Mechanism of Tsunamis and Recent Tsunami Disasters Introduction… · 2012-01-12 · Mechanism of Tsunamis and Recent Tsunami Disasters Akio OKAYASU, Department of Ocean Sciences,

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


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.


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


Distribution of tsunami height at Miyako Bay

Showa Sanriku Tsunami, 1933

Chilean Tsunami, 1960


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


Example of a hazards map (Tateyama City, Chiba, Japan)


Early warning system by GPS Buoys


Numerical simulation and CG for public awareness

3-D numerical simulation with Navier-Stokes Eq. + CG (by Arikawa, PARI)


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.

Documents

Mechanism of Tsunamis and Recent Tsunami Disasters Introduction… · 2012-01-12 · Mechanism of Tsunamis and Recent Tsunami Disasters Akio OKAYASU, Department of Ocean Sciences,