Comparison of direct and coda wave stress drop measurements for the Wells, Nevada, earthquake sequence

Abstract

I calculate the corner frequencies and stress drops of the seven largest earthquakes (M4-6) in the Wells, Nevada, sequence (2008) using both coda and direct waves. I use spectral ratio, empirical Green's function (EGF), and methods to investigate whether differences and uncertainties in the analyses could affect the calculated source parameters. I find that the source spectral ratios from the coda and direct S waves are similar for the same pairs of earthquakes but that the source spectra and corresponding source parameters depend systematically on whether the earthquake of interest is the larger or smaller within the spectral ratio. Mayeda and Malagnini (2009a) calculated coda spectral ratios between the M6 main shock and six large aftershocks (M > 4), and performed a combined inversion to calculate source parameters. They found that the main shock had a higher stress drop than the large aftershocks. I model the identical spectral ratios individually in the manner that I use for direct waves. I find that the choice of source model and fitting procedure produces significant random scatter but no systematic biases. I calculate direct wave ratios for the same earthquake pairs and find higher variability but no systematic difference in the results. Lastly, I use M2.8-3.2 aftershocks as EGFs for the large aftershocks. These spectral ratios yield significantly higher corner frequencies and stress drops for the large aftershocks than when the same earthquakes are the denominator in ratios with the main shock. To improve the quality of EGF analyses, I propose strict objective criteria for acceptance of a spectral ratio fit: (1) observable source pulse, (2) the amplitude ratio of the low- to high-frequency limits of the fit is at least 5, and (3) the corner frequency only varies by 50% when the variance is within 5% of the minimum. ©2012. American Geophysical Union. All Rights Reserved.