Why is the southern part of the San Andreas Fault so quiet?

The southern portion of the San Andreas Fault in California hasn’t experienced a major earthquake in 300 years, which is more than a century too long for a “major earthquake” to occur. To understand the “drought out” of this earthquake, the scientists used computer modeling and analyzed 1,000 years of old earthquakes. A new paper shows how lower water levels in the Salton Sea, once part of a much larger lake, could explain why earthquakes have decreased along this southern fault line.

The 800-mile-long San Andreas Fault is one of the largest fault lines in the world. A meeting of the North American and Pacific tectonic plates, this transform rift (where two tectonic plates move in front of each other) runs almost the length of California, from Cape Mendocino in the Redwood Country to the desert landscapes of Joshua Tree National Park. Scientists divide the rift into three distinct regions—North, Central, and South—and while the entire rift presents a clear and present earthquake risk, the southern portion bordering Los Angeles is the most worrisome.

Scientists estimate that over the past 1,000 years, this section of the fault has typically triggered a major earthquake every 180 years (more or less 40). But the Southern San Andreas Fault (SSAF) hasn’t had a good shake for over 300 years (despite what Hollywood movies of mediocre disasters have you believe).

Wanting to better understand this seismic anomaly, scientists from San Diego State University (SDSU) and the University of San Diego’s Scripps Institution of Oceanography investigated why the fault behaves differently today than it did in the past. their answer? Water, or to be more precise, the lack of it. According to their paper — published Wednesday in the journal Nature — declining water levels in the Salton Sea, located at the southernmost tip of the San Andreas fault, could explain why the movement of this part of the fault line has decreased over the past few years. Centuries.

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Mapping the extent of prehistoric Lake Cahuilla and its relationship to SSAF.

University of California San Diego

The Salton Sea is the recent remnant of a much larger prehistoric lake called Lake Cahuila. About 32 times larger than the current landlocked sea, Lake Cahuilla’s water levels have risen and fallen over thousands of years, and using new geological and biological data, scientists have confirmed that the past six major earthquakes along the SSAF occurred when the lake was at its fullest. The frequency of earthquakes in China and around the Dead Sea has also previously been linked to water levels in nearby lakes.

But how does the lake’s completion affect something like an earthquake? Using computer models, the scientists analyzed how the great ancient Lake Cahuilla affected the fault line, and discovered two major influences. First, large amounts of water can cause the Earth’s crust to bend, causing the two plates to open up and allow more seismic motion. Secondly, water can penetrate through cracks in the earth’s crust and increase fluid pressure, which also increases the likelihood of earthquakes.

In a press release, UC San Diego scientists described this hydraulic phenomenon as an air hockey table:

“The effect of increasing fluid pressure in the fault is a bit like an air hockey table. With air running, the disc slides easily, but when the air is cut off, friction makes it difficult to move the disc. Also, as fluid pressure inside the crack increases, water pushes sideways. crack, making it easier for them to overcome friction, slide past each other, and cause an earthquake.

This new research adds more complexity to proposed projects to transfer water from the Gulf of California to the Salton Sea, as adding more water too quickly could lead to deadly seismic activity.

Unfortunately, the dry-like state of the Salton Sea is only one cog in a very complex seismic machine, and not enough to completely stop earthquakes along the SSAF. Scientists estimate that an earthquake of magnitude 6.7 or higher is likely to hit the greater Los Angeles area within the next 30 years as the pressure continues.

When SSAF finally breaks its 300-year silence, it will likely do so with a huge bang.

Darren lives in Portland, has a cat, and writes/edits about science fiction and how our world works. You can find his past stuff at Gizmodo and Paste if you look hard enough.

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