In fiscal year 2006-2007, more than 23,000 earthquakes were detected and located by the automatic systems in northern California. This compares with over 30,000 in 2005-2006 and 38,800 in 2004-2005. Many of the large number of events in 2004-2005 are aftershocks of the 2004 Parkfield earthquake. The number of events continues to remain high, because we now receive and process data from a network of seismometers in the Geysers, a region with a high level of small magnitude seismicity. Of the more than 23,000 events, over 208 had preliminary magnitudes of three or greater. Thirty one events had greater than 4. The largest event recorded by the system occurred on 26 February 2007 with 5.4. This earthquake, as well as the five events with the next largest magnitudes, occurred off the coast of northernmost California.
As described in the 2003-2004 Annual Report, the BSL staff are no longer reading BDSN records for local and regional earthquakes (as of March 2004). This decision was in part intended to reduce duplication of effort between Berkeley and Menlo Park.
The BSL continues to focus on the unique contributions that can be made from the broadband network. From July 2006 through June 2007, BSL analysts reviewed more than 40 earthquakes in northern California and adjoining areas of magnitude 3.2 and higher. Reviewed moment tensor solutions were obtained for 37 of these events (through 6/30/2007). Figure 3.34 and Table 3.19 display the locations of earthquakes in the BSL moment tensor catalog and their mechanisms.
Berkeley Seismological Laboratory
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