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The Effect of Uncertainties on Estimates of Background Seismicity Rate

Thomas van Stiphout, Danijel Schorlemmer, & Stefan Wiemer

Published April 1, 2011, SCEC Contribution #1279

The analysis of seismicity rate changes (SRC) is very common in seismology because transients in activity rates can be related to changes in physical properties in the Earth's crust as well as to the recording homogeneity of seismic networks. In this study we propose an improved approach to estimate the significance of SRC. Using extensive Monte Carlo simulations over the range of free parameters, our approach is able to 1) include effects of epistemic and aleatory uncertainties on SRC and 2) translate the values of different SRC estimators (e.g. $- and $\beta$-values) and parameter settings (e.g. sample size or window length) into parameter-independent significances. As target region of our study we chose southern California, however, the method and results are universally applicable. We analyze both the real earthquake catalog and synthetic ones that are modeled after the real seismicity but free of earthquake clustering. We perform a sensitivity study of the various uncertainties that influence SRC. As the most important source of uncertainty we identify the selection of the seismicity declustering approach and its respective parameter settings. Seismicity declustering is commonly applied to separate dependent events (fore- and aftershocks, swarms) from independent events (background). The high sensitivity is a direct consequence of the fact that there is no unique or universerally applied approach to separating these two classes of events, but the two classes are rather statistically defined through the declustering procedure. The uncertainty in hypocenter location also influences the significance of SRC; its relevance depends on the ratio between the hypocenter uncertainties to the size of the anomaly. Uncertainties in magnitudes and declustering edge effect are found to be less relevant in this study, however, they should be considered in SRC studies. As an example application, we investigate the influence of uncertainties on estimating the significance of precursory rate changes reported in the literature prior to the 1992 Landers and Big Bear mainshocks. Our analysis shows that the significance of the precursory changes decreases significantly when uncertainties are correctly accounted for.

van Stiphout, T., Schorlemmer, D., & Wiemer, S. (2011). The Effect of Uncertainties on Estimates of Background Seismicity Rate. Bulletin of the Seismological Society of America, 101(2), 482-494. doi: 10.1785/0120090143.