An earthquake swarm started on the evening of July 5 at the southern margin of Coso Volcanic Field in Inyo County, California.
The swarm activity was triggered by a magnitude M5.4 earthquake at 9:19 PM PDT located 20 km (~20 miles) ESE of Little Lake, which itself was likely an aftershock of the M7.1 earthquake that occurred about an hour earlier 17 km NNE of Ridgecrest, and south of the Coso area.
An average of about 30 earthquakes per hour have been detected since, most within the range of magnitude M1 to M3.No ground deformation indicative of volcanic activity has been detected, and there is no imminent threat of an eruption.
The California Volcano Observatory will continue to monitor the situation for any sign of volcanic activity and provide updates as warranted.
The Coso Volcanic Field is located on the eastern side of the Sierra Nevada mountains at the northern end of the Mojave Desert, about 64 km (~40 miles) north of Ridgecrest.
The field covers about 150 square miles primarily within the Naval Air Weapons Station at China Lake and is comprised of lava domes, lava flows, and cinder cones erupted over the past 250,000 years.
The most recent eruption was about 40,000 years ago.
Crews in California continued to assess damage to cracked and burned buildings, broken roads, leaking water and gas lines and other infrastructure on Saturday after the largest earthquake the region has seen in nearly 20 years was felt from Sacramento to Mexico.
The governor, Gavin Newsom, declared a state of emergency and warned residents to be wary of new tremors, after the southern part of the state was hit by a second significant earthquake in as many days.
Residents of Ridgecrest, a city of about 27,000 150 miles north-east of Los Angeles, woke to new damage after the magnitude 7.1 tremor hit as darkness fell on Friday, jolting the area after a 6.4 quake struck 34 hours earlier. Kern county's fire chief, David Witt, said there were no known fatalities but the damage had not been fully assessed.
"We do feel like there is damage but we don't know the extent of it yet," Witt said at a news conference, according to CNN.
Witt said officials were inspecting buildings and the US army corps of engineers was inspecting a nearby dam.
Just north-west of Ridgecrest, US naval air weapons station China Lake was evacuated of all nonessential personnel.
The station said in a Facebook post normal operations were halted until further notice.
Experts warned that southern California could expect more significant shakes in the near future. There is about a one in 10 chance that another 7.0 quake could hit within the next week and the chance of a 5.0-magnitude quake "is approaching certainty", Lucy Jones, a seismologist at the California Institute of Technology and a former science adviser at the US Geological Survey, told reporters.
She added that the new quake probably ruptured along about 25 miles of the fault line and was part of a continuing sequence.
Jones told the Los Angeles Times the fault that caused the quakes appears to be growing.
"This happened at the end of the zone that moved previously," she said, adding that the fault is now 25 to 30 miles long and "growing".
In the hours after the 7.1 tremors, seismologists recorded more than 600 aftershocks.
The quakes were not expected to trigger larger faults including the San Andreas but Jones told the Guardian southern California should expect more quakes.
"This is the first magnitude 6 quake in 25 years.
It's the longest interval we've ever had," Jones said.
"We know that the last 20 years was abnormal ... we should expect more earthquakes than we've been having recently." She added: "Chances are, we're going to have more earthquakes in the next five years than we've had in the last five years."
In a state where a major earthquake disaster is always a possibility, the quakes put many on edge.
The two 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.
In Ridgecrest, most damages from Thursday's quake came from ruptured gas lines.
About 3,000 people were without power.
Many said they would sleep outside than risk staying in their homes.
In Los Angeles, 150 miles away, the quake was felt in offices in skyscrapers rocked for at least 30 seconds.
At Dodger Stadium, the press box lurched.
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 small town of Trona, Calif., suffered more damage from Friday night's 7.1 magnitude earthquake and was inaccessible because of rockslides, residents said.
Trona resident Ivan Amerson said there was "significant damage," with some houses knocked off their foundations.
State officials said they had heard reports of at least one structure collapse.
The San Bernardino County Sheriff's Department said there were numerous gas leaks and that deputies were helping with evacuations.