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Mapping Seismic Quiescence in California

GeoForschungsZentrum Potsdam
Telegrafenberg
14473 Potsdam, Germany

The decrease of microearthquake activity is a controversial possible precursor for mainshock prediction. The calculation of seismic quiescence is a convenient tool to quantify this decrease. However, the actual size of an anomaly significantly depends on the calculation parameters. The necessary averaging over space and time acts as 3D lowpass filter, the sliding statistics windows introduce highpass behavior, and the magnitude threshold shall ensure catalog completeness but also biases the data selection. To argue on any claimed anomaly, the only conclusive solution would be an exhaustive search over all parameters, over an area significantly larger than just the epicenter region, and against a test of the null hypothesis.

QMAP is a new mapping approach to process full catalogs, that is, some twenty years of data for areas of one million km², with one fixed set of parameters. It utilizes fractil statistics to avoid errors otherwise induced by clusters of aftershock series. QMAP features two different scaling modes, one to display absolute decrease and a second for weighted anomalies by erosion filtering. Applied to California, one can recognize quiescence anomalies of regional scale prior to the Coalinga and Loma Prieta earthquakes. But there are more and greater anomalies distributed all along the San Andreas fault system. Without further criteria to understand these patterns, the additional anomalies degrade seismic quiescence to an observation with high false alarm rate for earthquake prediction.