2 Earthquakes Strike Off Southern California Coast

Two moderate earthquakes have struck under the ocean near the Channel Islands off the Southern California coast.

The U.S. Geological Survey reports a magnitude 4.5 earthquake struck the sea floor about 14 miles east of San Nicolas Island at 2:45 p.m Sunday.

A magnitude 3.6 quake hit three minutes later in the same spot, 56 miles west of Santa Catalina Island, in an area where the sea floor is about 2,000 feet below the surface.

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Salton Sea Is Swarming With Earthquake Data

It’s one of the great mysteries of Southern California seismology: Every couple of years, the remote desert area around the Salton Sea is shaken by swarms of small to moderate earthquakes that often last several days.

The swarms returned this week, with the area recording more than 200 temblors since Saturday — including several that were felt Wednesday. But this time, scientists had sophisticated instruments in the ground to record the activity, helping them to better understand the swarms and how they can affect seismic risk elsewhere.

Scientists have noticed that the quakes appear to have a pattern, moving southeast as the days progress. But a bigger question remains: Can the quakes trigger larger — and potentially more destructive — quakes along the San Andreas fault, which terminates at the shore of the Salton Sea?

A creep meter on the San Andreas just north of the Salton Sea area, operated by the University of Colorado, found a 0.002-inch slip on the fault right after the largest earthquake in the swarm — a magnitude 4.8 on Tuesday.

Experts say that’s a tiny slip for a fault so large, but the novelty of having that kind of data is tantalizing for scientists.

“If you look at the statistics, they say the odds of something bigger happening is on the order of 1%,” said Susan Hough of the U.S. Geological Survey in Pasadena. “It raises your blood pressure as a seismologist, but we’re trying to read the tea leaves.”

Seismologists have long suspected that quakes in the Salton Sea area can trigger movement on other nearby faults, including the San Andreas. The strongest evidence of this occurred in 1987, when a magnitude 6.2 earthquake near the Salton Sea triggered a magnitude 6.6 quake 12 hours later on the Superstition Hills fault to the south.

Experts are still trying to figure out why the hard-scrabble landscape of desert expanse and small struggling towns around the sea is so seismically active.

The sea sits atop a very thin crust that is being constantly stretched as the North American and Pacific plates grind against each other. The area is also veined by dozens of fault lines that run parallel to and criss-cross one another.

The area experienced regular quake swarms until 1979, when a large temblor just south of the U.S.-Mexico border seemed to curb activity, said Doug Given, a U.S. Geological Survey scientist.

But this quiet period appears to have ended.

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Southern California Quake Swarm: A Precursor to the Big One?

There has been a swarm of earthquakes in one area of Southern California that scientists in Pasadena are watching closely, with more than 20 temblors hitting this morning.

The biggest of the 24 quakes recorded this morning was a magnitude-4.8 which struck at 4:55 a.m. near the Salton Sea in Imperial County, according to the U.S. Geological Survey.

The quake was centered three miles south of the small town of Bombay Beach and 90 miles east of San Diego.

It was followed by a swarm of smaller quakes, which were recorded between 4:58 a.m. and 6:14 a.m. around Bombay Beach. Most of those temblors registered lower than a 3.0-magnitude, officials said.

There were no immediate reports of any injury or damages.

Scores of small quakes have shaken the area in recent days.

The activity has sparked the interest of scientists who want to see if small faults crossing under the Salton Sea are transferring energy to a section of the more dangerous San Andreas fault, which has not popped in more than 300 years.

An earthquake that starts in Bombay Beach and ripples northwest along the San Andreas fault could be the Big One that devastates Los Angeles, Graham Kent, a research geophysicist at UC San Diego, told the Los Angeles Times.

The activity is being monitored by a system run the U.S. Geological Survey and Caltech in Pasadena.

In a 48-hour period starting Saturday morning, 42 quakes shook just south of Bombay Beach on the Salton Sea, ranging in magnitude from 0.5 to 3.3.

via Quake Swarm: A Precursor to the Big One?.

Study Finds Troubling Pattern of Southern California Earthquakes

Large earthquakes have rumbled along a southern section of the San Andreas fault more frequently than previously believed, suggesting that Southern California could be overdue for a strong temblor on the notorious fault line, a new study has found.

The Carrizo Plain section of the San Andreas has not seen a massive quake since the much-researched Fort Tejon temblor of 1857, which at an estimated magnitude of 7.9 is considered the most powerful earthquake to hit Southern California in modern times.

But the new research by UC Irvine scientists, to be published next week, found that major quakes occurred there roughly every 137 years over the last 700 years. Until now, scientists believed big quakes occurred along the fault roughly every 200 years.

The findings are significant because seismologists have long believed this portion of the fault is capable of sparking the so-called Big One that officials have for decades warned will eventually occur in Southern California.

“It’s been long enough since 1857 that we should be concerned about another great earthquake that ruptures through this part of the fault,” said Ken Hudnut, a geophysicist at the U.S. Geological Survey in Pasadena who was not involved in the study.

Many scientists thought the Carrizo area produced relatively infrequent but large-scale earthquakes such as the Fort Tejon temblor. The new work suggests the area produces more quakes but also ones of a smaller magnitude than Fort Tejon, said Ray Weldon, a University of Oregon geologist who was not involved in the research but reviewed the paper for the Journal of Geophysical Research.

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