RIDGECREST – According to the U.S. Geological Survey, Ridgecrest and the surrounding area is very seismically active; the area is situated in the Eastern California Shear Zone or ECSZ resulting in many large quakes even within the limited span of the historical era. The most notable quake is the 1872 Owens Valley Earthquake with an estimated magnitude 7.4 on the Richter Scale (other estimates put the quake at a 7.9); this quake is believed to be the largest earthquake on record not only in California but for all of the western United States.
The 1995 Ridgecrest Earthquake sequence started on Aug. 17th at approximately 3:39 p.m. when a 5.4 magnitude quake struck approximately 11 miles north of Ridgecrest with 2,500 aftershocks reported over the course of 5 weeks; then on Sept. 20th at around 4:28 p.m. another large quake shook the area measuring 5.8 on the Richter Scale centered 10 miles north of Ridgecrest and at that time, was the largest recorded quake to hit the area since the 1994 Northridge quake and is still the largest earthquake ever recorded in the immediate area though larger quakes are possible along some of the nearby fault zones.
After the Sept. 20th quake approximately 1,900 aftershocks were recorded over a 2 week span and several thousand more have been recorded in the area since; like previous quake activity in the area, the aftershocks showed some spatial migration with activity being triggered both northeast and southeast of the original epicentral region. At Least 4 small faults in the southern portion of the Airport Lake fault zone slipped in the course of the 2 main shocks and the larger aftershocks; about 1 centimeter of surficial triggered right-oblique slip was seen on a 3 kilometer long fault segment roughly 3 kilometers northwest of the epicenter area however, no surface rupture was found along the fault segment believed to have cause the 2 main shocks. Ridgecrest and the surrounding area experience numerous swarms of quakes in the past mostly under 5.0 and often with no obvious main shock.
On July 4 and 5, 2019, the Ridgecrest Earthquake Sequence occurred north and northeast of Ridgecrest and west of Searles Valley which included 3 initial main shocks (6.4, 5.4 and 7.1) and several aftershocks mainly within the area of the Naval Air Weapons Station China Lake; 11 months later, a 5.5 aftershock hit east of Ridgecrest which was the largest aftershock to hit the area since the main event. The first shock which is now being called a “foreshock” took place at 10:33 a.m. 11.2 miles east-northeast of Ridgecrest and 8.1 miles west-southwest of Trona on a previously unnoticed trending fault where it intersects with the Little Lake Fault Zone. Several smaller quakes were recorded and were followed by more than 1400 detected aftershocks. The 5.4 and 7.1 events happened on July 5th; the 5.4 hit at 4:08 a.m. and the 7.1 hit at 8:19 a.m. 6 miles to the northwest of Ridgecrest. The 7.1 is now considered to be the main shock and the most powerful quake to occur in the state in 20 years; aftershocks extended approximately 30 miles along the Little Lake Fault Zone and were felt throughout much of Southern California, parts of Arizona and Nevada. It was also felt as far north as San Francisco and as far south as Baja California, Mexico with an estimated 20 million people experiencing the foreshock and 30 million people feeling the main event.
The July 4th event did minor damage though some building fires were reported in Ridgecrest near the epicenter; the July 5th quake resulted in loss of power for about 6,000 residents in Ridgecrest and another 900 in Searles Valley; 3 minutes before the 7.1 event, a magnitude 5.0 quake shook the Ridgecrest area revealing the previous days 6.4 quake to have been a foreshock; all 3 events have been described by the USGS as occurring via shallow strike-slip mechanisms. By evening time on July 5th, more than 1400 aftershocks occurred ranging from 2-4 in magnitude; no additional damage happened from any of the aftershocks and geologists estimated that the total number of aftershocks were 3,000 by July 7th and 34,000 in the 6 months following the main quakes.
On June 3, 2020 at approximately 6:32 p.m. a 5.5 aftershock was recorded 11 miles south of Searles Valley and is tied for the strongest aftershock following the July 5th 7.1 main quake.
After the July 4th foreshock occurred, structural damage occurred in Ridgecrest; on mobile home was knocked off its foundation and deemed uninhabitable, 2 building fires occurred which destroyed half a home, around 20 injuries were reported from shattered glass and falling debris, gas lines broke, businesses suffered product loss and several apartment buildings were evacuated; 1 person died in Pahrump, Nevada when a jeep he was working on collapsed on top of him. Overall damage was estimated to exceed $5.3 billion.
Seismicity in the Indian Wells Valley has been characterized be swarms of quakes, some lasting for more than a year; the Ridgecrest sequences of 1995 and 2019 are similar in having 2 main shocks; a 5.4 on Aug. 17, 1995 and a 5.8 on Sept. 20, 1995; another similarity between the events is the first quake although occurring on a north-northwest trending fault may have involved a northeast striking fault and according to earlier studies, geologists noted that successive earthquake sequences here tend to increase in number and magnitude; they also tend to be moving southward.