2012/9/24 17:20-17:35 WCEE SS24.4

Special Session:
Great East Japan (Tohoku) Earthquake

Earthquake Source

Kazuki Koketsu
Earthquake Research Institute, University of Tokyo

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Names and features of the earthquake anddisaster The earthquake was officiallynamed “2011 off the Pacificcoast of Tohoku Earthquake” by
the Japan MeteorologicalAgency, but we abbreviate it as“2011 Tohoku earthquake”
because of its awkward length.

<General Feature>
Their hugeness The disaster was officiallynamed “Great East Japan
Earthquake Disaster” by theCabinet of Japan.

2. Severe tsunami damage
but moderate groundmotion damage<Specific Features>
1. Observed by dense
networks of geophysicalinstruments

3. They had not been
foreseen at all2

<General Feature> Their Hugeness The earthquake’s moment magnitude (Mw) of 9.0 (JMA) to 9.1(Global CMT Project) is the largest ever recorded in Japan andthe fourth in “Largest Earthquakes in the World Since 1900”
compiled by the United States Geological Survey (USGS).
 The disaster resulted in 16,131 fatalities, 3,240 missing, 5,994
injured, and 128,497 house collapses as of 13 January 2012
including the toll from the aftershocks and triggered events(Fire and Disaster Management Agency of Japan).
 The number of fatalities is the second for Japanese earthquakes
in “Earthquakes with 1,000 or More Deaths since 1900” (USGS),
preceded by only the 1923 Kanto earthquake.

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<First Specific Feature>

1. Observed by dense networks ofgeophysical instruments
The 2011 Tohoku earthquake is a megathrust event in a subduction
zone like most others in the list of “Large Earthquakes in the World
Since 1900” but the first that was observed by dense networks ofgeophysical instruments. Abundant data obtained from the dense
networks include teleseismic, strong motion, geodetic, and tsunami
datasets.

4Strong motion dataset
The strong motion records at
31 stations
selected along
the Pacific coast were doubly
integrated into grounddisplacements using highpass
filter with a corner period of
100 s. They were aligned
along the coast to form therecord section. Three pulses
can be seen in this record
section, and their arrival
times indicated by blue, red,and brown dots suggest thatthey were generated in thezones drawn with the samecolors. The first two zones
are located around theepicenter of the mainshock
determined by JMA, while thethird zone is 150 to 180 km
south of the epicenter.

5Geodetic dataset
The GEONET of Geospatial Information
Authority of Japan, which is a nation-wide
network of 1,200 GPS-based control
stations, observed crustal deformations
due to the earthquake as shown by static
displacements. These static displacements
point toward these zones, though theycannot distinguish the first two zones
because of their limited resolving power.

The Japan Coast Guard observed seafloor
displacements due to the earthquake.
6Tsunami and teleseismic datasets

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<Second Specific Feature>

2. Severe tsunami damage butmoderate ground motion damage
This disaster resulted in 16,131 fatalities, 3,240 missing, 5,994 injured,and 128,497 house collapses as of 13 January 2012 including the toll fromthe aftershocks and triggered events (Fire and Disaster Management
Agency of Japan).
Since more than 90% of the fatalities were from drowning (Kyodo News,2011), and the number of injured people is relatively small, the tsunami
damage was very severe.

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Back slip, earthquake, and tsunami(1) back slip
The both sides of source region(震源i域) are strongly coupled.
The Japan side is continuously
dragged in the direction ofsubduction.

subduction(2) earthquake
When the accumulated strainreaches the strength of thecoupling, the coupled area (固着域) is ruptured and anearthquake occurs.

(3) tsunami
The upheaval happens due tothe movement of the hanging
wall, and this upheaval
generates tsunamis in the sea(海).

9Slip distributions recovered by the source
inversions of Yokota et al. (2011)10

Rupture progress in the earthquake

Slow rupture propagationin the main zone
(Vr = 1.5 ∼ 1.8 km/s)→ Released energy waslargely consumed by fault
rupture and the radiatedenergy was reduced.
→ This caused moderate
ground motion damage.

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<Third Specific Feature>

3. They had not been foreseen at all12After the 1995 Kobe earthquake
The Japanese government built (1) a national program of seismic
hazard assessment and (2) nation-wide dense arrays of seismometers
and GPS receivers by establishing the Headquarters for Earthquake
Research Promotion (HERP).

Structure of the Headquarters for Earthquake Research Promotion(from Fujiwara et al., 2004)13

National program of seismic hazard assessment“National Seismic Hazard Maps for Japan” are being made by theEarthquake Research Committee (ERC) of HERP. They consist of“Probabilistic Seismic Hazard Maps” and “Scenario Earthquake Maps.”

Flowchart for Probabilistic Seismic Hazard Maps
Modeling of seismic activity

Evaluation of occurrence probability of earthquakes

Long-term
Evaluations

Probabilistic evaluation of strong-motion level

Evaluation of probabilistic seismic hazard for each earthquake

Evaluationsof StrongGroundMotion

Evaluation of probabilistic seismic hazard for all earthquakes
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Long-term evaluations of subduction zone earthquakes

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Long-term evaluations in Tohoku District
Tohoku earthquake

Expected
Magnitude

Probability
within 30years

Jogan earthquake

by Satake et al. (2008)

地震調査委員会資料より

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It had not been foreseen at all.
 Since the Tohoku earthquake was notforeseen and so its occurrence
probability was not evaluated, thenational seismic hazard maps did notinclude its effects; as a result, they were
not useful at all for the public
preparedness.

 Even if that the 869 Jogan earthquake

was taken into consideration, it could
not useful for the public preparedness
and the safety of the nuclear power
plant. Its magnitude was estimated to
be only 8.4 and the tsunami height close
to the plant was observed from tsunami
deposits to be only 4.6 m (Satake et al.,2008; Namegaya et al., 2010).

Seismic hazard map of probabilities ofground motions equal to or larger thanseismic intensity 6 Lower, occurring
within 30 years from January 1, 2010.
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Geller (2011)

Actually, two in green
circles could beforeseeable, but theothers occurred at
unknown active faults
or plate boundarysegments. So, they
are located at places
assigned a relatively
low probability.

However, since 1979, earthquakes that caused
10 or more fatalities in Japan actually occurredin places assigned a relatively low probability.
This discrepancy — the latest in a string ofnegative results for the characteristic
earthquake model and its cousin, the seismicgap model2–4 — strongly suggests that thehazard map and the methods used to produce
it are flawed and should be discarded.

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Implications of the previous

We think that the methods are not flawed and theprobabilistic approach is not necessarily inappropriate.
The reason why the probability maps were not useful wasthat our knowledge on past earthquakes is very limited.
For example, there remain many unknown active faults andplate boundary segments. This limitation leads to the large
uncertainties in scientific forecast.
A real problem is that the Japanese government has notclearly stated these limitations to the public.
Good communication of risk is indispensable in sciences
related to natural disasters.
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