MOMENT, ENERGY, STRESS DROP, AND SOURCE SPECTRA OF WESTERN UNITED-STATES EARTHQUAKES FROM REGIONAL CODA ENVELOPES

We present a new method to estimate stable seismic source parameters, such as energy, moment, and Orowan stress drop, using regional coda en velopes from as few as one broadband station. We use the method to com pute path- and site-corrected seismic moment-rate spectra for 117 rece nt western United States earthquakes. Empirical Green's function corre ctions were applied to our surface- and body-wave coda envelope measur ements to generate S-wave source spectra. These source spectra provide stable, single-station estimates of radiated seismic energy E(s) and seismic moment M(o) 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 result s, but teleseismic energy estimates appear to be nearly an order of ma gnitude low. We estimated the seismic moment of events ranging between M(w) 2.2 and 7.3, and energy estimates for which we had measured at l east 70% of the total energy, generally events above M(w) 3.3. We use these estimates to examine the behavior of derived parameters such as the Orowan stress drop (Delta sigma = 2 mu E(s)/M(o)). While the earth quakes 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 M(o)(0.25) We believe t his is a source effect and is not due to inadequate bandwidth or atten uation correction, and note that this trend appears to continue for mi croearthquakes as described in a recent deep borehole study in souther n California. Many of the large high stress drop earthquakes show comp lexity in their moment-rate spectra near the corner frequency and cann ot be fit by a simple omega-square model. Instead, above the first cor ner 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 omega-square model and has implications for seismic hazard es timation. Coda envelopes have three main advantages over direct arriva ls 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 measur ements 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 ho urs for large local and regional events, it allows the analysis of sei smograms with clipped early arrivals.