One day after Southern California experienced its strongest earthquake in 20 years, a magnitude 5.4 aftershock rattled the area once again early Friday morning.
The quake struck around 4:07 a.m. Pacific Time in the Mojave Desert, according to the U.S. Geological Survey. The aftershock was roughly 10 miles west of Searles Valley, California.
So far, it's the largest of a swarm of ongoing aftershocks to hit California following Thursday's magnitude 6.4 quake that led Gov. Gavin Newsom to declare a state of emergency for one county.
Dramatic video on Friday showed a Good Morning America reporter caught in the aftershock while her camera was rolling.
When the earthquake struck July 4, people from Las Vegas to the Pacific Coast reported feeling a rolling motion and took to social media to report it.
Communities in the Mojave Desert tallied damage and made emergency repairs to cracked roads and broken pipes Friday as aftershocks kept rumbling.
The town of Ridgecrest, close to the epicenter, assessed damage after several fires and multiple injuries that were blamed on the magnitude 6.4 quake. A shelter drew 28 people overnight but not all of them slept inside amid the shaking.
"Some people slept outside in tents because they were so nervous," said Marium Mohiuddin of the American Red Cross.
Damage appeared limited to desert areas, although the quake was felt widely, including in the Los Angeles region 150 miles (240 kilometers) away.
The odds of a quake of similar size happening in the next few days continued to dwindle and was only 6 percent on Friday, seismologists said.
There had been about 1,700 aftershocks since the Thursday quake, which was a bit higher than average, said Zachary Ross of the California Institute of Technology.
"An event of this size is going to keep producing aftershocks for years but the rates are going to decay with time," Ross said.
The quake involved two perpendicular faults in the area but it was unlikely to affect any fault lines away from the immediate area, seismologists said.
Damage in the town of Ridgecrest was relatively light because the city is relatively young, with growth coming in the 1940s and later so many buildings met upgraded building codes, said Susan Hough of the U.S. Geological Survey.
Ridgecrest Regional Hospital remained closed as state inspectors assessed it, spokeswoman Jayde Glenn said. The hospital's own review found no structural damage, but there were cracks in walls, broken water pipes and water damage.
The hospital was prepared to help women in labor and to give triage care to emergency patients. Fifteen patients were evacuated to other hospitals after the quake, Glenn said.
The quake did not appear to have caused major damage to roads and bridges in the area, but it did open three cracks across a short stretch of State Route 178 near the tiny town of Trona, said California Department of Transportation district spokeswoman Christine Knadler.
Those cracks were temporarily sealed, but engineers were investigating whether the two-lane highway was damaged beneath the cracks, Knadler said. Bridges in the area were also being checked.
The Ridgecrest library was closed as volunteers and staff picked up hundreds of books that fell off shelves. The building's cinderblock walls also had some cracks, said Charissa Wagner, library branch supervisor.
Wagner was at her home in the small city of 29,000 people when a small foreshock hit, followed by the large one, putting her and her 11-year-old daughter on edge.
"The little one was like, 'Oh what just happened.' The big one came later and that was scarier," she said.
The earthquake knocked over a boulder that sat atop one of the rock spires at Trona Pinnacles outside of Ridgecrest, a collection of towering rock formations that has been featured in commercials and films, said Martha Maciel, a Bureau of Land Management spokeswoman in California.
Meanwhile, the nation's second-largest city revealed plans to lower slightly the threshold for public alerts from its earthquake early warning app. But officials said the change was in the works before the quake, which gave scientists at the California Institute of Technology's seismology lab 48 seconds of warning but did not trigger a public notification.
"Our goal is to alert people who might experience potentially damaging shaking, not just feel the shaking," said Robert de Groot, a spokesman for the U.S. Geological Survey's ShakeAlert system, which is being developed for California, Oregon and Washington.
The West Coast ShakeAlert system has provided non-public earthquake notifications on a daily basis to many test users, including emergency agencies, industries, transportation systems and schools.
Late last year, the city of Los Angeles released a mobile app intended to provide ShakeAlert warnings for users within Los Angeles County.
The trigger threshold for LA's app required a magnitude 5 or greater and an estimate of level 4 on the separate Modified Mercali Intensity scale, the level at which there is potentially damaging shaking.
Although Thursday's quake was well above magnitude 5, the expected shaking for the Los Angeles area was level 3, de Groot said.
A revision of the magnitude threshold down to 4.5 was already underway, but the shaking intensity level would remain at 4. The rationale is to avoid numerous ShakeAlerts for small earthquakes that do not affect people.
"If people get saturated with these messages, it's going to make people not care as much," he said.
Construction of a network of seismic-monitoring stations for the West Coast is just over half complete, with most coverage in Southern California, San Francisco Bay Area and the Seattle-Tacoma area. Eventually, the system will send out alerts over the same system used for Amber Alerts to defined areas that are expected to be affected by a quake, de Groot said.
California is partnering with the federal government to build the statewide earthquake warning system, with the goal of turning it on by June 2021. The state has already spent at least $25 million building it, including installing hundreds of seismic stations throughout the state.
This year, Democratic Gov. Gavin Newsom said the state needed $16.3 million to finish the project, which included money for stations to monitor seismic activity, plus nearly $7 million for "outreach and education." The state Legislature approved the funding last month, and Newsom signed it into law.
Associated Press writers John Antczak, Brady McCombs in Salt Lake City and Adam Beam in Sacramento, California, contributed to this report.