Hundreds of aftershocks rumbled in Southern California on Friday, the day after the region was rocked by the strongest earthquake in two decades, igniting fires, triggering a hospital evacuation and raising concerns about an even more powerful jolt.

Most of the aftershocks ranked in the magnitude 2-to-3 range, with a few in the magnitude 3-to-4 range, well below the Fourth of July’s magnitude 6.4 earthquake centered near Ridgecrest, an inland Kern County city about 150 miles northeast of Los Angeles.

“We’re getting aftershocks every few minutes. There are hundreds of them,” Randy Baldwin, a geophysicist at the U.S. Geological Survey’s National Earthquake Information Center in Golden, Colorado, said early on Friday. “There’s a chance of some larger ones, in the magnitude 5 range.”

One aftershock struck even as Ridgecrest Police Chief Jed McLaughlin briefed news reporters about the emergency response to the earthquake on Thursday afternoon PDT.

A 5.4-magnitude aftershock rattled the nearby Searles Valley area Friday morning, less than 24 hours after the major earthquake. The aftershock occurred just after 4 a.m. about 10 miles west of Searles Valley, according to USGS data. It was the strongest of the more than 200 that followed Thursday’s seismic event.

Lucy Jones, a California-based seismologist, tweeted that the actual probability of the Big One, a far stronger earthquake, is about 2 percent per year.

An individual’s chances of being in a car accident today are about one in 7,000, Jones said, adding “I still wear my seat belt every day.”

Although Thursday’s earthquake was felt in Los Angeles and even in Las Vegas, more than 250 miles northeast of Los Angeles, it caused no damage in those locations. The earthquake was far enough away from the dangerous and well known San Andreas fault “that any impact on the system will be minimal,” Jones told the Los Angeles Times.

Aftershocks rumble from SoCal quake: Strongest earthquake to hit Southern California in 20 years rattles region, rumbles residents

Major aftershock rumbles near Ridgecrest: 5.4-magnitude aftershock shakes Searles Valley Friday morning following July 4 earthquake

The major seismic event marked the first time that Southern California has been struck by an earthquake above magnitude 6 since the 7.1 magnitude Hector Mine earthquake in October 1999, Jones and other seismologists said.

The latest Southern California earthquake triggered the evacuation of Ridgecrest Hospital patients and health care workers. The Kern County Fire Department reported that firefighters and other emergency personnel responded to roughly nearly two dozen incidents in and around Ridgecrest, ranging from medical assistance to structure fires.

Kern County Fire Chief David Witt said emergency officials in Ridgecrest responded to at least two house fires, a small vegetation fire, downed power lines and gas leaks. California Gov. Gavin Newsom and Ridgecrest Mayor Peggy Breeden declared a state of emergency in the area to speed response to the damage and any injuries.

‘Drop, cover and hold on’

Glenn Pomeroy, head of California’s Earthquake Authority, said the quake is “an important reminder that all of California is earthquake country.”

“It’s important to know what to do to stay safe when the ground starts shaking — drop, cover and hold on,” said Pomeroy. “And to take other steps to prepare to survive and recover from damaging earthquakes, such as to retrofit homes built prior to 1980 and the advent of modern building codes, which may be more vulnerable to earthquake damage, and consider earthquake insurance to protect ourselves financially.”

Are you covered for earthquakes?: California earthquake insurance: What does home insurance cover after a disaster?

The USGS reported that the earthquake quake started 11 miles outside Ridgecrest at 10:33 a.m. PDT. The agency originally reported it had a magnitude of 6.6, later scaling it down to 6.4. Aftershocks of 4.7 and 3.0 magnitudes were reported minutes later, and scores more of less strength followed.

Christine Goulet, executive director for Applied Science at the Southern California Earthquake Center, said the quake was felt over such a wide area because it was relatively shallow, only five or six miles deep. 

Contributing: Joseph Hong and Ricardo Lopez, Palm Springs Desert Sun