The Nature of Seismicity Patterns Before Large Earthquakes

Abstract

Various seismicity patterns before major earthquakes have been reported in the literature. They include foreshocks (broad sense), preseismic quiescence, precursory swarms, and doughnut patterns. Although many earthquakes are preceded by all, or some, of these patterns, their detail differ significantly from event to event. In order to examine the details of seismicity patterns on as uniform a basis as possible, we made space-time plots of seismicity for many large earthquakes by using the NOAA and JMA catalogs. Among various seismicity patterns, preseismic quiescence appears most common, the case for the 1978 Oaxaca earthquake being the most prominent. Although the nature of other patterns varies from event to event, a common physical mechanism may be responsible for these patterns; details of the pattern are probably controlled by the tectonic environment (fault geometry, strain rate) and the heterogeneity of the fault plane. Here a simple asperity model is introduced to explain these seismicity patterns. In this model, a fault plane with an asperity is divided into a number of subfaults. The subfaults within the asperity are, on the average, stronger than those in the surrounding weak zone. As the tectonic stress increases, the subfaults in the weak zone break in the form of background small earthquakes. If the frequency distribution of the strength of the subfaults has a sharp peak, a precursory swarm occurs. By this time, most of the subfaults in the weak zone are broken and the fault plane becomes seismically quiet. As the tectonic stress increases further, eventually the asperity breaks and sympathetic displacement occurs on the entire fault zone in the form of the main shock. Foreshocks do or do not occur depending upon the distribution of the strength of the subfaults within the asperity. Since the spatio-temporal change in the stress on the fault plane is most likely to dictate the change in seismicity patterns, detailed analysis of seismicity patterns would provide a most direct clue to the state of stress in the fault zone. However, because of the large variation from event to event, seismicity pattern alone is not a definitive tool for earthquake prediction; measurements of other physical parameters such as the spectra, the mechanism and the wave forms of the background events should be made concurrently.