Earthquake Pulsing on the San Andreas Fault

09 January 2004

UC Berkeley seismologist Robert Nadeau reports on the observation of aseismic pulsing along a 175 km segment of the San Andreas fault in the current issue of Science Magazine. Along with co-author Tom McEvilly, Dr. Nadeau reports on 16 years of data along a section of the San Andreas Fault between Loma Prieta in the north, near the city of Santa Cruz, and Parkfield in the south.

In addition to aseismic pulsing caused by larger events such as the 1989 Loma Prieta and 1998 San Juan Bautista earthquakes, Nadeau and McEvilly found that pulsing occurs over much of the central San Andreas in repeating cycles. In the northern half of the study region, for example, they found that aseismic pulsing occurs in 3 year cycles. They also found that the rate of small to moderate sized earthquakes during the first year of these cycles is 6 to 7 times greater than during the remaining 2 years. The Loma Prieta earthquake and the three largest earthquakes in the northern half of the study zone (magnitudes 4.7 to 5.4) also occurred during the first year of the three year cycles.

The mechanism responsible for the pulsing is not yet known, but indications are that it involves processes occurring below the seismogenic zone (i.e. the brittle upper crust where earthquakes occur).

Berkeley Seismological Lab
215 McCone Hall, UC Berkeley
Berkeley CA 94720-4760
www@seismo.berkeley.edu