3.9 Earthquake Hits South Bay

BAY AREA — An earthquake struck the Calaveras Fault in the South Bay at 9:39 Monday morning. Preliminary reports from the USGS pin the magnitude at 3.9. It was centered in the Alum Rock section of San Jose. More information about the Calaveras Fault is below.

There have been no reports of damage or injuries.

People took to social media to report feeling the temblor, especially in downtown San Jose high rise buildings.

Less than a half-hour after the earthquake hit, the USGS had received more than 800 responses on its "Did You Feel It" response form. People reported feeling it in San Francisco, inland to the Tri-Valley, and south to the Monterey Bay area.

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The earthquake came immediately after a rare thunderstorm moved through the area with lightning, thunder, hail, rain and gusty winds.

The Calaveras Fault connects to the Hayward Fault in the East Bay and is the least active part of the fault. "Historically, the southern half of the central segment of the Calaveras Fault has been the most seismically active segment of the fault," the USGS reports. "It produced the M6.2 Morgan Hill earthquake in 1984 and a M6.2 earthquake in 1911. The M5.9 Coyote Lake earthquake in 1979 ruptured slightly to the south of these other earthquakes." The Calaveras Fault is also responsible for the 2007 M5.4 Alum Rock earthquake.

-Image via USGS