hoda tsunami hazard-AUB 15oct hazard in Lebanon … · Our understanding of tsunami waves and...

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Dr. ELIAS A., International Day for Disaster Risk Reduction, AUB - 15 Oct 2012

Dr. Ata ELIAS, Geology department AUB


Outline

1.  Tsunami events:

2.  The physical and geological tsunami event

3.  Global tsunami mitigation efforts

Tsunami hazard

1.  The tsunami hazard in the Mediterranean

2.  Tsunami hazard in Lebanon

3.  Conclusion

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Generated by a sudden displacement of important volume of solids inside a large body of water. This geological event can result from

•  Submarine earthquakes •  Submarine (or coastal) landsides, •  Submarine (or coastal) volcanic eruptions •  Offshore impact of objects from the space


A Tsunami wave •  long wavelength (~200Km) •  Velocity is related to the water depth where wave is propagating •  Can show all patterns of regular wave interference, diffraction… •  Its amplitude is modulated by the characteristics of the seafloor (shape and depth) and the basin where it is propagating.

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Earthquakes that happen in the sea are the main tsunami source.

The extent of the inundation zone depends on different parameters such as:

•  the source strength, position, directivity

•  the seafloor characteristics •  the coastal area slope,

material, shape and position.

The runup can be very different along the same coast.


Dr. ELIAS A. – Workshop – Jan/05/2011

The tsunami propagates very long distances, Can go around coasts and islands, The height of the waves is always higher over the coasts.

Propagation of the Sumatra 2004 tsunami. Titov et al., Science 2005

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Our understanding of tsunami waves and hazard has largely increased since the 2004 Indian Ocean tsunami disaster. Tsunami research production has greatly increased in number and diversified including many aspects such as

•  reconstructing tsunami catalogs from historical and

geological records,

•  modeling tsunami waves propagation

•  establishing inundation maps,

•  building tsunami alert systems.


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Map of Earthquakes in the Mediterranean area between 1998-2003.

Known sources of tsunami in the Mediterranean: - Earthquakes, - Volcanic eruptions.


Crete

Sicily

Main tsunami sources in the Mediterranean basin: Hellenic area (Crete), Cyprus, and mainly Lebanese offshore.

Cyprus

365 and 1303

Some (historical) tsunamis in the area: Santorini (-1500), Crete 365, Lebanon 551, Crete 1303, Messina 1908, Rhodes 1956, Algeria 2003.

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PCTA: Peak Coastal Tsunami Amplitude 2.4. Generation of Hazard Results[18] At a given site, the annual probability of exceeding a

given PCTA was determined by studying all synthetic tsu-nami propagation scenarios and counting the number ofscenarios where the given PCTA was exceeded at the site ofinterest. Dividing the number of exceedances by the durationof the synthetic catalog (i.e., 100,000 years) then gives theannual exceedance probability.[19] The uncertainty associated with a given hazard curve

is expressed theoretically in terms of the range of possibletsunami heights corresponding to the annual probability ofexceedance, defined using a theoretical cumulative normaldistribution function within 2s. In this study, results arepresented as the median value, as well as the 15th and85th percentiles. Uncertainties arise owing to uncertainty inboth input parameters (e.g. zonation, activity rates, faultingregimes, estimation of sea bottom displacement etc.) andwave propagation calculations (bathymetry, estimation ofPCTA etc). The degree of variability among these parametersdiffers and is difficult to assess directly. However, moredetailed knowledge about the tectonics in the Mediterraneanregion and the associated earthquake activity can helpreducing the degree of epistemic uncertainty in the input data.In order to constrain better the uncertainties and to identifythe relative contributions of different sources of uncertainty,a systematic sensitivity study will be necessary. Such a studyis planned for the near future.

3. Tsunami Hazard Results

[20] The PTHA results can be expressed either as hazardcurves or as hazard maps. The hazard curve for a given sitepresents the annual probability of exceedance as a functionof PCTA. Hazard maps can present either the PCTA corre-sponding to a fixed annual probability or the probability of

exceeding a fixed PCTA threshold in some given timeperiod.[21] In Figure 4, we present the hazard curves for selected

locations in the Mediterranean region. The curves show thatthe hazard varies greatly throughout the region, but they alsoindicate that all sites can experience tsunami waves exceed-ing 1 m when considering sufficiently low probability levels.We emphasize that our results are a minimum estimate of thetsunami hazard because we only considered earthquake-generated tsunamis.[22] Figure 4 also shows two examples of the uncertainty

associated with our hazard curves, expressed in terms ofthe 15th and 85th percentiles. The relatively high vari-ability is common for probabilistic assessment of naturalhazards and is related to the relatively short time coverage ofthe observed catalog and uncertain knowledge of regionalgeology and tectonics. Furthermore, errors are associatedwith each calculation step of the PTHA method. A greaterconcern related to the uncertainties in the hazard assessmentis that owing to the finite duration of the synthetic catalog,it will contain only a subset of the events which couldpotentially occur in the Mediterranean region. In thisregard, it must be investigated whether the hazard results aresensitive to the choice of subset, that is, to the choice ofrandom seed in the synthetic catalog generation. In order totest this, the synthetic catalog has been split into 10 sub-catalogs of 10,000 years duration and hazard curves havebeen derived for each of these subcatalogs. The curvesobtained for selected sites are presented in Figure 5. It isfound that the hazard curves are extremely stable to thechoice of synthetic catalog, confirming that the chosen cata-log duration of 100,000 years is sufficient to derive stablehazard curves.[23] Hazard maps showing PCTA corresponding to a fixed

annual probability have been sampled from the median

Figure 3. Tsunami propagation scenario for a M = 8.5 earthquake in the eastern Hellenic Arc.

SØRENSEN ET AL.: TSUNAMI HAZARD IN THE MEDITERRANEAN SEA B01305B01305

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Sørensen et al 2010, JGR


The Lebanese coast has witnessed more than a tsunami throughout its history and before. The AD 551 earthquake has left the greatest impact in its historical record.

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As for the terrible disaster and the great and remarkable portent which happened in the city of Beirut in Phoenicia, when the earthquake took place and the cities collapsed, we have decided to make it a warning sign for the knowing of posterity.

For when the earthquake came from heaven, the sea withdrew and retreated from Beirut and the other coastal cities of Phoenicia for a distance of nearly two miles; the dreadful depths of the sea became visible and various and amazing sights were revealed: sunken ships full of different cargoes and other ones which suddenly, when the sea withdrew from the land, were moored in the harbours, settled on the ground and they were broken to pieces when the sea left them and withdrew on God’s command …Then, by a secret command, a tremendous surge of the sea rushed up to return to its original depth, overwhelmed and consumed all these wretched people in the depths of its swirling waters…. And God rolled the waters of the sea over them, as the flood burst forth and flowed back to its former abundance.

As the sea was rising up against them from behind, the earthquake brought down the city in front of them.

(John of Ephesus – (AD 507- 586))

A historical text about the 9July, AD 551 earthquake.


July 551AD Eq, M~7.5 The A.D. 551 earthquake in historical

records: - Severe damage along the coast from

Tripoli to Tyre: Beirut “jewel of Phoenicia”

destroyed, 30.000 pple of the inhabitants killed,

End of a flourishing Roman city

Tripoli “drowned”.

- less severe destructions inland.

- followed by a tsunami ©Elias et al, Geology 2007

Elias A. - UN workshop – Feb 2010 - Beirut

©Elias et al, Geology 2007

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Research results show evidence for tsunami potential in Eastern Mediterranean in general, and in Lebanon in particular.

The tsunami threat is real.

For more details check poster outside.


Transported boulders along the coast: a possible tsunami product.

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Difficulties in researching Tsunami hazards in Lebanon: -  The important man-made modification of natural sites.

-  The missing information on the shallow bathymetry of the offshore.


Important points to remember: -  A tsunamigenic earthquake can happen anytime without warning. -  A tsunami wave propagates very fast for long distances:

Based on the distances and the average water depths : We may have ~10mn before the wave reaches Beirut if the source is in Cyprus.

We may have less than 3 mn if the source is from within the Lebanese offshore.

Preliminary studies suggest that the wave height we should expect over the Lebanese coast in case of a near source is ~5-7m.(**)

(**)- This is a rough, but logical estimation. It needs more investigations.

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20.09.2012 – UNESCOPRESS Progress Towards a Tsunami Early Warning System in the North-eastern Atlantic, the Mediterranean and connected seas On 27 and 28 November 2012, the Tsunami Early Warning and Mitigation System in the North-eastern Atlantic, the Mediterranean and connected seas (NEAMTWS*) will conduct its first tsunami exercise since its inception in 2005 under the aegis of UNESCO’s Intergovernmental Oceanographic Commission (UNESCO-IOC).


Efforts that need to be done to increase the resilience of Lebanon facing tsunami hazards:

1- Establishing Tsunami alert system 2- Supporting and facilitating research on hazard and risk (exple: shallow bathymetry) 3- updating our hazard and risk models and preparedness plans according to new research results. 4- raise awareness among citizens and urban designers.

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Thank you.

hoda tsunami hazard-AUB 15oct hazard in Lebanon … · Our understanding of tsunami waves and...

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