Low-Frequency Properties of Intermediate-Focus Earthquakes

Intermediate-focus earthquakes are known to show features such as nondouble-couple focal mechanisms that indicate source complexities. To characterize these features, we have systematically studied the low-frequency radiation from 108 intermediate-focus earthquakes recorded by high-performance seismic networks from 1989–1997 whose total moment was 3 10 N m. We determined frequency-dependent focal mechanisms and source phase and amplitude spectra for each earthquake, estimating the uncertainties for all parameters. Frequency-dependent focal mechanisms were obtained from vertical-component, free-oscillation data in 1mHz bands over the range of 1–11 mHz. Source amplitude and phase-delay spectra were determined to 20 mHz from a combination of free-oscillation and surface-wave data. The population of intermediate-focus earthquakes in our catalog is not equally divided between compressional and tensional stress-release mechanisms; instead, 59% are down-dip tensional, 25% are down-dip compressional, and 16% are neither. We have assessed the statistical significance of any non-double-couple component of the source for every earthquake in the catalog. We represent a deviatoric focal mechanism by its principle axes and a scalar n that ranges from 1, for a compensated linear vector dipole (CLVD) with a compressional axis of symmetry through 0 for a double couple, to 1, for a CLVD with a tensional axis of symmetry. n varies from 0.91 to 0.64 in this dataset. Fifty-six of the 108 events have a significant CLVD component (n 0) at the 95% confidence level; 15 events have an unusually large CLVD component of the focal mechanism (|n| 0.4). We do not observe a correlation between the CLVD component and seismic moment or depth. T-type CLVD mechanisms correlate weakly with slab stress state, but no correlation was similarly found for P-type CLVD events. Seven earthquakes in the catalog are slow or compound, and three show strong frequency dependence of the moment tensor.