Mechanism of Tsunamis and Recent Tsunami Disasters Introduction… · 2012-01-12 · Mechanism of...
Transcript of Mechanism of Tsunamis and Recent Tsunami Disasters Introduction… · 2012-01-12 · Mechanism of...
Mechanism of Tsunamis and Recent Tsunami Disasters
Akio OKAYASU, Department of Ocean Sciences, TUMSAT
A Lecture of
Advanced Topics for Marine Scienceby
Tokyo University of Marine Science and Technology
The School of Internet (SOI) Asia 2011
Introduction: Overview of 2011 Tohoku Earthquake Tsunami
Introduction
Nearly 20,000 people were killed by 2011 Tohoku Earthquake Tsunami (Mw=9.0), Japan.
More than 220,000 were killed by 2004 Indian Ocean Tsunami (Mw=9.1)
A huge tsunami may attack any coast in Asia and Pacific areas. Other examples: 2010 Chilean Earthquake Tsunami (Mw=8.8) , Cascadian (North America) Earthquake Tsunami (not yet).
We need to learn about the nature of tsunamis and prepare for them to save people’s life (at least).
Measured Runup and Inundation Height for 2011 Tohoku Earthquake Tsunami
Japan
Distribution of Runup and Inundation Heights for 2011 Tohoku Tsunami
The 2011 Tohoku Earthquake TsunamiJoint Survey Group
Pacific Ocean
The 2011 Tohoku Earthquake Tsunami Joint Survey Group
Inter-Academic Society Group– http://www.coastal.jp/tsunami2011
Institutes and Members– Tsunami & Coastal Researchers, Physical Oceanographers, etc.
• Members of JSCE, SSJ, JGU, etc.– 300 participants from
64 Research Institutes and Universities + Central & Local Governments– Over 5,000 data for Runup and Inundation Heights
Secretary Office– Kansai University, Kyoto University
Activities– Coordinating survey areas, organization of survey teams– Mailing list and web maintenance, making survey manual– Data analysis and quick delivery to surveyors– Making survey data set for public
1854 Toukai, Nankai1896 Meiji Sanriku1933 Showa Sanriku1944 Tounankai1946 Nankai1960 Chile1983 Japan Sea1993 Okushiri2011 Tohoku
Tsunami around Japan
Eurasian plate
Philippine plate
Pacific plate
North American plate
1611 Keichou Sanriku
SanrikuTohoku Ria coastV-shape bay
The Sanriku Coast, Japan
Ria-type coast
Japan’s tsunami coast
Funnel-shaped bays
1896 Meiji tsunami38.2 m tsunami height in Ryouri
1933 Showa tsunami28.7 m tsunami height in Oofunato
2011 Tohoku tsunami~ 40 m tsunami height in Miyakothe highest recorded tsunamiheight to date
Name of city, town Number of dead and missing Population in 2010 Ratio of dead and
missing to population
Iwate Prefecture
Noda Vlg. 38 4,632 0.8%
Tanohata Vlg. 35 3,843 0.9%
Miyako City 614 59,442 1.0%
Yamada Town 845 18,625 4.5%
Otsuchi Town 1,609 15,277 10.5%
Kamaishi City 1,238 39,578 3.1%
Ofunato City 454 40,738 1.1%Rikuzen-TakataCity 2,116 23,302 9.1%
Ratios of Dead and Missing to Population (Iwate Prefecture, June 22) Tsunami traces at Rikuzen-takata City
11
Pre-Tsunami @Yahoo
200 m N
Post-Tsunami @Google
200 m N
Rikuzen-takata City Tsunami HeightDistribution
Meiji: ~ 4.6 m
Showa: 3.5-3.8 m
Chilean: 4.5-5.0 m
Tohoku: ~15 m
Inundation Area in Rikuzen-takata
Meiji: 1.56 km2
Showa: 1.34 km2
Chilean: 5.25 km2
Tohoku: 13.45 km2 Sports arena
Toppled RC building, Otsuchi Town
Runup height: T.P.+12.6m
Catamaran on top of 2‐story building
Part I : Mechanism of Tsunamis
Slide by F. Imamura, Tohoku Univ.
World Seismicity (1975 – 1995)
103km
L
h : Max. 4000m
100/1/ Lh
Long wave
Generation mechanism of subduction-zone earthquakes
plate
JapanPacificOcean
fault
→ Shallow water for tsunamis
sea bottom
Water surface
Slide by F. Imamura, Tohoku Univ.
Meiji Sanriku1896/06/1520:00 pmM8.2~8.5(tsunami Eq.)
Showa Sanriku1933/03/03M8.12:30am
Keicho Sanriku1611
Chilean Tsunami1960/05/24
地震調査委員会の図に加筆
Historical Earthquakes in Sanriku Area
22
Measured Co-seismic Displacement for 2011 Tohoku Earthquake
5.3 mOshika P.
Horizontal displacement Vertical displacement
24 mESE
-1.2 mOshika P.
+3 m
Land by GSISea by JCG
Land by GSISea by JCG
Displacement estimated by Meteorological Research Institute最大滑り量:30m,継続時間:約3分断層の長さ:約450km,幅:約150km
Displacement (m)Contour for every 4m
阿部・菅野・千釜(1990)Minoura・Nakaya(1991)
Satake, Namegaya,Yamaki(2008)
Jougan Tsunami(869)
・old document・tsunami sedimentsonce in 1000yrs
Slide by F. Imamura, Tohoku Univ. Slide by F. Imamura, Tohoku Univ.
Slide by F. Imamura, Tohoku Univ.
1960 Chilean Tsunami (Mw=9.5)
(H. Watanabe ,1998)
Definitions of tsunami height
Water level at tsunami
Tsunami height
Runup height
Inundation height
Inundationdepth
Tokyo Peil(T.P.+0m) = Mean water level
Modified on JMA figurehttp://www.jma.go.jp/jma/kishou/know/faq/takasa.png
Tsunami height dependency on resonant period of bays
Showa Sanriku, 1933 Chilean, 1960
(H. Watanabe ,1998)
Distribution of tsunami height at Miyako Bay
Showa Sanriku Tsunami, 1933
Chilean Tsunami, 1960
(H. Watanabe ,1998)
Non-linear shallow water wave equation or non-linear dispersive wave equation
A tsunami can be reproduced if you know the initial condition = initial form of the wave.
Tsunami propagation and deformation can be accurately calculated by these equations
Propagation of tsunamis can be described by wave equations
Linear Eq.
Non-linear 切り立った波形,大波高,複数の反射波
Non-linear + dispersive (Boussinesq Eq.) 入射波の分裂
Differences by wave equations Observed runup and inundation height around Yamada Bay, Iwate
Yamada Town
Pacific Ocean
Omoe Peninsula
Funakoshi Peninsula
Iwate Prefecture
10km
Miyako CityOtsuchi Town
Runup and Inundation Heights around Yamada Bay
Yamada Town
Funakoshi
Omoe Peninsula
Kiri-kiri
Aneyoshi
940m10km
Miyako City
Ohtsuchi Town
Yamada
Funakoshi Peninsula
Locality of tsunami
Aneyoshi, Omoe Peninsula, Maximum Runup of 38.8m
Damage at Osawa Area, Yamada Town Funakoshi Area, Funakoshi Bay side
7.4m above MWL
Inundation height: 9.8m above MWL
Outline of Numerical Simulation
Governing eq.: Non-linear shallow water equation
Coordinate, grid size: Cartesian coordinate, 50m
Bathymetry, topography: Charts, digital elevation map
Offshore boundary: Tsunami waveforms measured by GPS wave gages (buoys) at around20km offshore
GPS Wave Gages (Buoys) and Measured Tsunami Profiles
Snapshots of water surface elevation along the south-half of Iwate Prefecture
42min. later
60min. later
Calculated Maximum Runup and Inundated Areas
Y, k
m
X, km
130 135 140 145 150-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
4
6
8
10
12
14
16
18
20
22
24
Distribution of Maximum Surface Elevation above MWL
Yamada
Funakoshi
Part II : Counter-measures for Tsunamis
Sea Wall at Osawa Area, Yamada Town
6.6m
Bay-mouth breakwater at Kamaishi City
69m
The breakwater was partially broken by 2011 Tohoku Tsunami, but it is considered that it reduced the tsunami height and delayed the inundation.
For safe evacuation, “Soft-measures”
Temporal refuge building
Slide by F. Imamura, Tohoku Univ.
Example of a hazards map (Tateyama City, Chiba, Japan)
Slide by F. Imamura, Tohoku Univ.
Early warning system by GPS Buoys
Slide by F. Imamura, Tohoku Univ.
Numerical simulation and CG for public awareness
3-D numerical simulation with Navier-Stokes Eq. + CG (by Arikawa, PARI)
Slide by F. Imamura, Tohoku Univ.
Summary for counter-measures
Hard-type counter-measures, such as seawalls, tsunami breakwaters are quite effective measures, but have limitation for tsunamis larger than the assumed magnitude.
Soft-type counter-measures are needed for tsunamis larger than the design height of hard-type counter-measures. Prediction of the possible largest tsunami is very difficult with the current scientific knowledge.
People’s awareness of tsunami hazards is important factor for soft-type counter-measures. The early warning system should be provided and the information must be promptly and adequately transferred to people for effective evacuation.
Regional planning for tsunamis and other natural disasters (flood, storm surges, earthquakes etc.) are highly recommended. Local communities as well as local governments have an important role for preparation against disasters.