Complex source process of a small earthquake with M4.8.

Abstract

We observed the waveforms from a small earthquake (M=4.8), which corresponds to the largest aftershock of the 1983 Tottori earthquake, just above its hypocenter. The records have significant information on complicated source features. The P-waveforms recorded by a velocity seismograph are decomposed into a longer- (4 Hz) and a shorter-period (10-20 Hz) component. The source of the longer-period P-waves and S-waves can be theoretically explained by a uniform dislocation over a rectangular fault plane (1, 200 m × 800 m). Aftershocks accompanying this earthquake are distributed around the periphery of this fault plane. The source of the shorter-period P-waves corresponds to a multiple shock composed of two subevents whose source dimensions are each about 150m. Stress drops of the two subevents are about 10 to 100 times as large as that of the longer-period process. The distribution of aftershocks has a clustering structure. Events in each cluster appear to take place on a common fault plane appropriate to each cluster with a dimension of 100-200 m. The clustering structure shows heterogeneous distribution of fracture strength in the aftershock area. We may suppose such heterogeneities composed of pre-existing small weak zones also in the source area of this earthquake (M=4.8), in order to explain its complex source process. Namely, the rupture is considered to have extended over the rectangular fault including several weak zones, and the shorter-period waves with high stress drop mentioned above are expected to have been radiated from the fractures of strong patches among weak zones. The source process of micro to small earthquakes in a heterogeneous crust may be systematically explained as follows: The growth of rupture in the case of a microearthquake is confined within one or a few neighboring pre-existing weak zones (NISHIGAMI, 1987). On the other hand, the rupture in a small earthquake grows more extensively by fracturing not only several weak zones but also strong patches among them.