DIRECT AND INDIRECT AFTERSHOCKS

In this paper, aftershock sequences of 25 large earthquakes, M ≥ 6, which occurred in China continent since the M=7.2 Xingtai (邢台) earthquake of 1966, are analyzed. It is found that in these sequences a great number of aftershocks followed the main-shocks in two or three days. The frequencies of these aftershocks decreased rapidly and exhibited a time decay law different from that governing the later aftershocks. These early aftershocks governed the temporal variation of the later aftershocks and had already roughly sketched out the aftershock regions. Based on these characteristics, aftershocks are considered to be of two kinds, say direct aftershocks and indirect aftershocks. The characteristics of direct aftershocks are examined, and the relationships between indirect and indirect aftershocks are derived. A suggestion is made to estimate indirect aftershocks and predict the development of the aftershock sequences which rely on the direct aftershocks. Finally, a preliminary discussion is made on the machanism of the two kinds of aftershocks.