Comparing seismological interpretations of different source types produced in a rate-and-state fault model

Natalie Schaal, Nadia Lapusta, & Yen-Yu Lin

Submitted August 14, 2019, SCEC Contribution #9573, 2019 SCEC Annual Meeting Poster #085

Seismological methods rely on a number of simplifying assumptions about the earthquake source to interpret observed spectra of seismic signals in terms of event durations and stress drops. Following Madariaga [1976] and similar studies, these assumptions typically include a circular shape of the source, axi-symmetric rupture propagation with a constant rupture speed, and spatially uniform stress decrease within the ruptured area. However, this combination of assumptions is not realizable in dynamic finite-fault models and, furthermore, more realistic sources likely involve additional heterogeneity.

In order to explore how physically plausible complexity in source behavior translates into trends in seismological spectral properties, we create nine earthquake sources using simulations of spontaneous earthquake sequences on rate-and-state faults. We then study the resulting dependence in the normalized corner frequency k on the off-fault orientation θ for far-field synthetic seismograms over a focal sphere of stations and compare our findings to the results of Madariaga [1976], which are widely used to interpret microseismicity spectra. In doing so, we perform spectral fitting to synthetic p- and s-waveforms following the standard approach of using a Brune-type “omega squared” (i.e. high-frequency fall-off rate n is set equal to 2) spectral function as well as allowing n to vary to produce the best fit. The nine sources span a variety of microseismicity types, including Madariaga-like [Madariaga, 1976], asperity-type [Schaal and Lapusta, 2019], ring-like [Lin and Lapusta, 2018], and repeating earthquakes [Lui and Lapusta, 2016].

Our results show that the variation of k with θ and spread in the values of k for a given theta (representing dependence on the rotation about the source) appear to be source-type dependent. In part, the average value of k for some sources is significantly different from that of the Madariaga-like source and, hence, using the latter for converting the corner frequency into the event duration and then size can result in significant errors. Furthermore, both the values and trends in k versus θ can significantly depend on whether n was set equal to 2 or allowed to vary. Best-fit values of n tend to be above 2 for asperity-type sources, and below 2 for the ring-like source. Our findings have implications for improving the analysis of seismological data to appropriately determine the source parameters and type.

Key Words
microseismicity, seismological methods, spectral fitting, source models, rate-and-state friction, numerical simulations

Schaal, N., Lapusta, N., & Lin, Y. (2019, 08). Comparing seismological interpretations of different source types produced in a rate-and-state fault model. Poster Presentation at 2019 SCEC Annual Meeting.

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