Searching for hidden earthquakes in Southern California.

Earthquakes follow a well-known power-law size relation, with smaller events occurring much more often than larger events. Earthquake catalogs are thus dominated by small earthquakes, yet still missing a much larger number of even smaller events caused by signal fidelity issues. To overcome these limitations, we applied a template matching detection technique to the entire waveform archive of the regional seismic network in southern California. This effort resulted in a catalog with 1.81 million earthquakes, a factor of 10 increase, which provides important new insights into the geometry of fault zones at depth, foreshock behavior and nucleation processes, and earthquake triggering mechanisms. The extraordinary detail resolved in this type of catalog will facilitate a new generation of analyses of earthquakes and faults.