The quake occurred on a north-south oriented fault about 3 km to the east of the of the Calaveras fault as defined by the last 40 years of earthquake epicenters.
The moment tensor and first-motion mechanisms confirm this fault orientation.
The fault has no name and is not mapped at the surface of the Earth.
The background seismicity defines a trend that extends for about 10km. The fault is similar in orientation and tectonics to the fault which was ruptured by the Mt. Lewis sequence in 1986 (M5.7). However, the Mt. Lewis sequence lasted for 491 days and contained 1930 aftershocks. In contrast, as of Mar 30 17:10:53 PDT 2009 there were only 4 located aftershocks with the largest having a magnitude of 1.2. Even though this quake is on a similar structure, it is not exhibiting the similar behavior. The fault terminates to the south on the Calaveras fault in the middle of the 1984 M6.2 Morgan Hill rupture zone. The penultimate central Calaveras earthquake was in 1911, 73 earlier. It has been about 25 years since 1984, so the likelihood that the Calaveras fault has accumulated sufficient stress to sustain another M6 quake is quite low. The 30-year probability for a M>6.7 earthquake on the central section of the Calaveras fault is only a few percent (http://pubs.usgs.gov/of/2003/of03-214/WG02_OFR-03-214_Chapter6.pdf).
It is therefore unlikely that this M4.3 earthquake will trigger a significant earthquake on this section of the Calaveras fault.
Source: US Geological Survey, Menlo Park CA - Link
Update: 11:38:15 - 1.2 aftershock
Update: BART ten minute delay - BART is performing a system-wide check of all trains.
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Reported epicenter map: Google Map link - Shows a location just to the Northwest of Henry Coe State Park, A few miles South of Lick Observatory on Mt. Hamilton.
4.7 M Earthquake in Alaska 2 minutes before California Quake
at 17:38:13 UTC - Link
Update: 1111hrs - Originally rated at 4.4 M it was quickly upgraded to 4.6 M, then just as quickly downgraded to 4.3 M.
Earthquake was felt in a large area of Northern California centered in the the San Francisco Bay Area near Morgan Hill, the quake was felt over a wide distance with reports from San Francisco to Monterey, From Tracy to Salinas.
Did you feel it? Let the USGS know about it! - Link
|Depth||6.2 km (3.9 miles)|
|Location Uncertainty||horizontal +/- 0.1 km (0.1 miles); depth +/- 0.3 km (0.2 miles)|
|Parameters||NST=250, Nph=250, Dmin=7 km, Rmss=0.08 sec, Gp= 58°,|
M-type=local magnitude (ML), Version=2