Nonvolcanic Tremors Deep Beneath the San Andreas Fault

Abstract

We have discovered nonvolcanic tremor activity (i.e., long-duration seismic signals with no clear P or S waves) within a transform plate boundary zone along the San Andreas Fault near Cholame, California, the inferred epicentral region of the 1857 Fort Tejon earthquake (moment magnitude ~7.8). The tremors occur between 20 to 40 kilometers' depth, below the seismogenic zone (the upper ~15 kilometers of Earth's crust where earthquakes occur), and their activity rates may correlate with variations in local earthquake activity.