Background and Scope - Tohoku, Japan Earthquake and Tsunami of 2011 - page 23

Measurements of plate slip help characterize the seismic inputs for modeling the
tsunami generation. A number of estimates have been proposed for the amount and
pattern of slip at the plate boundary for the Great East Japan earthquake. Estimates of
maximum slip range from 30 meters (Ide 2011) to more than 50 meters (Simons
2011). Ammon (2011) used seismic data and GPS time series to estimate a zone of
large slip about 300 by 150 kilometers, offshore and slightly southward of the
earthquake hypocenter with a maximum slip value of about 40 meters. The source
area illustrated in Figure 10 (Simons et al. 2011) has a similar source distribution area
but higher slip values. The zone of maximum displacement occurs southeast (up-dip)
of the earthquake's epicenter. Simons included an analysis of tsunami wave
characteristics as well as GPS and seismic data to characterize the pattern and timing
of seismic displacement for the Great East Japan event.
1.5 Tsunami Generation, Propagation, and Runup
1.5.1HistoricalTsunamis
Japan's written history gives detailed accounts of many earthquakes and tsunamis.
Some of these events predate the use of modern seismographic and survey equipment,
but careful record-keeping allows for magnitude, source location, and runup estimates.
Table 6 gives a summary of the known tsunamigenic Japanese earthquakes above
magnitude 7 with reliable records and casualties over 1,000. The shaded portions of
the table indicate those events that happened in Sanriku along the Northeastern coast
of Honshu. While reports indicate the tsunami with the highest number of fatalities
(31,000) occurred in 1498, 240 kilometers southwest of Tokyo on the Nakai Trough,
Sanriku has also been the site of several destructive tsunamis. The dendritic coastline
allows the focusing of wave energy into small steep-sided coastal embayments,
amplifying the destructive effects of tsunamis. In addition, the short coastal shelf does
not allow for wave attenuation through bottom friction, as does the topography
further south in Sendai. Large tsunamis occurred in the area in 1611, 1896, and 1933.
The M 7.6 Meiji Sanriku subduction earthquake of 1896 caused over 27,000
casualties with high tsunami waves throughout the region. Kanamori (1979)
characterized this earthquake as a tsunami-earthquake because the shaking felt
onshore was relatively small compared to the size of the tsunami generated in the
event. As a result, many people were unaware of the potential danger from the
incoming waves. The tsunami attained a maximum height of 38 m at Ryori Bay in the
municipality of Shirahama. The 1933 Showa Sanriku tsunami, a shallow crustal
earthquake, produced tsunami waves as high as 29 m at this same location (at Ryori
Bay) and caused more than 3,000 fatalities throughout Sanriku.