Abstract

Two adjacent segments of Pacific-Eurasia plate boundary along the Kuril Islands arc were broken during two great shallow thrust earthquakes that occurred in 1963 (moment magnitude, Mw = 8.5) and 1969 (Mw = 8.2), respectively. In 1994 a great shock (Mw = 8.3) rebroke the ∼200-km-long plate boundary segment previously broken by the 1969 event, and a series of shocks in 1991 (Mw = 7.6), 1995 (Mw = 7.9) and 1996 (Mw = 7.2) rebroke major portions of the ∼300-km-long segment previously ruptured by the 1963 event. Due to the heterogeneities and incompleteness of the instrumental seismicity catalog for shallow (focal depth, h < 70 km) earthquakes with surface-wave magnitude Ms ≥ 6, particularly for the first half of the century, this is one of the first times that a complete history of instrumental seismicity at an energy level as low as Ms = 6, occurring in a focal region of great earthquakes during a full seismic cycle, has been observed. My findings indicate that significant activity at the Ms ≥ 6 and Ms ≥ 7 levels took place both within, and as well as along the downdip edges, of the forthcoming 1990's ruptures during most of the seismic cycle, whereas the adjacent regions of the plate boundary were relatively less active. The average displacements and repeat times (28.5 yr ± 3.5) observed for these great earthquakes along this section of the plate boundary as a whole, are consistent with the strain energy accumulated during that time by subduction of the Pacific plate under the Eurasian plate, at a rate of ∼10 cm/yr.

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