There is growing concern among seismologists that the 7.2 Mexicali earthquake on April 4 placed more pressure on faults in Southern California, resulting in increased quake activity over the last three months.
The latest evidence of this was Wednesday's magnitude-5.4 Collins Valley earthquake that rolled from the mountains south of Palm Springs, causing no major damage but rattling nerves across the region.
Wednesday's quake was centered in the San Jacinto fault zone — Southern California's most active — which runs 100 miles from the border northwesterly toward Riverside and San Bernardino. Scientists had warned for some time that the Mexicali quake had transferred pressure from the Mexican border area toward the San Jacinto fault and nearby Elsinore fault — which extends 110 miles and could cause major damage in urban areas — making quakes there more likely.
"The probability of a larger earthquake on those faults could be high within the next year or two," said John Rundle, a professor of physics and geology at UC Davis.
Rundle said the aftermath of the Mexicali quake is turning out to be significantly different from the aftermath of the two other large temblors to hit Southern California in the last two decades. Both the 7.3 Landers quake in 1992 and the 7.1 Hector Mine quake in 1999 in the Mojave Desert resulted in aftershocks that dissipated relatively quickly. By contrast, the Mexicali quake has been followed by aftershocks and "triggered earthquakes" that are showing no signs of ending.
"This thing seems to be popping off with lots of small earthquakes, and it's not decaying very quickly … which to me is worrisome, frankly," Rundle said.
Experts are particularly concerned because the northern edges of the Elsinore and San Jacinto fault zones line up, respectively, near the Whittier fault, which runs into Orange and Los Angeles counties, and the San Andreas fault. Both faults could produce catastrophic quakes.
Smaller earthquakes continue at an unexpectedly high level far north of the Mexicali quake in the Inland Empire.
"Under normal circumstances, you have a rather rapid die-off of activity after an earthquake. But in this case, the activity seems to be motoring along at a fairly high level," Rundle said.
In contrast, Rundle's analysis shows that the probability of a large earthquake — for example, a magnitude 7 — has decreased closer to the Mexicali quake area, such as near San Diego.
Rundle is involved in research that will take patterns of smaller earthquakes that have occurred after the Mexicali earthquake and use them to forecast probabilities of where larger earthquakes will be.
"You can actually mathematically map the transfer of stress from one fault to another after an earthquake," Rundle said. "Small earthquakes are a response to the underlying levels of stress. The higher the level of stress, the higher number of small earthquakes one typically finds. We use the patterns of small earthquakes to forecast the occurrence of larger earthquakes."
Another piece of evidence is science stations built on both sides of the earthquake faults. Satellite imagery of the stations shows that both sides of the faults are increasingly moving past each other, indicating growing strain.
Andrea Donnellan, a geophysicist at the Jet Propulsion Laboratory, likened the Mexicali quake to the initial crack in a car's windshield.
"Over time … it cracks more," she said. "The cracked head is where the stress is highest, and that will propagate."
Increased strain on a fault, Donnellan said, looks like a rubber band that is increasingly stretched.
"Stretch it for a while, it will break," she said.
The observations underscore that earthquake faults need to be looked at as a system — similar to how weather patterns in Alaska and on the other side of the Pacific Ocean can affect California.
"You can't look at it in a vacuum. If you look at it from a system perspective where all these faults interact with each other… you can get an idea how a fault can turn on or off an earthquake on another fault," Donnellan said.