Geological and historical evidence ofpaleotsunami

Geological evidence of paleotsunami in Tohoku
Huge boulders deposited by the 2011 Tohoku‐oki tsunami

Iwanuma Trench(interpreted by Iwanuma city)

Kazuhisa Goto
(International Research Institute of Disaster Science, Tohoku University)

Mw=9.0 earthquake on 11 March 2011

Tsunami warning issued on 11 March 2011

Moment magnitude (Mw=9.0)

JMA (2011)

JMA (2011)

Red and orange: tsunami warning
Yellow: attention

Tsunami generation mechanism

2011 Tohoku‐oki tsunami heights

ERI, Univ. Tokyo

Miyako City
http://www.mikawa.pa.cbr.mlit.go.jp/topics/img/press/kinuura4/06.siryou3.pdf

 Extremely large slip along the shallow
plate boundary (near trench axis)（e.g., Satake et al., 2011）．
 Submarine landslide(s) was theadditional source of tsunami at
Sanriku coast (Tappin et al., 2014) ?

Ishinomaki City

Tsunami Joint Survey Group

Tappin et al. (2014) Marine Geology

Pre‐2011 tsunami risk assessment
Tsunami hazard map and 2011 inundation area Tsunami hazard map and 2011 inundation areaat Miyako City, Iwate Prefecture
at Ishinomaki City, Miyagi Prefecture

Very high run‐up, but northern people experienced similar size of tsunamis past 120 years.

How to know past hazard/disaster?
Volcanic eruption
Typhoon/flood
Large Earthquake/tsunami

Meteorite impact

100000 year 10000 year

1000 year

100 yearpresent

2011 inundation area
Pre‐tsunami hazard map
10 yearmemorymeasurement
Historical document
Archeological record
Geological record
Geomorphological record

Modified after CDMC (2011)

Decreasing information
Modified after J. Komatsubara

Large difference of the tsunami hazard maps in areas prepared before the 2011 event

Evidence of paleotsunami decreases to the pastGeological evidence of paleotsunami

Historical record

Major objectives of the paleo-tsunami research are

Historical documents describe detail of earthquake andtsunami and consequent damages.
Japan

•

estimation of the long-term tsunami recurrence interval.

•

estimation of the inundation area, flow height, and magnitudeof earthquake.

for the tsunami risk assessments.

C. America
S. America
Philippines
Kuril‐Kamchatka
N. America‐Hawaii
NZ, Polynesia
AD600

AD1000AD1500AD2000Modified after report by B. Jaffe

Some countries have legend and/or oral tradition of tsunamis.

Tsunami recurrence estimated from tsunami
deposit in Sendai

Image of the tsunami sediment transportation (Sugawara et al., 2008)

Estimating magnitude of the AD869 Jogan earthquake
Distribution limit of sand deposit (>0.5 cm)
AD915 To‐a Tephra

Paleoshoreline at AD869
AD869 Jogan
depositCourtesy: Daisuke Sugawara

Mw>8.4It was not considered in the hazard map before the 2011 event
Minoura and Nakaya (1991), Sugawara et al. (2012)

Few hundred to thousand years interval.

・Tsunami inundated at least 3‐4 km from the paleo‐shoreline (e.g. Abe et al., 1990).
・Few hundred to thousand years recurrence interval (e.g. Minoura and Nakaya, 1991).

New policy for the tsunami risk assessment

Maximum possible…. How large and how to estimate?
Historical and geological evidence tell us approximate return period
and size of the rupture area of the fault.
 Magnitude of each seismic event is uncertain.
 Geological and historical researches take long time…
 Tsunami may not be generated only by the fault rapture (e.g. submarine
landslide).
Fault model

Nankai Trough Many big cities (Osaka, Nagoya, Tokyo)
 1300 years historical record
 100‐200 years interval of large event 1707 event was the largest (Mw=8.6~8.7)
 Many geological evidence
Cabinet Office, Government of Japan

Slip amount(m)Cabinet Office, Government ofJapan (2012)

The Japanese government made a new guideline for tsunami disaster prevention plansin December 2011, and recommended that local governments prepare for the“maximum possible earthquakes and tsunamis”.

In 2012, national government reported Mw=9.1 earthquake as the possible maximum
event along the Nankai Trough. Tsunami run‐up height will be approx. 30 m inmaximum. Human and economic losses will be 320,000 peoples and 220 trillion JPY(Asahi newspaper).

Maximum possible…. How large and how to estimate?

Post-2011 policy for the tsunami countermeasureFault model
Slip amount(m)Tsunami evaluation building

Level2 tsunami
Level1 tsunami
Sea level

Cabinet Office, Government ofJapan (2012)

Wave
breaker

Elevated
construction(e.g., road)

1. “The slip amounts were estimated based on the knowledge of 2004 Sumatra
earthquake, 2010 Chilean earthquake, and 2011 Tohoku earthquake (CDMC, 2012)”.
 Level 1 tsunami: One that occurs frequently every few tens of years to a hundred ormore years. The estimated tsunami height is the basis for construction of shore
protection facilities.
2. “It is very difficult to estimate the recurrence interval or timing of occurrence of
this maximum possible earthquake and tsunami (CDMC, 2012)”.

1. Do local governments need to prepare for such event?
2. Is this really the maximum? Overestimation or underestimation?

 Level 2 tsunami: One that occurs with an extremely low frequency, but generating
severe damage once it occurs. Shore protection facilities are probably insufficient toprotect against a tsunami of this size. Saving human life is, therefore, the top
priority, and hence so an awareness campaign including disaster education is highlyrecommended.
Goto et al. (2014) Episodes