Some people get their Fourth of July booms from fireworks. In Ridgecrest, Calif., it came from a pair of violent earthquakes.
The first struck just after rush hour last Thursday, when a 6.4-magnitude quake shook the quiet Mojave Desert community of 28,000. A swarm of aftershocks followed, beginning to peter out that evening. But that jarring jolt proved to be just a “foreshock.”
A day later, Ridgecrest was hit again — this time by a powerful 7.1-magnitude temblor. And a week after the Golden State’s worst quake in two decades, Southern California residents are still being jarred by relentless shaking.
At least 1,000 aftershocks of 2.5 magnitude or greater have occurred in the days since last week’s twin tremors, including a 5.5 magnitude just a half-hour after the main quake. And experts say it’s not over yet.
Last Saturday, Caltech seismologist Lucy Jones tweeted that there was “about a 1 in 10 chance that the Searles Valley [would] see another [7 magnitude].”
Since then, the trend has been encouraging — only one aftershock of 4.0 strength or greater has rattled the Ridgecrest area. But the U.S. Geological Survey (USGS) posted a bulletin on its website warning residents to “be ready.”
“It is likely there will be smaller earthquakes,” read the statement, citing a 99 percent chance of additional 3.0-magnitude quakes or greater. “As few as 40 or as many as 84 such earthquakes may occur.”
USGS estimated a little less than a 50/50 chance of a 5.0 magnitude or greater. The odds of a 6.0 are even slimmer — only 6 percent. And as of Tuesday afternoon, more good news — the chance of another 7.0 dropped to “1 in 200.”
Every earthquake is followed by aftershocks. The bigger the main quake, the bigger the aftershocks. With most earthquakes, the aftershocks are too small for us to feel, and sometimes even too minute to be detected. But with a main quake as big as last week’s, the concern is that even the aftershocks could be big enough to spur additional damage.
Experts say the aftershocks are fading faster than expected. The aftershocks -- some flirting with 5.0 magnitude -- from the Nov. 30 earthquake in Anchorage were felt nearly two weeks after the 7.1 quake But since Monday, only two aftershocks in the Ridgecrest sequence have topped 4.0 magnitude. Seismologists expect this decline to continue.
“The decay rate of this sequence is a bit faster than the average seen in California,” wrote Morgan Page, a seismologist with the USGS in Pasadena, Calif. “We don’t know the physical reason as to why the decay rate is different, but it is good news as far as aftershock probabilities go.”
Contrary to popular belief, last week’s quakes did not occur along the San Andreas fault. They peppered the landscape 85 miles to the northeast. Barbara Romanowicz, a professor of geophysics at the University of California at Berkeley, explained that a little-known entanglement of faults is what brewed the shaking.
“The earthquake occurred on a fault which is part of a messy system of faults called the ‘eastern California shear zone,’” Romanowicz wrote. “It runs northward from the Palm Springs Area through Owens Valley.”
The fault has been quiet lately, but hasn’t behaved. “It was the locus of largest known California earthquake,” according to Romanowicz. At 2:30 a.m. on March 26, 1872, a 7.8 to 7.9-magnitude quake thrust the ground up to 20 feet vertically and 40 feet horizontally. This time around, the lateral “slip” was closer to 18 to 20 feet.
Last week’s quakes triggered along one of these fault zones. The quakes were primarily “strike slip,” meaning that one tectonic plate is sliding past or against another.
“This fault is a right-lateral fault, which means if you stand on one side of the fault looking across it, the ground on the other side of the fault moved to the right in the earthquake,” Page said. “The San Andreas is not very close to the epicentral area.”
About 80 percent of the stress built up within the plates beneath California is expelled in earthquakes along the San Andreas fault. Less than a quarter is released along other fault lines.
Most of the big quakes in California spring from sideswipes at the San Andreas fault. But that may not always be the case. Seismologists think that the eastern California shear zone could take over in the distant future.
“It is thought that this relative wide zone of faults will eventually become the main plate boundary between the Pacific Plate and the North American Plate,” Romanowicz explained.
Shaking from Friday’s 7.1-magnitude earthquake was felt as far away as Sacramento and Las Vegas. Residents of both cities described the ground as “rolling.” Those are Rayleigh waves.
When an earthquake strikes, it sends out body waves and surface waves. The body waves propagate through the earth very quickly. We ordinarily don’t feel those much unless we’re extremely close to the epicenter — but seismometers do. Because they move faster than surface waves, it’s possible to process body wave data rapidly with computers and disseminate warnings “downwind” before the damaging surface waves arrive. An effort to build such a system in the United States — called “Shakealert” — is in its development phases.
Rayleigh waves are a form of surface wave. They move the ground in vertical circles. It’s what most folks felt last week.
“It is normal for Rayleigh waves to be the primary wave felt at large distances from a large earthquake,” Page wrote. “In this case, it was the primary wave felt in Los Angeles, 100 miles away from the epicenter.” And the farther one is from the epicenter, the longer the shaking lasts.
“The surface waves they felt were already ‘dispersed,’” Romanowicz wrote. “The different frequencies arrived at different times.”
In the meantime, those in California — which escaped the pair of quakes fatality-free — are counting their blessings that the quake didn’t strike near the more populous areas along the coast.
The USGS estimates a similar-magnitude quake along San Francisco’s Hayward fault could cause 800 or more deaths. Seismologists are pointing to this past week’s quakes as a wake-up call to be ready for the next big one.
The USGS echoed that sentiment last October. “Scientists are convinced that the Hayward Fault has reached the point where a powerful, damaging earthquake can be expected at any time.”