Californians woke early Friday to a 5.4 magnitude aftershock, renewing fears one day after Southern California experienced its strongest earthquake in two decades.
Susan Hough, a seismologist with the U.S. Geological Survey, said the 4:07 a.m. tremor, 9 miles northeast of the Mojave Desert city of Ridgecrest near where Thursday’s earthquake took place, would likely be the strongest aftershock, although more powerful ones could follow. There have been about 200 aftershocks since the 6.4 magnitude earthquake Thursday morning, which led to about two dozen fire and emergency medical calls but no serious injuries.
Thursday’s earthquake punctured a period of relative seismic calm in California, but scientists said it did not change their calculations of when the “Big One” might strike.
Although the faults around Ridgecrest are part of the larger San Andreas system that runs from the Gulf of California to Mendocino, north of San Francisco, Thursday’s earthquake did not relieve the stress on the San Andreas, said Paul Caruso, a geophysicist with the USGS in Golden, Colorado.
“We are not changing our forecast for the San Andreas,” Caruso said. “We still believe there’s a 70% chance of a magnitude 7 or greater in Southern California before 2030.”
The San Andreas fault, which runs near heavily populated areas and defines the boundary between the North American and Pacific tectonic plates, is considered the biggest seismic threat to California. Related cracks in the earth, like the Hayward fault that runs through Oakland and Berkeley in the San Francisco Bay Area, are also considered major threats.