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Space–time distribution of interplate moment release including slow earthquakes and the seismo-geodetic coupling in the Sanriku-oki region along the Japan trench |
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| Authors: | I Kawasaki Y Asai Y Tamura |
| Institution: | a Department of Earth Sciences, Toyama University, Gofuku 3190, Toyama, Toyama-ken 930-8555, Japan b Tono Research Institute of Earthquake Science, Mizunami, Gifu-ken 509-6132, Japan c National Astronomical Observatory at Mizusawa, Mizusawa, Iwate-ken 023-0861, Japan |
| Abstract: | 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, 12×1020 N m) and the 1968 Tokachi-oki earthquake (Mw8.2, 28×1020 N m), the Pacific plate is subducting under northeast Japan at a rate of around 8 cm/year. The seismic coupling coefficient in this region has been estimated to be 20–40%. In the past decade, three ultra-slow earthquakes have occurred in the Sanriku-oki region (39°N–42°N): the 1989 Sanriku-oki (Mw7.4), the 1992 Sanriku-oki (Mw6.9), and the 1994 Sanriku-oki (Mw7.7) earthquakes. Integrating their interplate moments released both seismically 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)×1020 N m, which was 20–35% of the accumulated moment (18.6–23.0)×1020 N m, in the region (39°N–40.6°N, 142°E–144°E) for the 21–26 years since the 1968 Mw8.2 Tokachi-oki earthquake. This is consistent with the previous estimates of the seismic coupling coefficient of 20–40%. On the other hand, the sum of the interplate moments including aseismic faulting is (11–16)×1020 N m, leading to a “seismo-geodetic coupling coefficient” of 50–85%, which is an extension of the seismic coupling coefficient to include slow events. (2) The time constants showed a large range from 1 min ( 102 s) for the 1968 Tokachi-oki earthquake to 10–20 min (103 s) for the 1896 Sanriku tsunami earthquake, to one day (105 s) for the 1992 Sanriku-oki ultra-slow earthquake, to on the order of one year (107 s) for the 1994 Sanriku-oki ultra-slow earthquakes. (3) Based on the space–time distribution, three “gaps of moment release,” (40.6°N–42°N, 142°E–144°E) 39°N–40°N, 142°E–143°E) and (39°N–40°N, 142°E–144°E), are identified, instead of the gaps of seismicity. |
| Keywords: | crustal deformation seismic coupling seismo-geodetic coupling slow earthquake Japan trench |
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