Here is a detailed map of historical earthquakes in Monterey County from the CAL-EMA, the California Emergency Management Agency.
San Ardo oil field area has, historically, been the epicenter of the strongest earthquakes in the Salinas Valley -- in the 5+ magnitude range. This could be due to the Los Lobos fault nearby. But it might also be due to the 80 wastewater injection wells found in the San Ardo oil field. USGS scientists have linked wastewater injection wells to earthquakes in Oklahoma, Texas, and Ohio. (http://earthquake.usgs.gov/research/induced).
There's a short Los Lobos fault that runs into San Ardo area. To the east of San Ardo is the San Andreas fault, which roughly parallels Monterey County's eastern border with San Benito and Fresno counties. To the west of San Ardo is the Reitz fault which runs length of Salinas Valley.
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