Dhe San Andreas Line in California is undoubtedly the most intensively studied earthquake fault in the world. Nevertheless, there are always new findings about this geological dividing line between the North American and the Pacific crustal plates, which surprise even experts. An American research group has now discovered that severe earthquakes can also occur in a segment of the fault that was previously considered harmless.
The San Andreas Fault stretches approximately 1,300 kilometers through most of California, from the Salton Sea in the far south to Cape Mendocino in the north. Along this stretch, seismologists have divided the fault into several segments, each corresponding in length to the extent of major historic earthquakes. For example, the northern segment is defined by the fault line of the major 1906 San Francisco earthquake, which stretched almost 500 kilometers from a point southwest of what is now Silicon Valley to Cape Mendocino.
Incessant drift of the Pacific plate
The fault is currently blocked in its northern and southern sections. Since the last major earthquakes, all of these segments have accumulated the mechanical stress caused by the incessant drift of the Pacific plate along North America. However, this does not apply to the central fault zone located roughly in the middle of California. There, the two flanks of the fault steadily crawl past each other at the snail-like speed of about 25 millimeters per year. Because of this extremely slow scraping, the rock does not absorb much mechanical stress, and therefore it cannot suddenly fracture in a catastrophic earthquake.
The area labeled “Creeping” corresponds to the apparently quiescent segment of the San Andreas Fault.
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Bild: Adapted from Coffey et al., Geology, 2022
Therefore, the previously accepted geological interpretation was that the seismic risk in the central part of the fault due to this so-called aseismic creep is very low. The geoscientists led by Genevieve Coffey from Columbia University in New York and Heather Savage from the University of California in Santa Cruz have now found clear geological evidence that this conventional wisdom is obviously not correct. The group examined drill core taken directly from the central segment of the fault at a depth of more than 2700 meters a few years ago.
“Cool” molecular structures
It contains sedimentary rocks that were deposited millions of years ago on the sea floor off what is now the west coast of North America. These sediments contain many organic substances that geologists use as so-called biomarkers. One of these substances is the crystalline organic solid phenanthrene, which changes its molecular structure under the influence of heat. As Genevieve Coffey and her colleagues now write in the journal “Geology”, “cool” molecular structures in the four-metre-long drill core repeatedly alternate with structures that could only have arisen under great heat.
Sedimentary rock under the microscope: The structure comes from previous earthquakes. The green area is a layer of rock that has been exposed to intense heat during plate movement.
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Bild: Kelly Bradbury/Utah State University
The question now is where this heat came from. Geoscientists attribute its origin to the well-known phenomenon that an earthquake generates a lot of heat. This is caused by friction when the two flanks of the fault slide past each other at high speed during an earthquake. A similar effect can be achieved, for example, by quickly rubbing the palms of your hands together in winter to warm your hands.
As with warming hands, the heat generated by an earthquake depends on the strength of the frictional force. The harder and faster you rub your hands, the greater the heat generated. Larger tremors release more frictional heat, and the biomarkers get hotter than smaller tremors. The measurements made by the researchers working with Coffey show that magnitude 8 earthquakes must have occurred in earlier times in the central segment of the San Andreas Fault, which was previously considered largely free of earthquakes. So there can be no talk of a low earthquake risk here.