A 4.9-magnitude earthquake in the Ridgecrest area early Friday drove home what seismologists have been telling Southern Californians for the past week:
In the wake of two strong quakes July 4 and 5, that area is likely to keep shaking for years.
And while the chance of another damaging aftershock is small, it will be elevated as long as that increased shaking continues.
“Each earthquake is like a roll of the dice,” said Nicholas van der Elst, a seismologist with the U.S. Geological Survey who’s an expert on aftershocks. “Each has the same probability of being large.”
A 6.4-magnitude earthquake July 4 and a 7.1-magnitude quake July 5 centered in the desert near Ridgecrest – but felt across hundreds of miles – set off a huge jump in seismic activity on that system of faults.
Through Friday morning, July 12, almost 1,400 quakes of magnitude 2.5 or higher (including 28 of magnitude 4.5 or stronger) had struck the area.
More than 10,000 small quakes were recorded as well – and seismologists say even more have occurred but aren’t in the data because they got lost in the noise of the biggest quakes.
The number of 2.5 or stronger earthquakes has been dropping throughout the week — and that’s actually happening at a faster rate than average, Caltech seismologist Lucy Jones has been saying.
Since peaking at 488 on Saturday, July 6, the number of 2.5 quakes dropped to 63 on Thursday, July 11.
In comparison, there were only three in that area in all of June.
So just like a gambler has more chance of winning (or losing) big the more they roll the dice, the Ridgecrest area’s chance of another strong earthquake is going down, but remains much higher than it was before July 4.
“It’s going to take years for aftershock rates to get down to where they were before, and months before local residents aren’t feeling at least small aftershocks regularly,” van der Elst said.
Friday’s 4.9 quake fits with seismologists’ expectations, he said, and doesn’t change expectations for the future. There’s about a 4-in-10 chance of another magnitude-5 aftershock in the next week, and an “almost certain” chance one will hit in the next year.
Remember, we said that the relative number of large magnitude quakes is constant, and more M5s in the sequence would be normal. This morning’s M4.9 is normal and it’s having its own aftershocks https://t.co/Q1FejGJvUI
— Dr. Lucy Jones (@DrLucyJones) July 12, 2019
For a magnitude-6 quake, the odds are about 1 in 100 on any given day in the near future, but there’s about a 6% chance one will hit sometime over the next week and a 10% chance over the next month.
Stronger than that would be a surprise. Van der Elst put the odds of the area having another magnitude-7 earthquake in the next year at about 2%.
“One in 50 is not huge, but it’s not nothing either,” he said.
The best rule of thumb for earthquakes is that they happen where they’re already happening, he said. Two past series of Southern California earthquakes serve as a good illustration of that, and of how long it takes for seismic activity to subside.
The Elmore Ranch-Superstition Hills earthquake series started Nov. 23, 1987 in the desert just south of the Salton Sea, about 80 miles east of San Diego. A 6.2 quake struck that evening, and — similar to how things played out in Ridgecrest — that turned out to be a foreshock to a 6.6 quake that hit a few miles away the next morning.
Aftershocks of 4.6 and 4.7 occurred in December 1987, January 1988 and March 1989. It took about two years for that fault zone to return to its former activity level.
A few years later, another sequence of related earthquakes that lasted even longer began in the San Bernardino County desert. It started April 22, 1992, with a 6.1 quake on the east side of Joshua Tree National Park.
Two months and hundreds of notable aftershocks later, an even bigger quake hit about 20 miles north: The 7.3-magnitude Landers earthquake on June 28. That triggered the 6.3-magnitude Big Bear quake three hours later on a fault about 20 miles west, van der Elst said.
By about 1996, seismic activity was getting back to historical normals. Then, in October 1999 came the 7.1-magnitude Hector Mine earthquake, on a fault about 30 miles northeast of Landers.
Van der Elst said that because of the long intervals between them, most seismologists don’t consider Joshua Tree a foreshock of Landers, or Hector Mine an aftershock.
“Instead, we use the more general term ‘sequence,’ acknowledging that each earthquake in the sequence passes along some tectonic stress to its neighbors, bringing them closer to the breaking point,” he said.
So while most large aftershocks are concentrated in the first few days after a big quake, the hazard does persist for months and years.
If Friday’s shaking – which people more than 200 miles away reported feeling – was a nerve-fraying reminder of that, van der Elst hopes it will also reinforce the importance of preparedness.
“Be prepared with food, water and a plan for reuniting with your loved ones,” he said.
