Data and Methodology

In this study we solved for four different source models: DC, deviatoric (DC+CLVD), DC+isotropic and the full moment tensor model (DC+CLVD+isotropic). The full moment tensor model can characterize source processes involving a combination of tensile and shear faulting (Julian et al., 1998). The deviatoric moment tensor model describes volume conserving source processes which deviate from a simple DC mechanism. DC+isotropic source mechanisms have been used to describe combinations of near-simultaneous faulting near an underground explosion source (Massé, 1981). The pure DC model assumes that the earthquake source is best modeled as shear along a linear fault plane and a priori sets the CLVD and volumetric components to zero.

For the DC and DC+isotropic models, a grid search method iterating over strike, dip, rake, DC moment and isotropic moment, which is equal to zero in the pure DC case, was used to find the solution which best fit the observed three-component Berkeley Digital Seismic Network waveforms bandpass filtered between 0.02 and 0.05 Hz. For the deviatoric and full moment tensor models, the second rank symmetric seismic moment tensor is solved by linearly inverting complete three-component filtered broadband seismograms in the time domain using a weighted least squares approach. Green's functions for all four models were computed utilizing a frequency wave-number integration method and the SoCal velocity model (Dreger and Helmberger, 1993) for source depths every 3 km between 2 - 17 km.

When testing more complex source models, the variance reduction usually increased with increasing complexity. F test statistics were performed to determine if the additional CLVD and/or volumetric components represented a true aspect of the source mechanism or if they were simply added non-physical parameters in the inversion.

Figure 13.32: DC solutions shown in black. DC+isotropic solutions shown as dark gray. Full moment tensor solutions shown as light gray. Date of event shown as YY.MM.DD.
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