Artefacts of earthquake location errors and short-term incompleteness on seismicity clusters in southern California

Yehuda Ben-Zion, & Ilya Zaliapin

Published July 23, 2015, SCEC Contribution #6204

We document and quantify effects of two types of catalog uncertainties – earthquake location errors and short-term incompleteness – on results of statistical cluster analyses of seismicity in southern California. In the main part of the study we analyze 117,076 events with m ≥ 2 in southern California during 1981-2013 from the waveform-relocated catalog of Hauksson et al. [2013]. We present statistical evidence for three artifacts caused by the absolute and relative location errors: (1) Increased distance between offspring and parents. (2) Underestimated clustering, quantified by the number of offspring per event, the total number of clustered events, and some other statistics. (3) Overestimated background rates. We also find that short-term incompleteness leads to (4) Apparent magnitude dependence and temporal fluctuations of b-values. The reported artifacts are robustly observed in three additional catalogs of southern California: the relocated catalog of Richards-Dinger and Shearer [2000] during 1975-1998, and the two sub-catalogs – 1961-1981 and 1981-2013 – of the Advances National Seismic System. This implies that the reported artifacts are not specific to a particular (re)location method. The comparative quality of the four examined catalogs is reflected in the magnitude of the artifacts. The location errors in the examined catalogs mostly affect events with m<3.5, while for larger magnitudes the location error effects are negligible. This is explained by comparing the location error and rupture lengths of events and their parents. Finally, our analysis suggests that selected aggregated cluster statistics (e.g., proportion of singles) are less prone to location artifacts than individual statistics like the distance to parent or parent-offspring assignment. The results can inform a range of studies focused on small-magnitude seismicity patterns in the presence of catalog uncertainties.

Ben-Zion, Y., & Zaliapin, I. (2015). Artefacts of earthquake location errors and short-term incompleteness on seismicity clusters in southern California. Geophysical Journal International, 202(3), 1949-1968. doi: 10.1093/gji/ggv259.

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