Implications of diverse fault orientations imaged in relocated aftershocks of the Mount Lewis, ML 5.7, California, earthquake

We use seismic waveform cross correlation to determine the relative positions of 2747 microearthquakes near Mount Lewis, California, that have waveforms recorded from 1984 to 1999. These earthquakes include the aftershock sequence of the 1986 M L 5.7 Mount Lewis earthquake. Approximately 90% of these aftershocks are located beyond the tips of the approximately north striking main shock, defining an hourglass with the long axis aligned approximately with the main shock. Surprisingly, our relocation demonstrates that many of these aftershocks illuminate a series of near‐vertical east‐west faults that are ∼0.5–1 km long and separated by as little as ∼200 m. We propose that these structures result from the growth of a relatively young fault in which displacement across a right‐lateral approximately north striking fault zone is accommodated by slip on secondary left‐lateral approximately east striking faults. We derive the main shock‐induced static Coulomb failure function (Δσ f ) on the dominant fault orientation in our study area using a three‐dimensional (3‐D) boundary element program. To bound viable friction coefficients, we measure the correlation between the rank ordering of relative amplitudes of Δσ f and seismicity rate change. We find that likely friction coefficients are 0.2–0.6 and that the assumed main shock geometry introduces the largest uncertainties in the favored friction values. We obtain similar results from a visual correlation of calculated Δσ f contours with the distribution of aftershocks. Viable rate‐and‐state constitutive parameters bound the observed relationship between magnitude of Δσ f and seismicity rate change, and for our favored main shock model a maximum correlation is achieved when Δσ f is computed with friction coefficients of 0.3–0.6. These values are below those previously cited for young faults.