San Andreas Fault: Tectonic Tremor Detected Deep Beneath Earth’s Surface Raises Risk of Massive Earthquake

America’s most famous fault line, the San Andreas, is known for its frequent earthquakes, but one part of the system, the San Jacinto Fault zone, in inland Southern California, has been surprisingly quiet for the last 200 years. Now new research has detected small tremors deep under the fault system, suggesting it’s not as calm as we once thought and may be ready to release a massive earthquake sometime soon.

The San Jacinto Fault zone in Southern California is not actually a plate boundary but rather serves as the stress release point between the North American Plate and the Pacific Plate as they grind together at the San Andreas Fault. An area of the San Jacinto Fault zone, known as the Anza Gap, is the main focus of the recent study.

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The researchers used a new highly sensitive detection method called multibeam back projection, which calculates plate movement while minimizing incoherent outside noise, to take measurements deep beneath the fault zone in inland Southern California. In doing so, they uncovered previously undetected tectonic tremors, which are likely a result of “slow slips” within the fault, Phys Org reported.

10_26_earthquake Earthquakes, such as the September 2017 earthquake in Mexico, can cause serious damage. New evidence has found that the San Jacinto Fault zone, in California, is not as quiet as we thought and may be due for a big quake. YURI CORTEZ/AFP/Getty Images

The tectonic tremors detected underneath the Anza Gap are the result of slow plate movement resulting in slow earthquakes anywhere from 8 to 15 miles beneath the Earth’s surface. The new research, now published online in the Bulletin of the Seismological Society of America, has revealed that at any given time the Anza Gap is spontaneously slipping at a far greater rate than researchers previously believed.

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The finding is significant because it’s the first time evidence of spontaneous tectonic tremors have been uncovered in this part of the fault line.

“While relatively little is known about tectonic tremors, in part because they have historically been difficult to detect, we know that these tremors are being caused by slow slips deep in the fault,” Abhijit Ghosh, an assistant professor of earth sciences at the College of Natural and Agricultural Sciences at University of California, Riverside and a researcher involved in the project, explained in a recent statement. “This may ultimately help to cause a damaging earthquake.”

The problem is, although Southern California is known for its frequent earthquakes, the Anza Gap has been relatively quiet for the past 200 years—too quiet, some researchers have suggested. According to Ghosh, such a period of tectonic peacefulness raises the question of how the gap has been releasing the stress it continues to accumulate from both the North American Plate and the Pacific Plate. “For that reason, many experts suspect that this area is ripe to produce a damaging earthquake,” the researcher explained.

According to the new report, the recent findings are not cause for imminent concern but rather may help geologists better predict earthquakes in the future. While we cannot prevent seismic activity, preparation can reduce its dramatic toll, allowing us to leave the catastrophic natural disaster scenes for the cinema.

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