An experiment in systematic study of global seismicity: Centroid‐moment tensor solutions for 201 moderate and large earthquakes of 1981

Abstract

Data from the Global Digital Seismograph Network were used to obtain ‘centroid‐moment tensor’ solutions using the method of Dziewonski et al. (1981). Results were obtained for 201 earthquakes ranging in seismic moment from 7×10 23 to 3×10 27 dyne‐cm. The wide dynamic range of the SRO/ASRO stations allows us to investigate, using the same algorithm, series of events among which the smallest and the largest may differ in moment by a factor as large as 1000. Among the events studied is a particularly interesting series, in January 1981, off the island of Honshu. The main shock was preceded by three foreshocks and followed by four aftershocks all falling within the range of our analysis. There is an indication that the source mechanism changes with the size of the event. Statistical analysis of the results for all earthquakes reveals that the half duration of shallow and intermediate events is approximated by 1.6×10−8 M 01/3. The shifts in origin time for deep‐focus earthquakes are indicative of the general complexity of these events, which depends only weakly on the seismic moment. The extreme departures from a double couple mechanism seem to depend both on the moment and on the focal depth. While for shallow earthquakes these departures decrease, from a maximum at about 10 25 dyne‐cm, with increasing moment, the reverse appears to be true with respect to deep‐focus earthquakes. Shallow earthquakes north of New Guinea and along the Solomon Islands show systematic and substantial departures from the double couple mechanism.