Quake swarm in California



A series of four earthquakes shook an area centered 24 km north of Bishop, Calif., on July 20–21, 1986. The earthquake swarm prompted the director of the U.S. Geological Survey (USGS) to send a letter to the California Office of Emergency Services, advising state officials of increased risk that additional earthquakes of the same or greater magnitude could soon follow. The earthquakes near Bishop had no apparent connection to two quakes in other parts of the state earlier in the month. As Eos went to press, the seismic activity near Bishop was tapering off, “as it does in a normal aftershock sequence,” according to David P. Hill, a geophysicist with USGS in Menlo Park, Calif.