Space-time distribution of interplate moment release including slow earthquakes and the seismo-geodetic coupling in the Sanriku-oki region along the Japan trench

Along the Japan trench where some Mw8 class interplate earthquakes occurred in the past century such as the 1896 Sanriku tsunami earthquake (M6.8, Mt8 .6, 12 x 10(20) Nm) and the 1968 Tokachi-oki earthquake (Mw8.2, 28 x 10(20) N m), the Pacific plate is subducting under northeast Japan at a rate of a round 8 cm/year. The seismic coupling coefficient in this region has been e stimated to be 20-40%. In the past decade, three ultra-slow earthquakes hav e occurred in the Sanriku-oki region (39 degreesN-42 degreesN): the 1989 Sa nriku-oki (Mw7.4), the 1992 Sanriku-oki (Mw6.9), and the 1994 Sanriku-oki ( Mw7.7) earthquakes. Integrating their interplate moments released both seis mically and aseismically, we have the following conclusions. (1) The sum of the seismic moments of the three ultra-slow earthquakes was (4.8-6.6) x 10 (20) N m, which was 20-35% of the accumulated moment (18.6-23.0)x 10(20) N m, in the region (39 degreesN-40.6 degreesN, 142 degreesE-144 degreesE) for the 21-26 years since the 1968 Mw8.2 Tokachi-oki earthquake. This is consi stent with the previous estimates of the seismic coupling coefficient of 20 -40%. On the other hand, the sum of the interplate moments including aseism ic faulting is (11-16) x 10(20) N m, leading to a "seiseismo-geodetic coupl ing coefficient" of 50-85%, which is an extension of the seismic coupling c oefficient to include slow events. (2) he time constants showed a large ran ge from 1 min (similar to 10(2) s) for the 1968 Tokachi-oki earthquake to 1 0-20 min (similar to 10(3) s) for the 1896 Sanriku tsunami earthquake, to o ne day (similar to 10(5) s) for the 1992 Sanriku-oki ultra-slow earthquake, to on the order of one year (similar to 10(7) s) for the 1994 Sanriku-oki ultra-slow earthquakes. (3) Based on the space-time distribution, three "ga ps of moment release," (40.6 degreesN-42 degreesN, 142 degreesE-144 degrees E) 39 degreesN-40 degreesN, 142 degreesE-143 degreesE) and (39 degreesN-40 degreesN, 142 degreesE-144 degreesE), are identified, instead of the gaps o f seismicity. (C) 2001 Elsevier Science B.V. All rights reserved.