California's San Andreas hit by swarm of earthquakes, raising concerns over bigger quakes within seven days
A swarm of earthquakes in San Andreas has raised the risk of bigger quakes hitting the region in the days to follow, the US Geological Survey (USGS) has warned. There is one in 1,000 chance of an earthquake with magnitude higher than seven striking the southernmost San Andreas Fault, they predicted.
According to the USGS, swarms are a sequence of mostly small earthquakes, which continue for days, weeks, or sometimes even months. Previously, they have hit the region, with most recent ones recorded in 2001, 2009 and 2016. They usually stayed active for 1 to 20 days.
USGS traced the origins of the swarm of earthquakes to the Salton Sea -- a saline lake sitting on the San Andreas Fault. The activity began on August 10, 2020, when experts recorded 54 quakes, with the largest having a magnitude of 4.3. The next day the region witnessed 10 earthquakes of magnitude 2 and above. "Today’s #saltonsea #earthquake swarm in CA raises the probability of larger earthquakes in the #sanandreas region. Medium-sized earthquakes can cause damage too," the scientific body tweeted on August 11.
"However, the risk of a large earthquake is still considerably elevated due to the swarm when compared to background levels," the USGS said in a statement. "The southernmost section of the San Andreas Fault is capable of rupturing in large magnitude earthquakes (magnitude 7+), the last of which occurred more than 300 years ago," they explained. "In a typical week, there is approximately a one in 10,000 chance of a magnitude 7+ earthquake on the southernmost San Andreas Fault."
USGS is currently monitoring the activity in the region. Three scenarios are likely to play out from 10 August to 18 August, they predict. The first one has a 98% odds of occurring. According to the forecast, none of the quakes will exceed the magnitude of 5.4. Moderate earthquakes (of range 4.5 to 5.4) can cause local damage, particularly in weak structures. People living close to the epicenters may feel the lower ones, they said.
The second scenario has about a 2% chance of occurring. The experts predict the possibility of a bigger earthquake of 5.5 to 6.9 magnitude within the next seven days. It is also possible that the region may witness a relatively bigger quake (of up to an M6.9) -- but that is unlikely, they add. "Earthquakes of this size could cause damage around the Salton Sea area and would be followed by aftershocks that would increase the number of smaller earthquakes per day," USGS noted. The third scenario, which forecasts a much larger earthquake of magnitude 7 or higher, has less than a 1% chance of occurring.
The experts have advised caution, especially when in or around vulnerable structures. "This swarm may lead to larger and potentially damaging earthquakes in the future, so remember to: Drop, Cover, and Hold on if you feel shaking or receive a ShakeAlert message. When there are more earthquakes, the chance of a large earthquake is greater, which means that the chance of damage is greater," they said.