Spectral characteristics and stress drop for microearthquakes near Fairview Peak, Nevada

Abstract

Frequency and amplitude characteristics were studoeed for microearthquakes recorded on a local network of vertical-component seismographs in the Fairview Peak area of central Nevada. Maximum trace amplitude o.f the events decreased with focal distance approximately as R -•'9. Site amplification, by a factor of 2, was observed for the only station not located on rock. Comparison of observed spectra for these events with theoretical spectra given by Brune indicates stress drops ranging from 0.04 to 0.60 bar. Calculated source dimensions and seismic moments for microearthquakes in this area are in agreement with results of earlier studies.When location of the events is considered, the variation in stress drop appears to be related to differences in the faulting mechanisms.In a recent study Brune [1970] used an earthquake dislocation model to derive nearand far-field theoretical spectra for seismic shear waves and suggested that comparisons of observed and theoretical spectra could be used to estimate the effective stress, stress drop, and source dimensions of earthquakes. In this paper we consider amplitudes and spectral characteristics of microearthquakes recorded in the Fairview Peak area of central Nevada and use the method outlined by Brune to estimate various source parameters of these events. Results of this study indicate that effective stresses vary within the Fairview fault zone, and this variation may be related to differences in focal mechanism for events in different parts of the zone. Previous investigations of microearthquakedistribution and focal mechanisms in the Fairview Peak area have been published by Stauder and Ryall [1967] and Ryall and Malone [1971], and average spectral characteristics of events in the same area were described by Douglas and Ryall [1970]. A discussion of previous theoretical work on elastic wave radiation for seismic dislocations, as well as the various assumptions Copyright •¸ 1972 by the American Geophysical Union. in his own derivation, are given by Brune and will not be repeated here except as they bear on the observations and analysis of this study. The data for this investigation were obtained in early 1970 near Fairview Peak. Eight earthquakes were included in the analysis; the locations of these events are shown in Figure 1 with the recording sites, recent fault breaks, and epicenters of small earthquakes recorded by stations of a regional network during late 1969 and 1970. Records used for amplitude and spectral analysis were obtained on portable tape-recording field seismographs designed by Hunt [1969]. This system uses an Electrotech EV-17 moving-coil seismometer with a free period of 1.0 sec and has the response characteristics shown in Figure 2. ANALYSIS AND RESULTS Focal coordinates for the eight earthquakes of this study were calculated by using times of P and S waves from four to six of the field stations in a computer location routine developed by Malone [1969]. In the process of focal determination, site corrections were obtained by a comparison of near-reg...

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