This earthquake occurred 5 km WSW of Yountville, CA, near the Napa Valley and approximately 5 km west of the West Napa fault. Although the earthquake was initially thought to have occurred on the West Napa fault, it is now thought that that it is located on an unnamed fault. It is not unusual for earthquakes to occur on unnamed faults in California - for example, the 1983 Coalinga and 1994 Northridge events. A magnitude 3.5 earthquake occurred in the same area as Sunday's quake in 1990.

The recent event showed the right-lateral strike-slip motion, typical of events in central California. As of 12:00 09/09/2000 (PDT) there have been over 300 small aftershocks, ranging in magnitude from -0.4 to 2.4. Only 11 have been larger than magnitude 1.6.

This earthquake injured 25 people, including 2 people critically. 70 people sought shelter at Red Cross facilities. Damage estimates are rising and range from $10 to $50M. On Wed, Sept 6, Governor Davis declared a state of emergency in the Napa Valley. A request has been made for federal funds.

The level of damage experienced in the city of Napa is somewhat surprising for an earthquake of this size. Preliminary analysis of the earthquake's records show that the waves in the valley were amplified compared to those in the nearby hills, probably due to the soft, young sediments along the Napa River. This figure is courtesy of the USGS and shows the location of the earthquake and seismograms recorded at various locations in the vicinity. More information on how soil type affects shaking is available at Soil Type and Shaking Hazard in the San Francisco Bay Area.

The M5.2 event is the largest earthquake in this area since the 1969 Santa Rosa events. On October 1st of that year, a magnitude 5.6 earthquake struck at 9:57 PM near the city of Santa Rosa. Just 80 minutes later, an M5.7 event followed, creating additional damage. These earthquakes were associated with the Rodgers Creek fault. 15 people were reported injured by these events, and 74 buildings were damaged (21 required subsequent demolition) and damage was estimated at approximately $6M (in 1969 dollars). For more information on the 1969 earthquakes, see the publication The Santa Rosa Earthquakes of October 1969 by Cloud et al., made available by John Martin and Associates.

Historically, the North Bay has experienced larger earthquakes. In 1898, a M6.5 event struck near Mare Island and in 1892, a M6.5 event shook Vacaville. According to Tousson Toppozada of the California Division of Mines and Geology,

"A stronger historical earthquake occurred in this vicinity on 1891 October 11th. It severely damaged chimneys and brick buildings at Sonoma and Napa, and cracked plastering and brick work as far away as Petaluma and San Francisco. We estimated an epicenter between Sonoma and Napa, and M 5.9 for the 1891 earthquake [Toppozada and others, 1981, 2000]."

The Rodgers Creek is a significant seismic hazard in the North Bay. According to the 1999 report by the USGS, the Rodger Creek-Hayward Fault system has 32% odds for one or more magnitude 6.7 or greater earthquakes from 2000 to 2030. The Concord-Green Valley Fault system has a 6% probability in the same time period.