Statistical features of foreshocks in comparison with other earthquake clusters

SUMMARY When earthquakes occur in a cluster, the probability that they will be foreshocks of a forthcoming significantly larger earthquake appears dependent on the magnitude differences, origin-time spans, and distances between several of the earliest events in a cluster. The earthquake catalogue of the Japan Meteorological Agency (JMA; 1926-91, MJ 2 4) is decomposed into numbers of clusters in time and space to compare statistical features of foreshocks with those of swarms and main shockaftershock sequences. Since the number of foreshocks in individual earthquake clusters is usually small, two types of data stacking are considered. First, we consider the spatial and temporal distribution of events relative to the location and time of the coming main shock. For the foreshocks, a relative doughnut-shaped pattern which converges in time to the main shock’s epicentre is seen, while this pattern is weak for swarms. Next, we consider the temporal, spatial and magnitude distributions of events relative to the time, location and magnitude of earlier events in every sequence. Then, a statistical discrimination of foreshocks from earthquakes of other types of clusters is explored. The search for significant trends of relative frequency of foreshocks in comparison with other cluster types revealed the following features of foreshocks: (1) their relative frequency is high when the inter-event time span is less than several days, but the maximum is attained at about several hours span; (2) their relative frequency is high at short distances between epicentres, especially within 10 km, and then decreases as the distance gets larger; and (3) their relative frequency increases as the magnitude increases among the earliest shocks in a cluster. The stability of the results is confirmed by finding similar trends using different cluster identification methods and different threshold magnitudes. Further, microearthquakes in the Japan University Network data file (1983-87; M 2 2) and moderate-to-large earthquakes in the International Seismological Centre world catalogue (ISC; 1979-90, Mh 2 5) are examined to see some effects of the magnitude-thresholds of these catalogues. Similar features to those obtained in the JMA catalogue are also found for these catalogues.

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