Robust retrieval of a seismic point-source time function

Details Robust retrieval of a seismic point-source time function Robust retrieval of a seismic point-source time function

Feb 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999geoji.136..385k&link_type=abstract

Geophysical Journal International, Volume 136, Issue 2, pp. 385-394.

Astronomy and Astrophysics

Astronomy

16

Inversion, Model, Seismic Structure, Synthetic Waveforms

Scientific paper

A comparison of two waveform-inversion methods designed to retrieve the mechanism of a seismic source and of its time function is presented using vertical-component synthetic signals, computed for velocity with a maximum frequency of 10 Hz. The geometry of the array of recording stations simulates the northeastern Italy Seismometric Network (OGS Trieste), consisting of 16 stations of which 12 are short period, vertical component only, three are short period, three components, and one is broad band. The synthetic seismograms are inverted using an inconsistent forward modelling technique; that is, by means of Green's functions (GFs) constructed for a structural model different from those used to generate the synthetic data. The approach based on `overparametrization' of the rupture process, by means of independent moment tensor rate functions (MTRFs), and their subsequent reduction to the source time function (STF) (Method I) is shown to be superior to a traditional approach where the rupture process is constrained a priori (Method II). With Method I, the effects of inconsistent structural modelling are partially absorbed into the uncorrelated parts of the MTRFs and their reverse slips, which allows us to eliminate them by subsequent retention of the STF as their positively constrained correlated part. Method I is shown to be able to yield a reasonable estimate of the STF even in the case when the traditional approach fails completely. Inadequacy of the GF, which may occur due to mislocation of the hypocentre, is taken into account by comparing the two approaches: the source depth is optimized simultaneously with the determination of the mechanism and the source time function. In addition to its capacity to handle inaccurate structural models, the overparametrization yielding a linear inverse scheme is completely independent from the starting model of the mechanism: Method II, using a gradient scheme, can proceed properly only if the starting source parameters are sufficiently close to the true ones. The extension of the comparison of the performances between the two methods to an M_d 3.0 earthquake near Friuli, in February 1988, recorded by seven stations of the OGS Trieste Network gives results in good agreement with the synthetic tests. The orientation of the nodal planes retrieved using Method I is in good agreement with the orientation of the source mechanism retrieved from the polarity of first arrivals, while Method II gives consistent results only when starting from the source parameters retrieved using Method I.

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