral microatoll slab (right) from Sumatra with a synthetic coral growth history (left). This modeling approach relates coral growth to progressive subsidence and episodic coseismic uplift and subsidence history (work of Sieh, Ward et al.), and is applicable to raised microatolls in particular but also to coral heads in raised reef platforms.



Figure 3. Coral boulder – cobble – gravel deposits on tops of raised reef barrier islands around Madang, Papua New Guinea: characteristics indicating deposition from tsunami waves (Morgan, Day et al., in prep.).

A: Example of the abundant unabraded displaced coral heads and broken coral boulders up to 1 m across that occur on top of the older surfaces of the raised barrier islands. C-14 ages of clasts like this cluster in groups, the most recent around 1500 AD, indicating deposition in discrete events that erode large coral blocks from living reef front.

B: Coral boulders in poorly sorted boulder – cobble – gravel sheet deposit, that extends up to 100 meters from the seaward side of the barrier islands, is distinct from well sorted storm ridge deposits up to 20 meters from seaward coast, and fines landwards over hundreds of meters into gravel – sand deposits (Fig. 2C). Poor size sorting of clasts in deposit and wide lateral extent are indicative of deposition from tsunami

C: Mixed gravel – sand deposit typical of landward limit of identifiable sheet deposit, found both on barrier islands and on landward side of Madang lagoon, 2 – 3 km from the open ocean, indicating widespread flooding in depositional event consistent with tsunami and inconsistent with storm waves breaking on barrier islands.

Figure 4. 2nd April Tsunami deposits, south Gizo. A: 12-cm sheet deposit of sand (enclosed by white dotted lines) at the Titiana village grounds. B: Boulders of massive coral eroded from the fringing reef slope and transported onto the reef flat at Pailongge, a horizontal distance of approx. 150m. Christmas tree worms near the base of the unabraded massive coral were still alive at the time of the survey, 40 days after the Tsunami (work of McAdoo, Jackson et al., in prep.).



Figure 5. Emerged coral reefs of Blache (Arairo Mboromani) Island east of Rendova Island (left), and Ranongga Island (right), Solomon Islands. The series of emerged wave cut notches on relatively sheltered Blache island indicates separate historic coseismic uplift events in the Western Province, similar to the 2nd April events that has uplifted Ranongga Island and exposed the fringing with devastating long-term impacts on subsistence fisheries and the livelihoods of local communities.




References Petterson, M.G., Babbs, T., Neal, C.R., Mahoney, J.J., Saunders, A.D., Duncan, R.A., Tolia, D., Magu, R., Qopoto, C., Mahoa, H., Natogga, D., 1999. Geological-tectonic framework of Solomon Islands, SW Pacific: crustal accretion and growth withing an intra-oceanic setting. Tectonophysics, 301, 35-60. Taylor, F.W., Papabatu, A.K., Briggs, R., Brown, A., 2007. Preliminary Report Regarding Coral Reef and GPS Measurements of Crustal Deformation Associated with the 2 April 2007 Gizo Great Earthquake. Institute for Geophysics, The Jackson School of Geosciences, The University of Texas at Austin, U.S.A., 6pp. Cummins, P.R., 2007. The 1 April, 2007 Solomon Islands Earthquake and Tsunami. Professional Opinion No. 2007/02. Risk Research Group, Geoscience Australia, 11pp. Kirby, S., Geist, E., Lee, W.H.K., Scholl, D. &, Blakely, R. (2006) Tsunami Source Characterization for Western Pacific Subduction Zones. Report of the Tsunami Subduction Source Working Group, USGS.

Solomon Islands Country Mission and Technical Advisory Report

Documents

Transcript of Solomon Islands Country Mission and Technical Advisory Report