- Southern California has seen a series of strong earthquakes: A magnitude 6.4 earthquake struck on July 4, followed by a magnitude 5.4 quake in the same region less than 24 hours after. Then a magnitude 7.1 quake hit just 16 hours later.
- The first earthquake in this set was the state’s first temblor in five years.
- Since then, seismologists in California have recorded more than 3,000 earthquakes of varying sizes in the past five days.
- But none in this series of earthquakes was the ‘big one’ that experts are concerned is overdue on the San Andreas fault.
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Independence Day celebrations in southern California were interrupted on Thursday afternoon when a magnitude 6.4 earthquake split the earth.
Before that temblor, which was centered in the city of Ridgecrest, the ground had not shook in California since a magnitude 6 quake struck Napa in August 2014. But since July 4, scientists have recorded more than 30,000 quakes in the area around the state’s Searles Valley.
A second earthquake struck Ridgecrest on July 5 – this time with a magnitude of 5.4 – and then a magnitude 7.1 quake ripped through the area just 16 hours later.
Measurements put that biggest temblor just shy of being the worst to hit California in two decades. No fatalities have been reported yet, and only minor injuries, but authorities have recorded a number of fires and buildings that sustained major structural damage.
Still, none of these quakes were the “big one” that seismologists have warned is coming in California.
Instead, the big one – in seismologist language – generally refers to a destructive earthquake of a magnitude 6.7 or higher that occurs along the San Andreas fault, which slices through 800 miles of the California coastline from Eureka to San Bernardino.
And according to experts, that big one long overdue.
“You realise the last big earthquake to hit the LA segment of the San Andreas fault was 1680,” physicist Michio Kaku told CBS News on July 5. “That’s over 300 years ago. But the cycle time for breaks and earthquakes on the San Andreas fault is 130 years, so we are way overdue.”
We’re still waiting for the big one
Seismologists initially labelled the magnitude 5.4 and 7.1 Ridgecrest quakes as aftershocks of the 4th of July event. But they now think the magnitude 6.4 temblor was a foreshock (a seismic precursor) to the magnitude 7.1 event – which was roughly 10 times bigger than the Independence Day quake.
Regardless of these designations, what remains certain is that none of these earthquakes are happening directly on the San Andreas fault line. Because they aren’t part of that fault system, their occurrence doesn’t change the chances that the big one could strike.
“We’re not anticipating these events will cause a big earthquake on the San Andreas Fault,” Paul Caruso, a geophysicist with teh US Geological Survey (USGS), told PBS News.
According to Lucy Jones, a seismologist from Caltech, they also don’t decrease the likelihood of the big one. Earthquakes are a fault’s way of relieving built-up seismic stress, but none of the 3,000 quakes happening near Ridgecrest are releasing any of the geologic tension along the San Andreas fault.
“There’s about a 2% chance of the Big One occurring each year,”Jones tweeted. That’s about a 1 in 20,000 chance of it happening on any given day.
‘A very energetic sequence’
Experts don’t know when the big one will hit California (though they generally agree that it’s a matter of when, not if). But seismologists did correctly warn that after the first quake, southern California residents should expect more ground shaking.
“There is about a 1 in 20 chance that this location will be having an even bigger earthquake within the next few days,” Jones told CBS News on July 5.
Jones’ forecast was right. And the quakes aren’t necessarily over: As of Saturday, the USGS said there’s a 96% chance that up to eight earthquakes with a magnitude of 5.0 or higher will hit California in the coming week.
According to Jones, however, it’s unlikely that the area would see another quake as bad as a 7.1. She told the LA Times that she couldn’t recall any pattern of quakes in which a 7.1 event came after a 6.4 foreshock and was then followed by an even more severe quake.
But it could happen, she warned.
She added: “One should always be preparing for a big one.”
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