Source scaling and moment estimation for the Pacific Northwest seismograph network using S-coda amplitudes

Earthquake coda waves can be used for investigating both source and medium parameters. In this investigation, we introduce a method for estimating the scalar seismic moment of local earthquakes using S-coda amplitudes recorded by regional network stations. Assuming a single-scattering coda model, more than 8000 coda amplitude measurements from 78 local western Washington earthquakes (with coda-duration magnitudes from 2.4 to 5.0) are used to estimate source spectra in the narrow frequency band 2 to 12 Hz. Using a transmission model based on previous studies, we find that our source modeling best supports an omega-square, constant stress drop, far-field scaling model. An average stress drop of 32 bars is inferred for the earthquakes studied. To calibrate the moment estimation procedure, a reference event with known moment and corner frequency is required. Using a suitable reference event, we estimate moments for the remaining 77 events in the data set. The moment values range from 5.1 × 10¹⁹ to 4.4 × 10²³ dyne-cm.