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Stopping phases in seismograms and the spatiotemporal extent of earthquakes

UNIVERSITY GADJAH MADA, FMIPA UGM, GEOPHYSICS SECTION, SEKIP UNIT III, P.O. BOX BLS 21, YOGYAKARTA, Indonesia
INSTITUTE OF METEOROLOGY AND GEOPHYSICS UNIVERSITY OF FRANKFURT, FELDBERGSTR. 47, D-6000 FRANKFURT, Federal Republic of Germany

Abstract

On the basis of long-period WWSSN and some broadband seismograms from Gräfenberg, we investigate the following earthquakes

P-wave seismograms or true ground displacements show clearly the directivity effect of rupture propagation and the multiple-event nature of these earthquakes. Fault-plane solutions of individual source shocks and their relative localization by the master-event technique yield fixpoints of rupture history. The most striking feature is a strong stopping phase, which is interpreted kinematically by abrupt termination of rupture expansion at the end of the fault. We use the stopping phase to determine total source extent in space and time, as well as the mean rupture velocity.

The results of this first stage of investigation are subsequently used for modeling the rupture according to the dislocation theory of shear faulting. Wave propagation through the earth's mantle is treated by ray theory. This forward modeling approach yields seismic moment, source dimensions, mean dislocation, and stress drop. Source complexities are related to unbroken fault gaps, bending of the fault plane, acceleration of rupture propagation in asperities, and, most impressive in its radiation effect, to the sudden termination of rupture in front of a barrier. Source complexities are also discussed with respect to tectonic implications.