時空間分布とサイスモ・ジオデティック・カップリング -中・長期予測の基礎-

Abstract

We had the 1989, 1992 and 1994 Sanriku-Oki earthquakes in the region from 39. 0°N to 40. 6°N along the Japan trench. Summarizing the aseismic nature of the earthquakes, we propose two new concepts“seismo-geodetic coupling“and”a gap of the moment release”. The sum of their seismic moments is (4. 8-6. 6) ×1020Nm. These events were associated with aseismic ultra-slow fautings of source times of a day to a year following the respective main seismic shocks. The ultra-slow faultings respectively released interplate momens, 1. 4, (1-4), 4. 2×1020Nm, equivalent to those of Mw7. 3 to 7. 7 seismic events. The sum of the moments released by both seismic and aseismic faultings amounts to (11-16) ×1020Nm. The interplate moment is accumulated as (19-23) ×1020Nm for the area of 180 km (39. 0°N to 40. 6°N) 150 km (width of high seismicity region between the Sanriku coast and the Japan trench), with the relative plate motion of 8 cm/yr and the interval of 21-27 years from the 1968 Tokachi-Oki earthquake of Mw8. 2 to the 1989-1994 ultra-slow earthquakes. The ratio of (11-16) × l020Nm to (19-23) ×1020Nm leads to the rough estimate of “seismo-geodetic coupling coefficient”of 50-85%, where the ordinary seismic coupling coefficient has been estimated to be 20-30% between the Okhotsk and subducting Pacific plates. Based on the spatiotemporal distribution of the interplate moments including the ultra-slow earthquakes, three gaps of the moment release are suggested for the Sanriku-Oki region. This would serve as a basis for the mid-term/long-term prediction of future interplate earthquakes in the Sanriku-Oki region.