Moment, energy, stress drop, and source spectra of western United States earthquakes from regional coda envelopes

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Abstract
We present a new method to estimate stable seismic source parameters, such as energy, moment, and Orowan stress drop, using regional coda envelopes from as few as one broadband station. We use the method to compute path‐ and site‐corrected seismic moment‐rate spectra for 117 recent western United States earthquakes. Empirical Green's function corrections were applied to our surface‐ and body‐wave coda envelope measurements to generate S‐wave source spectra. These source spectra provide stable, single‐station estimates of radiated seismic energy Es and seismic moment Mo that for common events are in excellent agreement with network‐averaged estimates obtained using local and regional data. Teleseismic moment estimates are compatible with our regional results, but teleseismic energy estimates appear to be nearly an order of magnitude low. We estimated the seismic moment of events ranging between Mw 2.2 and 7.3, and energy estimates for which we had measured at least 70% of the total energy, generally events above Mw 3.3. We use these estimates to examine the behavior of derived parameters such as the Orowan stress drop (Δσ = 2μEs/Mo). While the earthquakes we studied have a small range in Orowan stress drop, generally between 0.1 and 20 MPa, they show a strong tendency for Orowan stress drop to increase with moment, approximately as Mo0.25. We believe this is a source effect and is not due to inadequate bandwidth or attenuation correction, and note that this trend appears to continue for microearthquakes as described in a recent deep borehole study in southern California. Many of the large high stress drop earthquakes show complexity in their moment‐rate spectra near the corner frequency and cannot be fit by a simple ω‐square model. Instead, above the first corner frequency, the spectral decay ranges between f−1.0 and f−1.5. This leads to larger estimates of radiated energy than predicted with a simple ω‐square model and has implications for seismic hazard estimation. Coda envelopes have three main advantages over direct arrivals for estimating seismic moment and energy: (1) Coda amplitudes vary little with geology and source‐radiation anisotropy and allow accurate single‐station applications; (2) path‐corrected coda amplitude measurements can be applied to very large regions, allowing a comparison of source parameters throughout the western United States using a common methodology and stations; (3) because long‐period coda can last for hours for large local and regional events, it allows the analysis of seismograms with clipped early arrivals.