Regional studies with broadband data

Abstract : We have developed a new approach to the inversion of waveform data for the time-varying moment tensor. The method produces the source model which minimizes the modulus squared of any linear combination of moment tensor components, subjects to the constraint that the data are satisfied within specified confidence intervals. This method allows the determination of possible source models other than the least squares solution, enabling one to determine the significance of certain moment tensor properties, for example, the presence or absence of a volume change (isotropic component) in the source. Synthetic tests were used to examine the effect of microseismic noise and lateral heterogeneity on the extreme models of the isotropic component. Lateral heterogeneity is found to have a strong effect on the estimation of the isotropic component of the moment tensor. The method was tested by using long-period waveforms from the Global Digital Seismic Network to estimate the isotropic part of the moment tensor of a deep Bonin Islands earthquake. Modelling indicates that more than 10% of the mechanism might have to be isotropic for detection of volume change in the presence of 10% random noise and only 2% lateral heterogeneity. The least-squares solution indicates that a relatively large change in volume was involved in the source mechanism. However, the minimum extreme solution shows that this volume change is not actually required by the data and thus may not be significant. The method was also tested on near source data from the nuclear explosion Harzer. In this case, in spite of fairly large error bounds, it can be concluded that the source has a clear explosive component. (rh)

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