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NCSS event review threshold change

The Northern California Seismic System (NCSS), operated by the US Geological Survey in Menlo Park and UC Berkeley Seismological Laboratory, locates ~20,000 earthquakes per year in the Geysers geothermal region (Figure 1).

Since 2007/07/23, when data from the Calpine stations were integrated into Northern California Seismic System real-time operations, the magnitude (M) threshold for analyst review has been M1.0, or about 4000 earthquakes per year. Automatically determined, unreviewed locations below this magnitude threshold have been available the catalog. In reality, the number of events that have required review is higher because subnet triggers (detections without a location) require analyst review to determine a location and magnitude.

To accommodate reduced staffing levels, we are increasing the review threshold to M1.2 for earthquakes above 7km depth, which should reduce the number to be reviewed by ~1500 earthquakes per year. We anticipate that this change in policy will have little impact on the quality of the catalog because the magnitude detection threshold in the Geysers has been ~M1.2 since 1975 and the accuracy of the automatically located earthquakes since 2007 is quite high due to the large number of stations in the region contributed from the BG (LBNL Geysers) Network.

The coordinates of the exclusion area (Figure 1) are

polygon=38.83,-122.9,38.85,-122.9 38.9,-122.85,38.9,-122.78 38.78,-122.67,38.73,-122.67 38.73,-122.75,38.83,-122.9

Map of 2012 Geysers seismicity
Figure 1. Seismicity in 2012 above magnitude 1.0 (orange dots), and location Geysers exclusion polygon.