The quake was centered 13 miles south of San Francisco, about 5 miles west of the San Francisco International Airport, and 2 miles southeast of Pacific along the western slope of Sweeney Ridge just west of San Andreas Lake. (See a TopoZone location map or a MapQuest location map). The exact location is in the upper portion of the North Fork of San Padro Valley along the northwestern boundary to the San Padro Valley County Park.

The focal depth was at 9.3 km (5.8 miles) and a preliminary fault plane solution indicated thrust style motion, suggesting the quake did not occur along the San Andreas Fault because it shows predominently strike-slip-type mothion. It probably occurred along some nearby splay or fracture. Interestingly, this is the same style of movement that occurred with the August 17, 1999 Bolinas M5.0 earthquake.

This was the first M3.0+ event between Half Moon Bay and the Golden Gate Bridge in four years. See a list of M3s since 1970.

Felt Reports
The quake was widely felt on the northern San Francisco Peninsula but was too light to cause any damage or injuries and there were no reports of items toppling from tables or shelves. Did you feel this quake? Some people in Alameda, Contra Costa and Marin County also felt the quake.

Background Info
This portion of the San Andreas has shown an elevated level of seismicity during the past month and this is the largest event in the series thus far. During the last four years, activity has topped the 8 eq/mth mark four times: once in March 1997 (9 events), in July 1999, in June and July 2000 (each with 10 events), and most recently, in August 2001 with 15 events - the highest tally in the four year period. (See monthly activity graph from 1997 - present.)

Additional seismic information links:

• Weekly Seismo-Watch Central California Earthquake Report (Follow the weekly activity and see past reports)
• Seismo-Watch - links for more Northern California information
• Seismo-Watch Links to California Activity
• NCSN Recent Earthquakes Page.
• USGS Community Intensity Report

Mainstream media news reports:

• None at this time