Two Small Quakes Strike Off San Francisco Coast

Two small earthquakes hit just offshore of San Francisco on Saturday afternoon, minutes apart near the San Francisco Zoo. (usgs.gov 1) (usgs.gov 2) The U.S. Geological Survey recorded the first at magnitude 2.9 at 4:39 p.m. local time, 3 kilometers south-southwest of the zoo at a depth of 7.0 kilometers. (usgs.gov) A second quake, magnitude 2.7, followed at 4:41 p.m. in nearly the same spot at a depth of 6.9 kilometers. (usgs.gov) USGS “Did You Feel It?” reports showed weak shaking from the larger quake and lighter reports from the second, with responses coming from San Francisco and San Mateo County. (usgs.gov) By Sunday, ABC7 reported the first quake had been downgraded from a preliminary 3.0 to 2.9, and CBS reported no damage had been reported. (abc7news.com) (cbsnews.com) Earthquake magnitude measures energy release, while shaking intensity measures what people actually feel on the ground. A quake this small can be noticeable near the epicenter without causing structural damage. (usgs.gov 1) (usgs.gov 2) California’s public early-warning tools are aimed at stronger events, not quakes this size. The state says MyShake, Android Earthquake Alerts and Wireless Emergency Alerts are designed to give people seconds of notice before significant shaking arrives. (earthquake.ca.gov) (shakealert.org) The MyShake app listing says its ShakeAlert-powered warnings generally cover earthquakes of magnitude 4.5 or greater in California, Oregon and Washington. (myshake.berkeley.edu) (apps.apple.com) For this weekend’s pair, the main public record is the felt reports: nearly 1,000 responses for the 2.9 quake and dozens for the 2.7. The tremors were minor, but they were close enough to the coast to get people’s attention. (usgs.gov)