Northern California Seismicity Report for the week ending May 16, 2002

This report provides a brief summary of earthquake activity recorded by the U.S. Geological Survey Northern California Seismic Network in Menlo Park and the University of California Berkeley Seismological Laboratory. It is automatically generated each Wednesday shortly after midnight, Universal time (UT). UT (ie., Greenwich Mean Time) is 8 hours ahead of PST or 7 hours ahead of PDT.

Because it is not possible to review all of the earthquakes before this report is generated, it may contain errors. Please report errors or provide suggestions and comments to ncsn@andreas.wr.usgs.gov

The report no longer includes maps showing earthquakes for the entire US or world. Those maps are now available from the U.S.G.S. National Earthquake Information Center. In addition, all earthquake data can be retrieved from the Northern California Earthquake Data Center.

Please note that some of the earthquake data in this report have been reviewed by seismic analysts. Consequently, the parameters that describe the hypocenter, magnitude, origin time, and solution statistics may differ slightly from the information contained on the real-time earthquake web page. The differences reflect revised arrival times at seismic stations and the analysts' decisions to include or exclude information from seismic stations.

The minimum magnitude ( M) values in the following bullets reflect the smallest earthquake which the network can routinely record within the region. During the previous seven-day period ending at midnight (UT) Wednesday, our seismic networks recorded

• 21 M >2.0 earthquakes in Northern and Central California (map) (within dashed polygon of figure).

The largest event to occur this week in this region was a moderate earthquake of magnitude 4.9, 3 miles SW of Gilroy, CA at 05:00 AM UTC Tuesday, 05 14, 2002 (10:00 PM PDT Monday, May 13, 2002).

The largest earthquake of the week was a Mw 4.9 event near Gilroy that was felt throughout the Bay Area. Details, maps and photographs can be found at http://earthquake.usgs.gov/eqinthenews/nc40133364/ The May 13, 2002 Gilroy earthquake occurred near the south end of the Sargent fault and was followed by several small aftershocks located nearby. This moment magnitude M 4.9 earthquake was not unusual, but it ended a post-1989 Loma Prieta earthquake period of relative seismic quiescence in the south San Francisco Bay region. The energy released in an M4.9 event is about 1/1000 that of the M6.9 Loma Prieta event. The M 5.3 Pacheco Pass earthquake near San Juan Bautista in 1998 was the only nearby M> 5.0 shock since 1990. The Gilroy earthquake occurred at normal mid-crustal depth and had a typical strike-slip faulting mechanism. The region where the earthquake occurred has been seismically active at the magnitude 5 level at least since detailed recording began about 50 years ago. The epicenters of the earthquake and its aftershocks are located near the Castro fault, a minor mapped fault which parallels the Sargent fault. The mapped length of the Castro fault is about 15 km and the maximum magnitude earthquake on a fault of that length is about 6.2. The largest known event that might have occurred on the Castro fault is the magnitude 5.3 earthquake that occurred on March 2, 1959. The Gilroy earthquake was reported as felt from near San Miguel on the south to Santa Rosa on the north but did little damage. Magnitude 5 earthquakes in the San Francisco Bay region are typically felt throughout the region by people in tall buildings and at sites susceptible to ground amplification. In comparison, ground motion recordings in the epicentral region by newly-installed instruments in the ANSS network are about 20% as strong as those recorded in the epicentral region of the 2000 magnitude 5.2 Yountville earthquake. The strong-motion coverage of the Gilroy earthquake was doubled by the records from ANSS instruments installed during the past three years. This coverage was especially improved in the Silicon Valley area. ANSS funding has been limited to date; implementation of full ANSS funding will provide more than a five-fold increase in the number of new instruments that would have recorded this earthquake. Fulfilling the promise of ANSS is a high priority for the USGS. Dense urban recording of earthquakes provides a detailed description of shaking severity, enabling rapid identification of locales of potential damage to structures toward which emergency response efforts can be directed to saving lives. After well-recorded earthquakes, analysis of the dense data sets leads to more accurate inputs to seismic design codes and practices.

• 21 M >1.2 earthquakes in the San Francisco Bay area (map).

The largest event to occur this week in this region was a moderate earthquake of magnitude 4.9, 3 miles SW of Gilroy, CA at 05:00 AM UTC Tuesday, 05 14, 2002 (10:00 PM PDT Monday, May 13, 2002).

• 10 M >1.2 earthquakes in the Long Valley region (map).

The largest event to occur this week in this region was a microearthquake of magnitude 2.5, 9 miles SE of Mammoth Lakes, CA at 04:07 AM UTC Tuesday, 05 14, 2002 (09:07 PM PDT Monday, May 13, 2002).

The following table provides a listing of all M >2 earthquakes that occurred this week. An explanation of the entries in the table is available.

---ORIGIN TIME (UT)--- -LAT N-- --LON W-- DEPTH  N RMS ERH ERZ LOC    PREF AZ MIN S. PAGE   1
YEAR MON DA HRMN  SEC  DEG MIN  DEG  MIN    KM  RD SEC  KM  KM REMKS  MAG  GAP DS PF

2002 MAY  9 0556 35.76 40 18.87 124 28.08  7.26 23 .05  .5  .4 MEN    3.2D 232 15 NN
2002 MAY  9 1107 55.84 38 47.86 122 43.72  3.95 43 .10  .1  .4 GEY    3.6L  30  3 NN
2002 MAY 10 0343 19.11 38 48.00 122 48.09  1.07 28 .04  .1  .2 GEY    2.1D  35  1 NN
2002 MAY 10 0539 30.76 38 47.99 122 47.87  1.08 23 .03  .1  .2 GEY    2.0D  38  1 NN
2002 MAY 10 2345 11.58 38 46.25 122 43.41  2.15 22 .05  .2  .3 GEY    2.1D  43  1 NN
 
2002 MAY 11 0042 54.70 36 28.45 120 53.17 11.19 17 .08  .4 1.3 CRV    2.7D 130 13 WW
2002 MAY 11 0042 57.70 36 28.95 120 51.90 15.81 54 .12  .2  .4 CRV    2.5D 148 13 NN
2002 MAY 12 0557 38.83 37 35.16 118 48.20  5.79 30 .07  .2  .4 MOR    2.4D  90  2 NN
2002 MAY 12 1033 31.84 36 34.94 121  3.85  5.21 48 .13  .2  .4 BVL    2.2D  41  2 NN
2002 MAY 13 1734 28.68 40 38.02 121 58.51 12.92  8 .06  .6  .7 SHA    2.1D 167  2 WW
 
2002 MAY 14 0407 32.07 37 34.10 118 50.81 10.33 33 .08  .3  .5 MOR    2.5D 102  3 NN
2002 MAY 14 0500 29.49 36 58.03 121 35.99  7.63 87 .17  .2  .5 SAR    4.9W  30  2 WW
2002 MAY 14 0505 35.48 36 58.34 121 36.68  7.85 46 .12  .2  .7 SAR    2.5D  33  8 WW
2002 MAY 14 0540 26.49 36 57.90 121 36.15  6.60 65 .13  .2  .4 SAR    2.4D  31  2 WW
2002 MAY 14 0552 19.73 36 58.49 121 37.06  6.60 77 .14  .2  .5 SAR    2.8D  35  4 WW
 
2002 MAY 14 0620 35.87 36 57.87 121 35.73  6.78 82 .16  .2  .5 SAR    3.2L  29  1 WW
2002 MAY 14 2132  5.39 38 48.61 122 47.92  2.22 39 .07  .1  .8 GEY    3.5L  37  7 WW
2002 MAY 14 2248 29.08 38 48.55 122 48.46  2.71 38 .06  .1  .4 GEY    3.2L  34  1 WW
2002 MAY 14 2253  3.91 38 48.19 122 48.74  1.90 16 .03  .2  .4 GEY    2.0D  38  2 WW
2002 MAY 15 1422 29.31 38 49.84 122 48.14  2.15 20 .03  .2  .4 GEY    2.1D  39  3 WW
 
2002 MAY 15 1807 22.30 35 30.28 121 13.33  1.06 15 .05  .6 2.0 SSM    2.5D 194 15 WW