Monday, March 30, 2009

Earthquake 4.3 M - San Francisco Bay Area - Morgan Hill, CA

Final Update:
Tectonic Summary
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 (

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.

View Larger Map
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

Earthquake Location Map - San Francisco Bay Area

Earthquake Details

  • Monday, March 30, 2009 at 17:40:29 UTC
  • Monday, March 30, 2009 at 10:40:29 AM at epicenter
Location37.285°N, 121.620°W
Depth6.2 km (3.9 miles)
  • 18 km (11 miles) N (7°) from Morgan Hill, CA
  • 19 km (12 miles) E (91°) from Seven Trees, CA
  • 20 km (13 miles) ESE (117°) from Alum Rock, CA
  • 25 km (16 miles) ESE (104°) from San Jose City Hall, CA
Location Uncertaintyhorizontal +/- 0.1 km (0.1 miles); depth +/- 0.3 km (0.2 miles)
ParametersNST=250, Nph=250, Dmin=7 km, Rmss=0.08 sec, Gp= 58°,
M-type=local magnitude (ML), Version=2
Event IDnc40234037


  1. This comment has been removed by a blog administrator.

  2. I recently came accross your blog and have been reading along. I thought I would leave my first comment. I dont know what to say except that I have enjoyed reading. Nice blog. I will keep visiting this blog very often.



- Due to rampant abuse, we are no longer posting anonymous comments. Please use your real OpenID, Google, Yahoo, AIM, Twitter, Flickr name.

Twitter Buttons

****REMINDER**** Every fire has the ability to be catastrophic. The wildland fire management environment has profoundly changed. Growing numbers of communities, across the nation, are experiencing longer fire seasons; more frequent, bigger, and more severe, fires are a real threat. Be careful with all campfires and equipment.

"I am a firm believer in the people. If given the truth, they can be depended upon to meet any national crisis. The great point is to bring them the real facts, and beer." --Abraham Lincoln

View blog top tags

Subscribe via email to California Fire News - Keep track of Cal Fire News

Enter your email address:

Delivered by FeedBurner