Fault parameters of the 1896 Sanriku Tsunami Earthquake estimated from Tsunami Numerical Modeling

The June 15, 1896 Sanriku earthquake generated devastating tsunamis with the maximum run-up of 25 m and caused the worst tsunami disaster in the history of Japan, despite its moderate surface wave magnitude (Ms=7.2) and weak seismic intensity. This is a typical tsunami earthquake, which generates anomalously larger tsunamis than expected from its seismic waves. Previously proposed mechanisms of tsunami earthquakes include submarine slumping and slow rupture in the accretionary wedge or in the subducted sediments. In this paper, we estimate the fault parameters of the 1896 tsunami earthquake by numerically computing the tsunami and comparing the waveforms with those recorded at three tide gauge stations in Japan. The result indicates that the tsunami source is very close to the Japan trench and the fault strike is parallel to the trench axis. The fault width is about 50 km, suggesting that the tsunami earthquake is a slow rupture in the subducted sediments beneath the accretionary wedge. caused no damage. On the other hand, the seismic intensity along the Sanriku coast from the 1896 event was much smaller, only 23 (weak shaking) on the modem JMA scale corresponding toIV or V on the MM scale. However, the maximum tsunami run-up height was 25 m and caused about 22,000 casualties. Why was the 1896 tsunami so large for its relatively small M s and seismic intensity? Kanarnori [ 1972] indicated that the large discrepancy between seismic and tsunami waves is explained by a slow and long rupture process. Okal [1988], using the normalmode theory, showed that an earthquake source in a shallow sedimentary layer excites much larger tsunamis than in solid rock. Fukao [1979] and Palayo and Wiens [1992] studied other tsunami earthquakes using seismic wave analysis and concluded that tsunami earthquakes are slow thrust events in the accretionary wedge. Satake [1994] modeled the recent 1992 Nicaragua tsunami earthquake and concluded that a slow rupture on a shallow fault in the subducted sediments is responsible for