The foreshock originated along a strike-slip fault in the Eastern California Shear Zone, a region frequented by earthquake swarms, near the edge of Death Valley National Park. The rupture occurred along a 10 mi (16 km) section of an unspecified fault line. Earth on either side of the fault was deformed, with lateral shifts of 6–8 in (15–20 cm) in the immediate vicinity.
Officials in southern California’s high desert were braced for strong, potentially dangerous aftershocks after a major earthquake damaged buildings, ruptured gas lines and sparked fires near its remote epicenter.
As darkness fell on Friday, the magnitude 7.1 tremor rocked the Mojave desert town of Ridgecrest near Death Valley National Park, jolting the area with eight times more force than a 6.4 quake that struck the same area 34 hours earlier.
Southern California can expect more significant shaking in the near future, said Lucy Jones, a seismologist at the California Institute of Technology and a former science adviser at the US Geological Survey.
According to our forecast, over the next 1 Week there is a 4 % chance of one or more aftershocks that are larger than magnitude 7.1. It is likely that there will be smaller earthquakes over the next 1 Week, with 360 to 660 magnitude 3 or higher aftershocks.— USGS (@USGS) July 6, 2019
Jones told the LA Times the fault that caused the quakes appears to be growing.
“This happened at the end of the zone that moved previously,” Jones said, adding that the fault is now 25 to 30 miles long.
“The fault is growing,” she said.
California governor Gavin Newsom requested federal assistance and placed the state Office of Emergency Services (OES) on its highest alert.
At a late-night news conference, OES director Mark Ghilarducci said: “We have significant reports of fires, structural fires, mostly as a result of gas leaks or gas line breaks.”
The quake caused water main breaks and knocked out power and communications to parts of Ridgecrest, a city of about 27,000 about 125 miles north-east of Los Angeles. No fatalities or serious injuries were reported, police said. But Ghilarducci said the full damage would not be known until Saturday.
“This was a very large earthquake, and we also know there’s going to be a series of aftershocks as a result of the main quake,” he said, adding his agency faced a “challenge” getting needed resources to the isolated quake zone.
“This is not going to be something that’s going to be over right away.”
In the hours after the 7.1 tremor, seismologists recorded more than 600 aftershocks. The quakes were not expected to trigger larger faults including the San Andreas.
Ridgecrest residents were still recovering from Thursday’s quake. Most damage came from ruptured gas lines. About 3,000 people were left without power, according to Southern California Edison. Many said they would sleep outside than risk staying in their homes.
San Bernardino county firefighters reported cracked buildings and one minor injury. In Los Angeles, 150 miles away, offices in skyscrapers rocked for at least 30 seconds. Brian Humphrey of the Los Angeles fire department said more than 1,000 firefighters were mobilized. At Dodger Stadium the press box lurched for several seconds.
As far away as Las Vegas, players and staff left the court after the earthquake was felt during an NBA summer league game between the New York Knicks and the New Orleans Pelicans. The US Geological Survey said it was felt in Mexico too.
The chance of an earthquake of magnitude 6 or higher is 39 %, and it is most likely that as few as 0 or as many as 3 such earthquakes may occur.— USGS (@USGS) July 6, 2019
Communities in the Mojave were assessing damage after Thursday’s quake, which set fires and opened three cracks across a short stretch of state route 178 near the tiny town of Trona, said California transportation spokeswoman Christine Knadler. Bridges were being checked.
The quakes were the most powerful in the region since 1994, when the 6.7 magnitude Northridge quake hit the heavily populated San Fernando Valley, causing 57 deaths and billions in dollars of damages.
Southern California residents should expect more earthquakes in coming years, experts warned.