Ambient Noise and Fault Zone Healing
Ambient Noise and Fault Zone Healing Cross-correlating long records of microseismic noise between two stations can determine properties of the intervening medium, providing a powerful probe of structure, and when performed over time, temporal changes in the structure. An exciting application of this approach is to monitor temporal changes in velocity structure in and around fault zones, especially when a large earthquake occurs. The figure shows changes in the medium along the San Andreas Fault, near Parkfield, California, with velocity reductions correlating with the San Simeon earthquake (off the fault) and the Parkfield earthquake (on the fault). The gradual increase in along-fault seismic velocity following the earthquakes suggests that the fault is damaged by the shear strain and seismic waves from the earthquake, and the medium recovers (heals) over time. The red line shows an empirical correspondence with Global Positioning System (GPS) measured displacement along the fault; the filled-in black lower plot is the amount of nonvolcanic tremor (low-frequency vibrations from nearly continuous fault sliding) occurring over time as measured by local seismometers. The correspondence of the along-fault velocity structure, surface displacement, and nonvolcanic tremor suggests stress relaxation in the part of the fault zone at greater depth than the coseismic rupture. (Image from M. Brenguier, M. Campillo, C. Hadziioannou, N.M. Shapiro, R.M. Nadeau, and E. Larose, 2008. Postseismic relaxation along the San Andreas Fault at Parkfield from continuous seismological observations, Science, 321(5895):1478– 1481, doi:10.1126/science.1160943. Reprinted with permission from AAAS.)