6.0 Napa Earthquake

At 3:21 am, Sunday morning an earthquake struck about six miles south of Napa and lasted 10 to 20 seconds depending on proximity to the epicenter. According to the U.S. Geological Survey (USGS) the earthquake occurred near the north shore of San Pablo Bay and measured 6.0 on the Richter scale. The bayshore areas in the San Francisco Bay region are underlain by landfill and bay mud and have experienced disproportionately greater damage during historic earthquakes. Such damage is caused by soil failure in the fills and amplification of ground shaking by the soft bay mud and magnified the damage of this moderate earthquake. (The Richter scale is logarithmic.) The earthquake occurred near the north shore of San Pablo Bay an area underlain by landfill and bay mud and have experienced disproportionately greater damage during historic earthquakes. Such damage is caused by liquefaction, soil failure in the fills and amplification of ground shaking by the soft bay mud.

This was the largest earthquake recorded in the San Francisco Bay Area since the magnitude-6.9 Loma Prieta quake struck in 1989,collapsing part of the Bay Bridge roadway and killing more than 60 peopleAccording to Richard Allen, director of the University of California, Berkeley Seismological Laboratory, an earthquake early warning system currently being tested issued a 10-second warning before the quake struck. Though in 10 seconds I would not have been able to even wake up..

According to a study done in 1999 the Hayward-Rodgers Creek Fault system has a 32 % probability of generating a large earthquake (magniture 6.7 to 7.4) by the year 2030, and the Concord-Green Valley Fault system has a 6 % chance of generating a large earthquake (magnitide ≥6.7) in the same time period.

Earthquakes are know to impact groundwater wells. The most common ground-water response is an instantaneous water-level fall or rise. This response can both near and far from the epicenter of the quake without significant change to the rock formation. Recovery to the pre-earthquake water level can be so rapid as to be almost unnoticeable, or it may take as long as several days or months. Water level changes can be large enough to have the well flow to the land surface, or render a well dry.

Well water can also become cloudy or take on a different color, smell and feel. The water can become contaminated with dirt, minerals and other solids, as well as bacteria due to damage to the casing and grouting. To see if your well has been impacted, you will have to empty your pressure tank and see what pumps out of the well. Turbidity could move through the system and pass in a short period or not depending on the specific geology, soil type and hydro geology. For more information see "Your Water Well After the Earthquake."