Seismicity Seismic monitoring systems detect and locate thousands of earthquakes globally every year, most of which go unnoticed by the public. The resulting earthquake catalogs prepared by universities, the USGS, the International Seismological Centre (ISC), and many international organizations are extensively used for seismic hazard analysis and basic research applications. The map on the left shows locations of 82,000 earthquakes along the San Andreas Fault system between 1984 and 2003, detected bya dense regional seismic network. The seismicity distribution reveals properties of faults and how these change in space and time. Many events occur where no faults are mapped at the surface, revealing hidden fault zones. New procedures for locating these events provide very precise relative locations, sometimes with uncertainties as small as a few meters, thus illuminating faulting process in unprecedented detail. For the region of central California shown in the map on the right, 12% of all recorded earthquakes occurred on faults that failed multiple times during the 19-year observation period. Repeating earthquakes tend to concentrate along faults that are largely creeping. This observation suggests that tiny asperities on these otherwise steadily sliding faults strengthen rapidly—within days to a few years—so that they can become re-stressed by the nearby ongoing slip. (Image modified from F. Waldhauser and D.P. Schaff, 2008. Large-scale relocation of two decades of Northern California seismicity using cross-correlation and double-difference methods, Journal of Geophysical Research, 113, B08311, doi:10.1029/2007JB005479.)


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