Short term and short range seismicity patterns in different seismic areas of the world

The aim of this work is to quantitatively set up a simple hypothesis for oc currence of earthquakes conditioned by prior events, on the basis of a prev iously existing model and the use of recent instrumental observations. A si mple procedure is presented in order to determine the conditional probabili ty of pairs of events (foreshock-mainshock, mainshock-aftershock) with shor t time and space separation. The first event of a pair should not be an aft ershock, i.e., it must not be related to a stronger previous event. The Ita lian earthquake catalog of the Istituto Nazionale di Geofisica (ING) (1975- 1995, M greater than or equal to 3.4), the earthquake catalog of the Japan Meteorological Agency (JMA) (1983-1994, M greater than or equal to 3.0) and that of the National Observatory of Athens (NOA) (1982-1994, M greater tha n or equal to 3.8) were analyzed. The number of observed pairs depends on s everal parameters: the size of the space-time quiescence volume defining no naftershocks, the inter event time, the minimum magnitude of the two events , and the spatial dimension of the alarm volume after the first event. The Akaike information criterion has been adopted to assess the optimum set of space-time parameters used in the definition of the pairs, assuming that th e occurrence rate of subsequent events may be modeled by two Poisson proces ses with different rates: the higher rate refers to the space-time volume d efined by the alarms and the lower one simulates earthquakes that occur in the nonalarm space-time volume. On the basis of the tests carried out on th e seismic catalog of Italy, the occurrence rate of M greater than or equal to 3.8 earthquakes followed by a M greater than or equal to 3.8 mainshock w ithin 10 km and 10 days (validity) is 0.459. We have observed, for all thre e catalogs, that the occurrence rate density lambda for the second event of a couple (mainshock or aftershock) of magnitude M-2 subsequent to a nonaft ershock of magnitude M-1 in the time range T can be modeled by the followin g relationship: lambda (T, M-2) = 10(a)' (+ b(M1) (- M2)) with b varying fr om 0.74 (Japan) to 1.09 (Greece). The decrease of the occurrence rate in ti me for a mainshock after a foreshock or for large aftershocks after a mains hock, for all three databases, obeys the Omori's law with p changing from 0 .94 (Italy) to 2.0 (Greece).