An extract from Professor Koji Okumura’s (Hiroshima University) report on the recent earthquake & tsunami:
A M 9.0 (JMA: Japan Meteorology Agency) or Mw 8.9 (U.S. Geologcial Survey) gigantic earthquake occurred at 11:46 AM (JST) on March 11, 2011 on the interplate megathrust interface between Northeast Japan and Pacific Ocean. The seismogenic fault plane 500 km long along the trench and 200 km wide toward under Northeast Japan ruptured by three sub-events in 5 minutes (according to JMA) generating the largest known earthquake in and around Japan.
The earthquakes expected in the source area had much smaller magnitude of M7.4 to M 8.2 from 6 smaller fault segments based on historic seismicity (HERP: Headquarters for Earthquake Research Promotion, 2009a). The March 11 earthquake ruptured all 6 segments in a single earthquake. This conjunctive gigantic earthquake seems to be far beyond seismologists’ imagination as the 2004 Sumatra-Andaman earthquake. The segments are shown in online annex documents based on HERP (2010, p. 27 and p.28; 2009a).
The first tsunamis reached to the coast from Choshi (36° N) to Miyako (40° N) simultaneously within 10 minutes as minor (10s of centimeter) backwash in three tide-gauges. Then, about 30 to40 minutes after the earthquake huge (3 m+) run- up first attacked coastal lowlands. The short time between earthquake and the first wave indicates sea-bottom deformation by underlying seismogenic fault reached very close to the coastline. The simulutaneity of first arrival at Choshi (36° N) to Miyako (40° N) demonstrate the both ends of the fault plane are located offshore of these two tide gauges. See the online annex documents based on tide-gauging by Maritime Safety Agency.The Sanriku coastal areas between Ayukawa and Hachinoe have suffered repeatedly from recent tsunamis by 1896 subduction earthquake, 1933 outer-rise earthquake under east slope of Japan Trench, and distal 1960 Chile earthquake. The preparedness has certainly reduced fatalities but banks and watergates did not stop tsunami run-ups. During recent tsunamis, tsunami height in Sanriku aeas in Iwate prefecture exceeded 10 meters and enormous damages took place. On the other hand, tsunami height in Miyagi prefecture to the south was mostly less than 5 meters and damages were relatively light. Further south in Sendai area and Joban coast of Fukushima and Ibaraki, only few locations experienced tsunamis more than 2 meters (Watanabe, 1985).
Download the full report here:
