2019 Ridgecrest earthquake
Coordinates: 35.705°N 117.506°W
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  Ridgecrest Los Angeles Las Vegas Sacramento Tijuana  | |
| UTC time | 2019-07-04 17:33:49 |
|---|---|
| ISC event | 616042637 |
| USGS-ANSS | ComCat |
| Local date | July 4, 2019 |
| Local time | 10:33 a.m. PDT |
| Magnitude | Mw 6.4 (mainshock) |
| Depth | 10.7 km (6.6 mi) |
| Epicenter | 35.705°N 117.506°W |
| Type | Strike-slip |
| Areas affected | California, Nevada, Arizona, Baja California |
| Max. intensity | VIII (Severe)[1] |
| Foreshocks | 2 (≥2.5 MW) |
| Aftershocks | >200 (≥2.5 MW)[2] |
| Casualties | 20 injured |
On July 4, 2019, a series of earthquakes occurred near Searles Valley and Death Valley National Park in California. A 6.4 MW mainshock occurred at 10:33 a.m. PDT (17:33 UTC) in a remote area of San Bernardino County. The mainshock was preceded by several foreshocks, and hundreds of aftershocks were detected after the mainshock. Relatively minor damage resulted, though some building fires were reported in Ridgecrest, California, near the epicenter. Effects were felt across much of southern California, parts of Arizona and Nevada, and as far south as Baja California, Mexico. An estimated 20 million people experienced the earthquake.
Background
Although California is frequented by earthquakes, the state has been in an "earthquake drought" for decades..[3] Prior to the 2019 earthquake, the last 6 MW or stronger event was the 2014 South Napa earthquake.[4][3] 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.[3] An average of three to four ground-breaking events are expected each century.[5] The 2019 earthquake is the largest in Southern California since a 7.1 MW in October 1999;[6] 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.[3]
Earthquakes
At 10:02 a.m. PDT (17:02 UTC) on July 4, 2019, a 4.0 MW foreshock occurred about 6.8 mi (10.9 km) southwest of Searles Valley, California.[7][8] A mainshock registering 6.4 MW occurred at 10:33 a.m. PDT (17:33 UTC) 7.5 mi (12.1 km) southwest of Searles Valley.[1] The most populated area near the epicenter was Ridgecrest, home to 27,000 people.[9] The earthquake 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.[9][10] 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.[11] Focused at a relatively shallow depth of 6.6 mi (10.7 km), the earthquake affected a large region populated by 20 million people.[1][9] Residents near the epicenter reported that shaking lasted approximately 30 seconds.[11] 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.[9][12]
A significant series of aftershocks followed the main earthquake, with the majority of magnitudes ranging approximately 2–4 MW.[13] By the morning of July 5, more than 200 aftershocks of at least 2.5 MW occurred,[2][14] the strongest of which measured 5.4 MW at 4:07 a.m. PDT (11:07 UTC) on July 5.[15] The number of aftershocks within hours of the earthquake is unusually high, but not unprecedented.[16]
Impact
Structural damage and two building fires—one of which destroyed half of a home—occurred in Ridgecrest.[9][17] Around 20 injuries were reported, primarily from shattered glass and falling debris.[18][17] Some gas lines broke, prompting utility companies to shut off service. Businesses experience product loss, with goods falling off shelves.[19] Emergency personnel responded to nearly two dozen incidents in the city and opened two shelters.[9] Fifteen patients at Ridgecrest Regional Hospital and residents in several apartment buildings were evacuated.[20] Approximately 6,900 customers lost power near the epicenter: 6,000 in Ridgecrest and 900 in Searles Valley.[21] A 4 in (10 cm) wide crack occurred along Highway 178 near Searles Valley and debris covered part of Route 395.[22][17] 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.[23] In Los Angeles proper, power outages affected the Fashion District and Granada Hills.[24]
Aftermath
Hours after the earthquake, California Governor Gavin Newsom declared a state of emergency for Ridgecrest.[25] 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.[26] Routine protocol inspections of major infrastructure, including the Los Angeles Aqueduct, were conducted across the Los Angeles Metropolitan area.[4] The Ventura County Fire Department dispatched personnel to survey the county for damage.[22] Naval Air Weapons Station China Lake brought in additional personnel to conduct damage assessments.[17] The USGS sent geologists to Kern County soon after the earthquake to look for a surface rupture and collect data.[4] Disneyland suspended ride operations for inspection.[27]
According to seismologist Dr. Lucy Jones, there is a "1 in 20 chance" that this event will be 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 MW.[4] On July 5 at 11:07 UTC (4:07 a.m. PDT) a 5.4 MW aftershock took place.
USGS's ShakeAlert issued a warning 6.9 seconds after the earthquake, about 50 seconds before the mainshock was felt at the Caltech Seismological Laboratory.[28] 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 MW in the Los Angeles County area, which is below the activation threshold of 5.0 MW 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.[24][29] According to the City of Los Angeles's Twitter account, the threshold will be lowered after this event.[30][31]
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