1096 Bulletin of the Seismological Society of America, 90, 4, pp. 1096–1100, August 2000 Short Notes Kuril Islands Arc: Two Seismic Cycles of Great Earthquakes during which the Complete History of Seismicity (M s  6) Is Observed by Omar J. Pe ´rez Abstract Two adjacent segments of Pacific-Eurasia plate boundary along the Ku- ril Islands arc were broken during two great shallow thrust earthquakes that occurred in 1963 (moment magnitude, M w  8.5) and 1969 (M w  8.2), respectively. In 1994 a great shock (M w  8.3) rebroke the 200-km-long plate boundary segment pre- viously broken by the 1969 event, and a series of shocks in 1991 (M w  7.6), 1995 (M w  7.9) and 1996 (M w  7.2) rebroke major portions of the 300-km-long segment previously ruptured by the 1963 event. Due to the heterogeneities and in- completeness of the instrumental seismicity catalog for shallow (focal depth, h  70 km) earthquakes with surface-wave magnitude M s  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 M s  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 M s  6 and M s  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. The purpose of this article is to report the spatial- temporal distribution of seismicity (h  70 km; Ms  6) that occurred along a 1000-km-long segment of Pacific- Eurasian plate boundary in the Kuril Islands arc, during the intervening time between a series of great (M w  7.8) shocks that broke two nearby portions of it, first in the 1960s and then again in the 1990s, that is, during two complete seismic cycles of great earthquakes. Because the world instrumental seismicity catalog (1900–1999) for shallow depth (h  70 km) shocks with M s  6 is complete only for the second half of the century (Pe ´rez and Scholz, 1984, 1997; Pe ´rez, 1999), and because repeat times of great earthquakes are typically on the order of 30 to 150 years (Fedotov, 1965, 1967; Kelleher et al., 1973; McCann et al., 1979; Sykes and Quitmeyer, 1981; Thatcher, 1990), it has been difficult to instrumentally observe the seismic behavior at an energy level as low as M s  6, of segments of plate boundaries broken during successive great shocks of comparable size, for example, shocks of comparable seismic moment (M o ) or area of rupture. The cases reported here are among the few for which such observations can be made because they fulfill the condition of having both, the great event and its repeat, occurring after 1950. Figure 1 shows the rupture zones (outlined areas) of the largest (M w  7.6) events that occurred along the southern Kuril Islands between 1963 and 1999. The main epicenters corresponding to the 1994 (M w  8.3), 1991 (M w  7.6), and 1995 (M w  7.9) shocks are indicated by closed stars. Those corresponding to the breaks in 1963 (M w  8.5), 1969 (M w  8.2), 1973 (M w  7.7), and 1978 (M w  7.6) are indicated by open stars. These ruptures were obtained by plotting all the aftershocks with body-wave magnitude m b  4.5, occurring during the next 15 days following each main event, as listed by the International Seismological Cen- tre (ISC) or its predecessor, the International Seismological Summary (ISS). An examination of the focal mechanism data compiled by Pacheco and Sykes (1992), and of Har- vard’s Centroid Moment Tensor Solutions (1976 to 1999) shows that these earthquakes, as well as the great majority of the strong (M s  6) shallow shocks occurring along the Kuril Islands in Figure 1, are of the shallow thrusting type, indicating that they were interplate events. An examination of the aftershock data reported by ISC and ISS shows that the larger events, that is, the 1963, 1969, 1973, 1994, and 1995 shocks, broke the entire lithospheric plate boundary down to depths of 60 to 65 km, whereas the smaller ones,