Source parameters of the 15 November 1995 Border Town, Nevada, earthquake sequence

Abstract

AbstractThe November 1995 Border Town, Nevada, earthquake sequence occurred near the California-Nevada border approximately 20 km northwest of Reno, Nevada. The largest earthquake of the sequence (Mw = 4.5) was widely felt throughout the Reno-Sparks-Truckee region. This event occurred on a west-dipping high-angle fault at a depth of 14 km and shows dip-slip motion on a preferred fault-plane orientation of strike N10°E, dip 70° W, and rake −75°. We have relocated 27 aftershocks and one foreshock of the event using records from the local network and two portable digital instruments. The largest aftershocks also align along a N10°E trend and define the preferred fault plane. All of the aftershocks occur within a small volume with a 2- by 2-km horizontal extent and between depths of 10 and 14 km.Simultaneous determination of M0, fc, and κ is made by fitting spectra using various starting models (i.e., initial Δσs and κ) based on an f−2 spectral shape. M0 ranges from 1018 to 1023 dyne × cm for 14 events and hypocentral distances were less than 15 km. We fit S-wave spectra with different starting models to test the stability of Δσs. The results show that Δσs and κ converged to different values depending on the starting model and the magnitude because of an attenuation-source dimension trade-off. For example, a model starting with a stress drop of 10 bars produced an average Δσs of 27 bars and an apparent scaling breakdown below Mw 3. Starting the search at 50 bars produced an average Δσs of 69 bars and no magnitude dependence. We cannot resolve any breakdown in self-similarity because we find plausible spectral models for all magnitudes where Δσs is nearly constant at 60 bars.Seismograms of the four largest aftershocks, Mw 3.0 to 3.4, were deconvolved using smaller aftershocks as empirical Green's functions. Event radii estimated from the pulse widths yield stress drop averaging 34 bars, within the range of the frequency-domain results.