TEMPORAL AND SPATIAL FEATURES OF AFTERSHOCK SEQUENCES

In studying the overall features of nine aftershock sequences of large earthquakes, M 7, occurred in the eastern part of China since 1966, we found that aftershock frequencies agree well with the kw, n = n1 t-h (for 1t 1000 days), which differs somewhat from K. Mogis relation in that frequencies and magnitudes of aftershocks decrease slowly with time; That for the aftershock sequencies of the swarm type and a portion of the main-shock-aftershock type, the outlines of the zones of distribution of aftershocks which occur within the first three or four days after the mainshock or the largest shock is always smaller than that of the final aftershocks of the sequences; that most aftershock foci are distributed at the depth range from 5 to 10 kilometers in the crust, but the relation between the focal depths of the mainshocks and aftershocks has not been found.Temporal and spatial features of small and moderate aftershocks before some strong aftershocks are as follows: (1) The rate of decrease of the frequencies of the aftershock sequences deviates from the normal, few to more than ten days before a strong aftershock, but it becomes normal when the times of occurrence of certain strong aftershocks are taken as origin; (2) The epicenters of small and moderate aftershocks before a strong aftershock tend to spread toward the epicenter of this strong aftershock or to generate a seismic gap; (3) The magnitude sequence of the aftershocks before a strong aftershock reveals seismic deficiency; (4) Aftershocks indicate alternatively high and low seismic activities on the M-log t diagrams, and such pseudo-periodility may represent activities of aftershocks in clusters. Result of our investigation gives us an impression that strong aftershocks are generated by the same process as the mainshocks and those temporal and spatial features as mentioned above of an aftershock sequence may imply possibly subsequent occurrence of a strong aftershock. These phenomena may also indicate that an aftershock sequence is the combination of the aftershock sequence of the mainshock and the aftershock sequences of strong aftershocks, both temporally and spatially.This paper ends with a comparison of certain of those features of aftershock sequences with the results of rock sample experiments and discussions.