Source characteristics of moderate size events using empirical Green funclions: an application to some Guerrero (Mexico) subduction zone earthquakes

Abstract

The records of an aftershock (M ~ 4) of a moderate size event (M = 5.9) which occurred along the subduction zone of Guerrero (Mexico), are used as empirical Green functions (EGF) to determine the source characteristics of the mainshock and of its smaller size (M = 5.5) foreshock. The data consist of accelerograms recorded by the Guerrero Accelerograph Array, a high dynamic range strong motion array. The three events appear to be located close to each other at distances much smaller than the source to receiver distances. The fault mechanism of the mainshock is computed by non-linear inversion of P polarity readings and S wave polarizations determined at two near source stations. The foreshock and aftershock fault mechanisms are similar to that of the mainshock as inferred from long period data and shear wave polarization analysis. The maximum likelihood solution is well constrained, indicating a typical shallow dipping thrust fault mechanism, with the P-axis approximately oriented in a SSW direction. The source time functions (STFs) of the mainshock and foreshock events are determined using a new method of deconvolution of the EGF records at three strong motion sites. In this method the STF of the large event is approximated by a superposition of pseudo triangular pulses whose parameters are determined by a non-linear inversion in frequency domain. The source time function of the mainshock shows the presence of two separate pulses, which can be related to multiple rupture episodes. The relative location of mainshock sub-events is done by using plots of isochrones computed from measurementes of the time delay between pulses on the STF records at each station. The first sub-event is located no more than 2.5-3 km away from the other along the fault strike. The STF retrieved from foreshock records shows single pulse waveforms. The computed STFs are used to estimate seismic moments, source radii and stress release of the events assuming a circular fault model and constant rupture velocity. The final rupture model for the mainshock indicates the successive breaking of two nearby large slip patches having sizes of the order of a few kilometers and with the rupture propagating at an average speed close to the shear wave velocity.