Rupture mechanics of complex earthquakes in southern Mexico

Abstract

Long-period P waveforms of the three most important events of the sequence of earthquakes which occurred near the coast of Chiapas, Mexico, commencing on April 29, 1970, were analysed using synthetic seismograms. Those synthetics were computed by the Haskell-Thomson matrix method which allows an investigation of reflections from near-source layering. Some perturbations in the wave forms are shown to be caused by near-source structure, while other are produced by complexities in the earthquake source time functions. The results show that the aftershock of April 30, 1970, at 0833 GMT ( M s = 6.4) is a simple earthquake and consequently can be modelled by a single point source with a seismic moment of 6.6 × 10 25dyn cm. The foreshock of April 29 at 1122 GMT ( M s = 6.3) consisted of two simple events having approximately the same seismic moment of 4.1 × 10 25 dyn cm. Four events were identified within a 15 second interval prior to the main rupture ( M s = 7.3) for which the seismic moment was 9.9 × 10 26dyn cm. Rather small fault areas and low rupture velocities seem to be characteristic of these earthquakes. The direction of rupture does not show a clear pattern although there is a tendency for it to occur approximately in a NE-SW direction.