A Time-Domain Approach for Accurate Spectral Source Estimation with Application to Ridgecrest, California, Earthquakes

We determine the seismic moment and corner frequency of earthquakes using narrow-band, time-domain measurements of the maximum shear-wave displacement amplitude. Each S wave is passed through a set of one-octave band-pass filters between 0.25 and 32 Hz to measure the peak amplitude on the horizontal components. Following the development of Richter (1935) local magnitude scale, we derive a functional form of maximum amplitude versus distance, but separately for each frequency band, and use them to estimate wave attenuation as a function of frequency and distance. Attenuation is a strong function of frequency, approximately proportional to 1/root f, with path averaged Q increasing (attenuation decreasing) with propagation distance. We use these empirical attenuation curves to correct the time-domain peak amplitudes to a reference distance of 10 km to recover the source spectrum. We find that the spectral shape is consistent with the Brune model, showing a well-defined plateau at low frequency and a decay at high frequency proportional to omega(-2). We use the median spectrum for stations at distances up to 120 km for each event to measure the moment and corner frequency. This time-domain method provides a reliable and efficient way to accurately measure earthquake source parameters.