California Earthquakes

  • Large Swarm of Earthquakes Strikes Southern California

    An intense earthquake swarm started near Salton Sea, California on September 30, 2020, with 421 earthquakes detected by 07:00 UTC on October 1.

    • The swarm is centered just southeast of Salton Sea, near Westmorland in Imperial Valley, in an area with a history of intense swarms. The most notable earthquakes in the region include M5.8 in 1981 and M5.4 in 2012.
    • During this earthquake swarm, the probability of larger earthquakes in this region is significantly greater than usual, the USGS said.
    • The swarm is taking place in Brawley Seismic Zone, a network of small faults that connect the San Andreas and Imperial faults.

    The USGS registered a total of 440 earthquakes from 10:06 UTC on September 30 to 07:33 UTC on October 1, 2020, with the largest M4.9 at 00:31 UTC on October 1, followed by M4.5, M4.4, M4.2, and two M4.1.

    "This earthquake [M4.9] and the associated swarm are located in an area of diffuse seismic activity between the San Andreas fault in the north and the Imperial fault to the south," USGS seismologists said in a statement.

    "This area has also seen swarms in the past – notably the 1981 Westmorland swarm, which included a M5.8 earthquake, and the 2012 Brawley swarm, which included a M5.4 earthquake. Past swarms in this region have remained active for 1 to 20 days, with an average duration of about a week. The current swarm is occurring about 40 kilometers (25 miles) to the south of the swarm that occurred near Bombay Beach in August 2020."

    According to the USGS, there is approximately a 3 in 10 000 chance of a magnitude 7+ earthquake in the vicinity of this swarm in a typical week.

    "[This is] one of the largest swarms we have had in the Imperial Valley," seismologist Dr. Lucy Jones said.

    "None of the earthquakes that have been happening in the Imperial Valley are anywhere near the San Andreas fault," Jones added.

    "They are in the Brawley Seismic Zone – a network of small faults that connect the San Andreas and Imperial faults. Historically largest Brawley event was M5.8."


    The following three scenarios describe the possibilities of what could happen from September 30 to October 6, according to the USGS forecast released on September 30.

    Only one of these scenarios will occur within the next week, USGS said. 

    These scenarios include the possibility of earthquakes on and off the Imperial and San Andreas Faults.

    1. Scenario 1 (most likely): Earthquakes continue, possibly including earthquakes up to magnitude 5.4.

    The most likely scenario is that the rate of earthquakes in the swarm will decrease over the next 7 days. Some additional moderately sized earthquakes (M4.5 to 5.4) may occur, which could cause localized damage, particularly in weak structures. Smaller magnitude earthquakes (M3.0+) may be felt by people close to the epicenters.

    2. Scenario 2 (less likely): A larger earthquake (magnitude 5.5 to 6.9) could occur within the next 7 days.

    A less likely scenario is a somewhat larger earthquake could occur (up to M6.9). Earthquakes of this size could cause damage around the area close to the earthquakes that have already occurred and would be followed by aftershocks that would increase the number of smaller earthquakes per day. This scenario occurred in a previous swarm in the area – in 1981, when a swarm in this region included a magnitude 5.8 earthquake.

    3. Scenario 3 (least likely): A much larger earthquake (magnitude 7 or higher) could occur within the next 7 days.

    A much less likely scenario, compared with the previous two scenarios, is that the ongoing swarm could trigger an earthquake significantly larger than the M4.9 that occurred on September 30 (i.e., M7.0 and above). While this is a very small probability, if such an earthquake were to occur, it would have serious impacts on communities nearby and would be followed by aftershocks that would increase the number of smaller earthquakes per day.

    Featured image credit: TW/SAM, Google

  • Southern California is rocked by a 4.5-magnitude earthquake - with tremors felt from Los Angeles to San Diego

    A 4.5-magnitude earthquake rocked Southern California, with tremors felt from Los Angeles down to San Diego.

    The quake hit 10 miles west of L.A. at 11.39 pm local time on Friday, according to the US Geological Survey.

    It occurred at a depth of nearly 11 miles, and lasted for 30 seconds. It was reported to be one of the biggest earthquakes to hit the L.A. area in years.

    There have been no reports of injuries or damage to property, but authorities warned locals to prepare for aftershocks.

    The Los Angeles Fire Department posted a tweet shortly after the quake, which read: 'If Inside When Shaking Starts: DROP, COVER, HOLD ON! Protect Your Head & Neck While Taking Cover Under Sturdy Furniture or Near a Sturdy Interior Wall, Away From Windows and Doorways Until Shaking Stops.

    One seismologist stated that Friday's quake occurred in almost the exact same location as the Whittier Narrows earthquake in October of 1987.

    That 5.9-magnitude earthquake left eight people dead and a further 200 injured. The damage bill totaled more than $213 million.

    Meanwhile, on Friday night, several people posted videos that showed their homes shaking as the quake occurred.

    One TikTok user was partway through a performance, when her house began to rattle as the tremors hit.

    Another wrote on Twitter: 'Felt the biggest earthquake in Los Angeles yet. It was so big I ran out of my apartment with my purse and no shoes.'

    Residents live in fear of a giant earthquake jolting through the center of the city.

    Seismologists say a dormant fault line runs directly beneath Los Angeles that is well overdue for rupture.

    The experts fear the result could be a catastrophic 7.4-magnitude earthquake that would cause massive loss of life and billions of dollars in damage.

    It's been more than a generation since a giant earthquake shook the city.

    The 6.7-magnitude Northridge earthquake rocked the L.A's suburbs on January 17, 1994, causing more than $20 billion in damage and killing 60 people.

    'Hidden' fault line directly under Los Angeles threatens a devastating magnitude 7.4 earthquake

    A fault line, long believed to be dormant underneath Los Angeles, could link with others and cause a major magnitude 7.4 quake, according to a report published in September 2019.

    The 'Wilmington Fault,' was so deep below the Earth's surface that it was difficult to study, but researchers from Harvard, the USC and the US Geological Survey last year imputed a 'cluster of clues' into a three-dimensional model that revealed activity not previously detected.

    Research indicates that the Wilmington Fault is usually supposed to rupture every 3,200 to 4,700 years - however it has been dormant now for millions of years.

    There are fears for the communities of Los Angeles and Long Beach, which are built on the Wilmington Fault.

    'I hope bringing attention to it can potentially increase safety in the region,' study author Franklin Wolfe, a doctoral candidate who is part of Harvard's structural geology and Earth resources group, said at the time of publication.

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