Download Lessons learned from the Tohoku earthquake / tsunami and

Lessons learned from the Tohoku earthquake / tsunami
and preparation for future hazard
Kenji Satake
Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032 JAPAN
E-mail: [email protected]
1. The 2011 Tohoku earthquake and tsunami
The 2011 Tohoku earthquake (Magnitude 9.0) was the largest earthquake in Japanese history. It caused
nearly 20,000 casualties, mostly from devastating tsunamis, and serious damage to Fukushima Dai-ichi
nuclear power station (NPS). The earthquake was officially named as “off the Pacific coast of Tohoku
earthquake” by the Japan Meteorological Agency (JMA), but often abbreviated as the Tohoku (-oki)
earthquake. The earthquake and tsunami disaster, including the NPS accident, was named as “the Great
East Japan Earthquake disaster” by the Cabinet Office.
The nation-wide GPS network recorded large displacements in directions opposite to the previous
observations, with a maximum of 5.3 m eastward and 1.2 m downward motions. In addition, marine
geophysical measurements started before 2001 detected huge (30 to 50 m) seafloor displacements. By using
these geodetic data, as well as seismological and tsunami data, numerous source models have been proposed.
The seismic moment is approximately 4 x 1022 Nm (corresponding to Mw 9.0). Another common feature
is that huge (~50 m) slip occurred on a shallow plate interface near the trench axis. This is a surprising
result, because the shallow plate interface has been considered to be weakly coupled and unable to
accumulate strain. Only unusual “tsunami earthquakes,” that generate weak seismic waves but large tsunamis,
occur at the shallow plate interface.
The tsunami heights were large along the Sanriku coast with a maximum of nearly 40 m. The tsunami
inundated nearly 5 km from the coast in the Sendai plain. Generation and propagation of the tsunami were
recorded at offshore gauges. The ocean bottom pressure gauge, located ~ 70 km off Sanriku coast and
connected with seafloor cable, recorded ~2 m water rise starting immediately after the earthquake, followed
by an impulsive wave with additional 3 m rise within 2 min. Similar tsunami waveforms were recorded on
another pressure gauge and offshore GPS gauges before their arrival on the Sanriku coast.
Figure 1. (left) The regions for long-term forecast and the locations of four nuclear power stations. (right)
The focal mechanism solution of the mainshock and the locations of earthquakes with M ≥ 5.0 (JMA) occurred
within a week (yellow circles). The slip distribution estimated from the tsunami waveforms is shown. Coseismic
movements of land-based GPS stations (red arrows, Ozawa et al., 2011) and marine GPS/acoustic
measurements (black arrows, Sato et al., 2011), and the proposed fault models (white rectangles) of the 1896
Sanriku earthquake and the 869 Jogan earthquake (Satake et al., 2008) are shown.
2. Long-term earthquake forecast
Was such a gigantic earthquake foreseen in Japan? The Japanese government made long-term forecast of
large earthquakes in and around Japan based on past earthquake records (Figure 1). Off Miyagi prefecture
(Miyagi-oki region), to the west of the 2011 Tohoku earthquake epicenter, the probability of a great
(M~7.5) earthquake occurrence between 2010 and 2040 was estimated as 99 %, highest in the Japanese
islands. This estimate was based on the facts that large (M~7.4) earthquakes have repeatedly occurred there
with an average interval of 37 years since 1793, and that 34 years have passed since the last earthquake
occurred in 1978.
The 2011 Tohoku earthquake was much larger than the forecast, both in magnitude and the source
area. The rupture started in the southern Sanriku region and propagated into the neighboring regions.
The source area was about 500 km long and 200 km wide, including the central Sanriku, Miyagi-oki,
southern Sanriku, Fukushima-oki regions as well as parts of Ibaraki-oki and along the Japan Trench
regions. The long-term forecast thus failed to predict the size (M) and the source region of the 2011
Tohoku earthquake, because it was based on a characteristic earthquake model and relatively short (a few
hundred years) historical data.
3. Tsunami warning
The JMA issued tsunami warning at 3 min after the earthquake. The estimated tsunami height, based on
the initial estimate of magnitude (M=7.9) and tsunami numerical simulation results, were underestimated.
Nevertheless, very strong ground shaking and the rapid tsunami warning urged many coastal residents to
evacuate to high ground and thus saved their lives. After detecting the large offshore tsunami on the bottom
pressure and GPS wave gauges, JMA upgraded the tsunami warning messages at 28 min after the
earthquake (before the tsunami arrival on coast) to higher level of estimated tsunami heights. However,
the updated tsunami warning message did not reach all the coastal communities, because a power failure
occurred and many people had already started evacuation.
4. Past tsunamis
On the Sanriku coast, the 1896 Sanriku earthquake caused very large tsunami, with a maximum height of 38
m, with 22,000 casualties. It was a typical tsunami earthquake; the preceding earthquake shaking was very
weak. Study of tsunami waveforms indicate that the 1896 earthquake was generated from a fault motion near
the trench axis.
In the Sendai plain, a large earthquake and tsunami occurred in AD 869 in Jogan era on the Japanese calendar.
A national history book depicts strong shakings, large tsunami inundation and the associated damage
including 1,000 drowned people. In addition, sand brought by this tsunami, called tsunami deposits, had
been found in geological layers in coastal lowland, more than 5 km from the current coast in the Sendai plain.
Based on the distribution of tsunami deposits, the 869 Jogan earthquake was modeled as an interplate
earthquake with M=8.4.
5. The 2011 tsunami source model
The tsunami modeling indicates that the 2011 earthquake was a combination of a 1896-type “tsunami
earthquake” and a “Jogan-type” deeper interplate earthquake. A huge slip near the trench axis (Figure 1),
similar to the 1896 Sanriku tsunami earthquake, caused the impulsive tsunami waves recorded on the
bottom pressure gauges and were responsible for the high tsunami along the Sanriku coast. The fault
motion and large slip along the deeper plate interface, similar to the previous model of the 869 Jogan
earthquake, produced a long-wavelength seafloor deformation and caused the first gradual rise at the
gauges, as well as the large tsunami inundation in the Sendai plain.
6. Lessons learned
The lessons learned from the 2011 Tohoku earthquake and tsunami disaster include importance of
geophysical observations, improvement of long-term forecast of earthquakes, effective tsunami warning,
assessment for future tsunami hazard, and preparedness and education for infrequent hazards.