Paleoseismology Changes the San Andreas Game

An analysis of rocks drilled from deep beneath the central portion of the San Andreas fault seem to indicate a potential for larger quakes than previously thought.

Beth Johnson

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IMAGE: The San Andreas Fault, on the Carrizo Plain, about 100 miles from Los Angeles. CREDIT: NASA Jet Propulsion Laboratory

A new study in the journal Geology presents an analysis of rocks drilled from a section of the San Andreas fault here in California showing that the slow creep of the faultline may be more dangerous than previously thought.

The San Andreas faultline is nearly 1300 kilometers long and runs the length of the state of California, from Baja in the south to the Mendocino Triple Junction in the north. It’s the dividing line between the Pacific Plate and the North American Plate, which are rubbing alongside each other as one moves north and the other moves south. It has been the site of several major earthquakes in the past couple of centuries, including the 1906 San Francisco earthquake, the 1989 Loma Prieta quake, and the 1994 Northridge quake. All three of these earthquakes resulted in death and destruction and were the result of a sudden, fast, and extreme slip between the two plates. Basically, they stick for a long time and then suddenly slide and cause a massive quake.

But if you know your California geography, you might have noticed something about the locations of those three quakes — two were in northern California and one was in southern California. So what happens in the middle of the state and the middle of the faultline? Not much. It’s not that quakes don’t happen. It’s that they happen pretty regularly but on a smaller scale. The central section of the San Andreas moves at a nice, steady 26-millimeters per year pace. It’s a process known as aseismic creep, and it has generally been thought to be a safe process than the massive slippages of the northern and southern sections.

In fact, the quakes that do occur in this central section tend to happen at the southern end of it and are only around magnitude 6, which is pretty mild by large earthquake standards. For contrast, the 1906 quake was a 7.9 magnitude, and the most deadly quakes around the ring of fire tend to be even greater than that, with several in excess of magnitude 9, including the Tohoku quake in Japan in 2011.

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Beth Johnson

Planetary scientist, podcast host. Communication specialist for SETI Institute and Planetary Science Institute. Buy me a coffee: https://ko-fi.com/planetarypan