5.5 Aftershock Rocks Bishop, Chalfant Valley

Another strong aftershock of last week’s 6.1 Chalfant Valley earthquake, measuring 5.5 on the Richter scale, struck Bishop early Thursday, breaking a plate-glass window in a Safeway store and forcing closure for the day of a National Weather Service office after several lighting fixtures fell.

Meanwhile, another mild tremor, the second in three days, was felt in the Culver City area. The noon quake measured only 2.8 and did no damage.

The month of July saw the most seismic activity in California in the last three years, with seven earthquakes measuring more than 5.0. Five of these occurred in the Bishop area, one near Palm Springs and one off the San Diego County coast. Seismologists said the Palm Springs and San Diego quakes were unrelated to the ones near Bishop.

The U.S. Geological Survey, which had advised last week that more quakes in the 6.0 range were possible in the Bishop area, said that the latest aftershock there was centered southwest of the hamlet of Chalfant Valley and about nine miles north of Bishop. This was several miles closer to Bishop than the main quake July 21. Bishop, with a population of 3,550, is the largest population center in the Eastern Sierra region.

USGS seismologist David Hill said the number of strong aftershocks of the Bishop quake is “a little bit unusual.” For a 6.1 quake, one would usually expect one aftershock over 5.0, he said, but there have now been three. The main shock was preceded the previous day by a 5.5 foreshock.