Tremor activity changes following two San Andreas Fault earthquakes

Large earthquakes change the stress on faults. This can either accelerate or decelerate deep fault creep, as suggested by changes in tremor activity rates. Analyzing tremor data from the San Andreas Fault in central California, Shelly and Johnson found that the 2003 San Simeon earthquake, with a magnitude of 6.5, created a “stress shadow” north of Parkfield, causing tremors to stop abruptly there for about a month, while tremors south of Parkfield continued. On the other hand, tremors increased sharply following the 2004 magnitude 6.0 Parkfield earthquake. The authors examined the spatial and temporal variability of postearthquake effects on the San Andreas Fault and noted a depth dependence of tremor recurrence patterns. The study provides new information on the properties of these sections of the fault as well as new insight into how earthquakes affect stress on a fault and deep fault motion. ( Geophysical Research Letters , doi:10.1029/2011GL047863, 2011)