Earthquakes occurred north and northeast of the town of Ridgecrest, California, located in Kern County and west of Searles Valley (approximately 200 km [122 mi] north-northeast of Los Angeles) on July 4-5, 2019. They included two initial main shocks of magnitudes 6.4 and 7.1, and many perceptible aftershocks, mainly within the area of the Naval Air Weapons Station China Lake. Eleven months later, a 5.5 aftershock took place (the largest aftershock of the sequence) to the east of Ridgecrest. The first main shock (now deemed to be a foreshock) occurred on Thursday, July 4 at 10:33 a.m. PDT, approximately 18 km (11.2 mi) ENE of Ridgecrest, and 13 km (8.1 mi) WSW of Trona, on a previously unnoticed NE-SW trending fault where it intersects the NW-SE trending Little Lake Fault Zone. "Tectonic Summary". This quake was preceded by several smaller earthquakes, and was followed by more than 1,400 detected aftershocks. The M 7.1 quake struck on Friday, July 5 at 8:19 p.m. PDT approximately 10 km (6 miles) to the northwest. The mainshock, was the most powerful earthquake to occur in the state in 20 years (after the 1999 Hector Mine earthquake). Subsequent aftershocks extended approximately 50 km (~30 miles) along the Little Lake Fault Zone.
Relatively minor damage resulted from the initial foreshock, though some building fires were reported in Ridgecrest near the epicenter. The main quake on July 5 cut power to at least 3,000 residents in Ridgecrest. Effects were felt across much of Southern California, parts of Arizona and Nevada, as far north as the San Francisco Bay Area and Sacramento, and as far south as Baja California, Mexico. An estimated 20 million people experienced the foreshock, and approximately 30 million people experienced the mainshock.
Earthquakes
At 10:02 a.m. PDT (17:02 UTC) on July 4, 2019, a 4.0 foreshock occurred about southwest of Searles Valley, California. A foreshock registering 6.4 occurred at 10:33 a.m. PDT (17:33 UTC) southwest of Searles Valley. The most populated area near the epicenter was Ridgecrest, home to 28,000 people. 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 section of an unspecified fault. Earth on either side of the fault was deformed, with lateral shifts of in the immediate vicinity. The exact fault it occurred on is uncertain with many small faults encompassing the region, though United States Geological Survey (USGS) seismologist Susan Hough stated it may have been the Little Lake Fault. Focused at a relatively shallow depth of , the foreshock affected a large region populated by 20 million people. Residents near the epicenter reported that shaking lasted approximately 30 seconds. Shaking was felt as far north as Sacramento, California, eastward to Phoenix, Arizona, and as far south as Baja California, Mexico, with evacuations taking place in Mexicali and Tijuana.
At 8:19 p.m. PDT on July 5, a larger 7.1 earthquake occurred in the Ridgecrest area after being preceded by a 5.0 foreshock 3 minutes prior, revealing the previous day's 6.4 earthquake to have been a foreshock. All three earthquakes have been described by the United States Geological Survey as occurring via shallow strike-slip mechanisms. Subsequent seismic activity occurred along two intersecting faults in the Little Lake Fault Zone.
Aftershocks A significant series of additional earthquakes followed the foreshock, with the majority of magnitudes ranging approximately 2–4 . By the evening of July 5, more than 1,400 earthquakes occurred, the strongest of which measured 5.4 at 4:07 a.m. PDT (11:07 UTC) on July 5. Shaking from this aftershock was felt as far north as Fresno, as far south as Laguna Hills (including Los Angeles), and as far east as Las Vegas, Nevada. No additional damage occurred from any of the aftershocks. The number of aftershocks within hours of the earthquake were unusually high, but not unprecedented.
Thousands more aftershocks occurred following the 7.1 event, with the total number of aftershocks exceeding 3,000 by the morning of July 7. Geologists at the USGS estimate an additional 34,000 aftershocks in the six months following the earthquakes.
On June 3, 2020, at 6:32 PM PST, a 5.5 aftershock was recorded 11 miles south of Searles Valley. This aftershock is tied for the strongest aftershock following the 7.1 mainshock.
+ Events at or above 4.5 associated with the 2019 Ridgecrest earthquakes Date Time (UTC) Magnitude MMI Depth Location 17:33:49 6.4 VIII SW of Searles Valley 18:39:44 4.6 VI ESE of Ridgecrest 18:56:06 4.6 VI NE of Ridgecrest 19:21:32 4.5 VI SSW of Searles Valley 11:07:53 5.4 VII W of Searles Valley 03:16:32 5.0 VII WSW of Searles Valley 03:19:53 7.1 IX W of Searles Valley 03:22:03 4.6 VII E of Little Lake 03:22:48 5.0 VI ESE of Little Lake 03:23:50 5.4 VII WNW of Searles Valley 03:25:27 5.0 VI ESE of Little Lake 03:27:06 4.7 VI E of Little Lake 03:29:29 4.5 VI SW of Searles Valley 03:47:53 5.5 V ESE of Little Lake 03:50:59 4.9 V ESE of Little Lake 04:13:07 4.8 VI SE of Ridgecrest 04:18:55 5.4 VII E of Little Lake 04:36:55 4.9 VII ESE of Little Lake 06:01:51 4.6 VI E of Little Lake 08:32:57 4.6 VI SSW of Searles Valley 09:28:28 4.9 VII ESE of Little Lake 13:06:55 4.5 VI E of Little Lake 23:50:41 4.5 VII N of Ridgecrest 05:38:15 4.5 VI W of Searles Valley 00:14:37 4.5 VI NNE of Coso Junction 13:11:37 4.9 VI ENE of Ridgecrest 20:15:36 4.5 VI NNE of Ridgecrest 03:59:14 4.9 VI NE of Coso Junction 00:42:47 4.7 VI E of Little Lake 20:49:50 5.0 VII E of Little Lake 01:32:11 5.5 VIII S of Searles Valley 01:19:07 4.6 V NE of Ridgecrest
Impact July 4 foreshock
Structural damage and two building fires—one of which destroyed half of a home—occurred in Ridgecrest. One mobile home was knocked off its foundation and deemed uninhabitable. Around 20 injuries were reported, primarily from shattered glass and falling debris. Some gas lines broke, prompting utility companies to shut off service. Businesses experienced product loss, with goods falling off shelves. Emergency personnel responded to nearly two dozen incidents in the city and opened two shelters. Fifteen patients at Ridgecrest Regional Hospital and residents in several apartment buildings were evacuated. Approximately 6,900 customers lost power near the epicenter: 6,000 in Ridgecrest and 900 in Searles Valley. A wide crack occurred along State Route 178 near Searles Valley and debris covered part of U.S. Route 395. In Trona, several buildings sustained damage, water and gas lines broke, and power lines fell. Rockslides covered multiple roads around the community, blocking off mountain roads, including the section of State Route 178 that connects Bakersfield with Lake Isabella. In Los Angeles proper, power outages affected the Fashion District and Granada Hills.
One person died in Pahrump, Nevada, over 150 miles away from the epicenter, when a Jeep supported by jackstands he was working under fell upon him.
July 5 mainshock Several fires broke out and five injuries were reported after the mainshock hit, most of them in Ridgecrest and Trona. 3,000 people were left without power in Ridgecrest and the rest of Kern County. In Trona, severe damage was reported, with houses knocked off their foundations and numerous gas leaks. Roughly 50 homes were damaged in the town. Multiple rockslides also occurred, making all roads to the city impassable. Water was also in high demand. Caltrans once again closed State Route 178 after new cracks and rockslides had occurred.
Overall damage is estimated in excess of $5.3 billion.
Aftermath July 4 foreshock Hours after the 6.4 foreshock, California Governor Gavin Newsom approved an emergency proclamation for Kern County. Local emergency responders such as the Los Angeles Police Department used social media to alert the public that they were aware of the earthquake and to only use 9-1-1 if there was dangerous conditions or injuries. Routine protocol inspections of major infrastructure, including the Los Angeles Aqueduct, were conducted across the Los Angeles metropolitan area. The Ventura County Fire Department dispatched personnel to survey the county for damage. Naval Air Weapons Station China Lake brought in additional personnel to conduct damage assessments. The USGS sent geologists to Kern County soon after the foreshock to look for a surface rupture and collect data. Disneyland suspended ride operations for inspection.
On July 4, seismologist Dr. Lucy Jones said there was a "1 in 20 chance" that the shock would be followed by larger earthquakes over the next few days. Jones also noted that the magnitude of additional shocks would also likely increase, with some possibly exceeding 5 .
July 5 mainshock
The State Operations Center for the California Governor's Office of Emergency Services (Cal OES) was raised to its highest level by Governor Newsom to coordinate resources. The agency distributed cots, water, and food. Due to temperatures exceeding , Cal OES established cooling stations in the affected areas. Newsom proclaimed a state of emergency for San Bernardino County on July 6. The California National Guard deployed 200 personnel to assist in relief operations. Power and water service was restored to Ridgecrest and roads were deemed safe by July 7; however, residents were advised to boil water for several days.
Following the shock, (NAWS) Naval Air Weapons Station China Lake evacuated all non essential personnel and declared the base as "not mission capable until further notice" in a statement published on the base's Facebook page on July 6.
In addition, Dr. Lucy Jones said that the odds of another 7 or above earthquake was a "1 in 10 chance", with a "50-50" chance of a 6 hitting the Owens Valley. The NW-SE fault is believed to now have increased to between 25 and 30 miles in length.
Earthquake early warning Many residents of Los Angeles County were not pleased when an earthquake early warning mobile app, ShakeAlertLA, did not alert them to either of the Ridgecrest earthquakes, which many people assumed to be a failure.; This app, developed by the City of Los Angeles in partnership with the USGS, and released at the beginning of 2019, is intended to give users located in Los Angeles County some tens of seconds of warning of possibly damaging shaking from a non-local earthquake.More distant earthquakes provide more warning, but are less damaging, than local earthquakes. ShakeAlertLA is triggered by the ShakeAlert system being developed by the USGS that monitors earthquakes across Southern California. When that system detects an earthquake of magnitude 5.0 or greater it calculates the intensity of shaking to be expected in Los Angeles County, factoring in the calculated magnitude and distance of the source..
At the time of the Ridgecrest earthquakes the threshold for issuing an alert was an expected shaking intensity in Los Angeles County of 4 (IV) on the Modified Mercalli Intensity Scale, what is considered "light shaking" with only slight damage (such as cracked plaster). The intensity expected for Thursday's quake (on the 4th) was calculated to be less than that, so no alert was issued. While the actual shaking reached intensity 4 in some areas in the northern part of the county, in other parts the intensity reached only a level 2 or 3, not noticed by many people.. The second quake, having a greater magnitude, was felt more strongly, with an intensity of 4.5 in some places, and 3.9 downtown..
According to the City of Los Angeles's Twitter account, the threshold will be lowered after this event.
Tectonic setting