On the Seismicity of Central Kamchatka before the 1997,M = 7.9 Kronotskii Earthquake

摘要

We report results from a detailed study of seismicity in central Kamchatka for the period from 1960 to 1997using a modified traditional approach. The basic elements of this approach include (a) segmentation of the seis-mic region concerned (the Kronotskii and Shipunskii geoblocks, the continental slope and offshore blocks),(b) studying the variation in the rate of M = 4.5-7.0 earthquakes and in the amount of seismic energy releaseover time, (c) studying the seismicity variations, (d) separate estimates of earthquake recurrence for depths of0-50 and 50-100 km. As a result, besides corroborating the fact that a quiescence occurred before the Decem-ber 5, 1997, M = 7.9 Kronotskii earthquake, we also found a relationship between the start of the quiescenceand the position of the seismic zone with respect to the rupture initiation. The earliest date of the quiescence(decreasing seismicity rate and seismic energy release) was due to the M = 4.5-7.0 earthquakes at depths of0-100 km in the Kronotskii geoblock (8-9 years prior to the earthquake). The intermediate start of the quies-cence was due to distant seismic zones of the Shipunskii geoblock and the circular zone using the RTL method,combining the Shipunskii and Kronotskii geoblocks (6 years). Based on the low magnitude seismicity (M 2.6)at depths of 0-70 km in the southwestern part of the epicentral zone (50-100 km from the mainshock epicenter),the quiescence was inferred to have occurred a little over 3 years (40 months) before the mainshock time and alittle over 2 years (25 months) in the immediate vicinity of the epicenter (0-50 km). These results enable a morereliable identification of other types of geophysical precursors during seismic quiescences before disastrousearthquakes.