2019 Ridgecrest earthquakes

2019 Ridgecrest earthquakes

File:2019-07-06 Ridgecrest, CA M7.1 earthquake shakemap (ci).jpg ShakeMap for M7.1 earthquake
Ridgecrest Los Angeles Las Vegas Sacramento San Diego Tijuana
Depth	10.7 km (6.6 mi) 17.0 km (10.6 mi)
Epicenter
Type	Strike-slip
Areas affected	California, Nevada, Arizona, Baja California
Max. intensity	IX (Violent)
Foreshocks	2 (≥2.5 ṃ) (including before main foreshock)
Aftershocks	>1,400 >200 (≥2.5 ṃ) (including after main foreshock)
Casualties	none 25 injured (20 on July 4, 5 on July 5)

From July 4 to July 5, 2019, a series of earthquakes occurred between Searles Valley and Ridgecrest, California, between the southern perimeters of Seqouia National Park and Death Valley National Park. On July 4, a 6.4 ṃ foreshock occurred at 10:33 a.m. PDT (17:33 UTC) in a low population density area of San Bernardino County. The swarm occurred approximately 180 miles east of the San Andreas Fault that runs the length of California. This quake was preceded by several smaller earthquakes, and was followed by more than 1,400 detected aftershocks. On July 5, a 7.1 ṃ mainshock occurred near the same location, at 8:19 p.m. PDT (3:19 UTC on July 6), which was the most powerful earthquake to occur in the state in 20 years.

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 the 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 as far south as Baja California, Mexico. An estimated 20 million people experienced the foreshock, and approximately 30 million people experienced the mainshock.

Background

California lies across the mainly transform boundary between the North American and Pacific Plates. About 75% of the displacement between the two plates is taken up by the San Andreas Fault system, with the remaining 25% being accommodated by faults within the Walker Lane/East California Shear Zone.

Although California frequently experiences earthquakes, the state has been in an "earthquake drought" for decades. Prior to the 2019 earthquakes, the last 6 M_W or stronger event was the 2014 South Napa earthquake. The San Andreas, San Jacinto, and Hayward faults in particular have been exceptionally quiet since 1919, with geologists surmising this to be the only 100-year period in the last 1,000 years with no ground-breaking earthquakes. An average of three to four ground-breaking events are expected each century.

The 2019 earthquakes are the largest in Southern California since the 7.1 M_W Hector Mine Earthquake in October 1999, 20 years earlier; however, that event occurred away from the main plate boundary. The lack of major events in the region is increasing the strain along multiple faults, raising the probability of major earthquake events.

Earthquakes

At 10:02 a.m. PDT (17:02 UTC) on July 4, 2019, a 4.0 ṃ foreshock occurred about 6.8 mi (10.9 km) southwest of Searles Valley, California. A foreshock registering 6.4 M_W occurred at 10:33 a.m. PDT (17:33 UTC) 7.5 mi (12.1 km) southwest of Searles Valley. The most populated area near the epicenter was Ridgecrest, home to 27,000 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 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. The exact fault it occurred on is uncertain with many small faults encompassing the region, though United States Geological Survey (USGS) seismologist Susan Hughs stated it may have been the Little Lake Fault. Focused at a relatively shallow depth of 6.6 mi (10.7 km), 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.

A significant series of additional earthquakes followed the foreshock, with the majority of magnitudes ranging approximately 2–4 M_W. By the evening of July 5, more than 1,400 earthquakes, the strongest of which measured 5.4 ṃ 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 is unusually high, but not unprecedented.

At 8:19 p.m. PDT on July 5, a larger, 7.1 ṃ earthquake occurred in the Ridgecrest area, revealing the 6.4 ṃ earthquake to have been a foreshock. Both 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 Airport Lake Fault Zone.

Foreshocks, as well as aftershocks at or above ṃ 4.5

Date (UTC)	Magnitude	Depth (km)
2019-07-04 17:02:55	4.0	10.3
2019-07-04 17:09:20	2.5	11.6
2019-07-04 17:33:49	6.4	10.7
2019-07-04 17:35:01	4.5	4.6
2019-07-04 18:39:44	4.6	2.8
2019-07-04 18:56:06	4.6	1.9
2019-07-04 19:21:32	4.5	3.0
2019-07-04 22:12:08	4.5	2.0
2019-07-05 11:07:53	5.4	7.0
2019-07-05 11:07:53	7.1ṃ Mainshock
2019-07-06 03:16:32	5.0	-1.0
2019-07-06 03:19:52	7.1	17.0
2019-07-06 03:22:03	4.6	9.1
2019-07-06 03:23:50	5.4	12.4
2019-07-06 03:25:27	5.0	11.2
2019-07-06 03:27:05	4.5	4.1
2019-07-06 03:47:53	5.5	3.4
2019-07-06 03:50:59	4.9	6.6
2019-07-06 04:13:07	4.8	7.9
2019-07-06 04:18:55	5.4	2.2
2019-07-06 04:36:55	4.9	1.5
2019-07-06 06:01:51	4.6	4.0
2019-07-06 08:32:57	4.6	2.5
2019-07-06 09:28:28	4.9	3.9
2019-07-06 09:29:20	4.5	2.9
2019-07-06 09:30:40	4.5	-0.6
2019-07-06 13:06:55	4.5	1.5
2019-07-06 23:50:41	4.5	3.0

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 4 in (10 cm) 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.

July 5 mainshock

Several fires broke out and five injuries were reported after the mainshock hit, most of them in Ridgecrest and Trona. One fire broke out in a restaurant. 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. 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.

Aftermath

July 4 foreshock

Hours after the ṃ 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 is a "1 in 20 chance" that the foreshock would have been followed by larger earthquakes over the next few days. Jones also noted that the magnitude of aftershocks was also likely to increase, with some possibly exceeding 5 ṃ. On July 5 at 11:07 UTC (4:07 a.m. PDT) a 5.4 ṃ aftershock took place, with the 7.1 ṃ mainshock occurring later that day at 8:19 p.m. PDT (3:19 UTC).

USGS's ShakeAlert issued a warning 6.9 seconds after the foreshock, about 50 seconds before the foreshock was felt at the Caltech Seismological Laboratory. However, the phone app ShakeAlertLA, which is used to alert residents of Southern California of impending earthquakes, did not activate. This occurred due to the earthquake having a predicted magnitude of 4.5 ṃ in the Los Angeles County area, which is below the activation threshold of 5.0 ṃ and below level 4 on the Modified Mercalli intensity scale ("light shaking"). Due to this, residents were not informed of the earthquake despite the advance notice given to seismologists. According to the City of Los Angeles's Twitter account, the threshold will be lowered after this event.

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. Newsom proclaimed a state of emergency for San Bernardino County on July 6.

After the mainshock, Dr. Lucy Jones said that the odds of another 7 ṃ or above earthquake was a "1 in 10 chance", with a "50-50" chance of a 6 ṃ hitting the Owens Valley. The fault is believed to now have increased to between 25 to 30 miles in length.

See also

• 2010 Baja California earthquake
• List of earthquakes in 2019
• List of earthquakes in California

External links

• The International Seismological Centre has a bibliography and/or authoritative data for this event. - M6.4
• The International Seismological Centre has a bibliography and/or authoritative data for this event. - M7.1