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Differential analysis of coda Q using similar microearthquakes in seismic gaps. Part 1: Techniques and application to seismograms recorded in the Anza Seismic gap

Department of Earth and Environmental Sciences and Geophysical Research Center New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801
School of Earth and Ocean Sciences University of Victoria, Victoria, B.C. Canada, V8W 2Y2
Seismographic Station University of California, Berkeley, California 94720

Abstract

A large but conflicting literature exists that suggests that the scattering properties of the Earth in seismogenic areas may be temporally variable and that this variability could be correlated with the nucleation times of moderate to large earthquakes. To search for temporal variations in coda Q in the vicinity of an expected moderate earthquake nucleation region along the San Jacinto fault zone, pairs and clusters of earthquakes recorded by the Anza Seismic Network between 1982 and 1992 are examined. To minimize differences arising from path and source variability, only the most similar examples of microearthquake wave fields are intercompared. To assess coda Q differences between similar microearthquake signals, we search for systematic temporal changes in moving-window spectral amplitude ratios. For a single-scattering coda model with an envelope decay function of the form t^–me^–ft/Qc, the relationship between the natural log amplitude spectral ratio and Q^–1 = Q^–1_c2 – Q^–1_c1 is linear; i.e., ln r(f, t) = ftQ^–1(f) + W(f), where W(f) is a time-independent relative source term. To obtain robust uncertainty estimates of Q^–1(f) and W(f), we utilize multi-taper spectral analysis coupled with a nonparametric Monte Carlo confidence interval estimation procedure. For the most similar events in the Anza region (coda cross-correlation values of approximately 0.7 for 16 sec of coda), we find that coda Q as a function of time is stable at a typical 1 resolution of only between –25% and +50% of the reference coda Q value for the area, even in the best-constrained frequency bands. Comparison of similar microearthquake coda signals with both short- and long-temporal separations indicates that all of this variability can be reasonably attributed to random fluctuations in the coda that are driven by source variability and is not indicative of any systematic temporal variability in coda Q. To obtain estimates of relative coda Q that are more precise than this level, the spectral ratio analysis technique requires significantly more similar earthquakes than are produced by seismogenic processes in the Anza region. Such earthquakes have been observed to occur in other regions of the San Andreas Fault system and are analyzed in Antolik et al. (1996).

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