On the influence of earthquake energy range on the estimated seismicity parameters before the magnitude 7.9 Kronotskii earthquake of 1997

摘要

Results are reported from a detailed study of central Kamchatka seismicity for the period 1962-1997 based on a modification of the traditional approach. The approach involves (a) a detailed structure of the seismic region that recognizes the Kronotskii and Shipunskii geoblocks and two further blocks, the continental slope, and the offshore portion, (b) a study of variations in the rate of M = 3.0-7.2 earthquakes and the amount of seismic energy released at depths of 0-50 and 51-100 km, (c) a study of seismicity variability, and (d) separate estimates of the recurrence of crust-mantle earthquakes (depths 0-50 km) and mantle events (51-100 km). As a result, apart from corroborating the fact of a quiescence preceding the December 5, 1997 Kronotskii earthquake (M 7.9), we also found that a relationship exists between its beginning and the position of the earthquake-generating region relative to the mainshock epicenter. The quiescence dominates the seismic process during the pre-mainshock period and is characterized by a decreased rate of earthquakes (the first feature) and a decreased amount of seismic energy release (the second feature). Based on the first feature, we found that the quiescence started in 1987 throughout the entire depth range (0-100 km) in both parts of the Kronotskii geoblock close to the rupture zone of the eponymous earthquake. As to the Shipunskii geoblock, which is farther from the rupture zone, the quiescence began in the mantle of the inner area first (1988) and somewhat later at depths of 0-50 km within the continental slope (1989). By the second feature, the quiescence began at shallower depths in the inner area of the Kronotskii geoblock at the same time and later on (a year later) in the mantle (1988). Under the continental slope of the trench in the Shipunskii geoblock the shallower quiescence also began in 1987, while it was 3 years late in the inner zone (1990) and involved the earthquake-generating earth volume at depths of 0-100 km. These data are identical with or sufficiently close to the estimate for the beginning of this quiescence using a circular area of radius 150 km that combines the Kronotskii and Shipunskii geoblocks by the RTL method (1990). Based on the data on small earthquakes (M ≥ 2.6) at depths of 0-70 km mostly for the continental slope of the Kronotskii geoblock in the southwestern part of the epicentral area (at distances of 50-100 km from the mainshock epicenter), the quiescence began a little over 3 years (mid-1994) before the mainshock time and a little over 2 years (late 1995) in an immediate vicinity of the mainshock epicenter (0-50 km). These results provide evidence of different times when the stress buildup began in different earth volumes during the precursory period of this catastrophic earthquake; this should be borne in mind when examining the data for the other kinds of geophysical precursors.[PUBLICATION ABSTRACT] Show less