They say quakes along the central section may have been similar to other large San Andreas events, including the one that destroyed San Francisco. But many could well have been larger, say the researchers, because their method of estimating earthquake magnitude is still evolving. This would translate to at least a magnitude 6.9 quake, the size of the destructive Loma Prieta and Northridge events. In most, the fault appears to have jumped more than 1.5 meters (5 feet). In all, they say the blackish, crumbly stuff shows signs of more than 100 quakes. In the new study, the researchers found many such altered compositions in a band of highly disturbed sedimentary rock lying between 31 meters below the surface. At Lamont-Doherty, they refined the method in the U.S. Recently, study coauthors Pratigya Polissar and Heather Savage figured out how to take advantage of these so-called biomarkers, using the altered compositions to map prehistoric earthquakes.They say that by calculating the degree of heating in the rock, they can spot past events and estimate how far the fault moved from this, they can roughly extrapolate the sizes of resulting earthquakes. This cooks the rocks, altering the makeup of organic compounds in any sedimentary formations along the fault path. When earthquake faults slip, friction along the moving parts can cause temperatures to spike hundreds of degrees above those of surrounding rocks. The San Andreas Fault, on the Carrizo Plain, about 100 miles from Los Angeles. It was rock from near the bottom of the borehole that Coffey and her colleagues analyzed. It features a 3.2-kilometer-deep borehole from which rock cores have been retrieved, and monitoring instruments above and below ground. Because of their regularity, scientists hoping to study clues that might signal a coming quake have set up a major observatory atop the fault near the city of Parkfield. There, magnitude 6 events-not that dangerous by most standards-occur about every 20 years. Only one small area, near its southern terminus, is known to produce any real quakes. The central section, by contrast, appears harmless. Many scientists believe it is building energy for a 1906-scale event. The southern section caused the 1994 M6.7 Northridge earthquake near Los Angeles, also killing about 60 people. Also, the 1989 M6.9 Loma Prieta quake, which killed more than 60 and collapsed a major elevated freeway. The northern section hosted the catastrophic 1906 San Francisco magnitude 7.9 earthquake, which killed 3,000 people and leveled much of the city. The threats of the San Andreas are legion. “We should be aware that there is this potential, that it is not always just continuous creep.” “This means we can get larger earthquakes on the central section than we thought,” said lead author Genevieve Coffey, who did the research as a graduate student at Columbia University’s Lamont-Doherty Earth Observatory. The study, which uses new chemical-analysis methods to gauge the heating of rocks during prehistoric quakes, just appeared in the online edition of the journal Geology. Now, a study of rocks drilled from nearly 2 miles under the surface suggests that the central section has hosted many major earthquakes, including some that could have been fairly recent. (Adapted from Coffey et al., Geology, 2022)Īt least that is the story most scientists have been telling so far. Rock samples from almost 2 miles down were taken at the San Andreas Fault Observatory at Depth, or SAFOD, marked by the red star. The “creeping” central section, subject of a new study, is in yellow. This is called aseismic creep.Ĭalifornia’s San Andreas Fault. This prevents stresses from building, and there are no big quakes. However in the central section, which separates the other two, the plates slip past each other at a pleasant, steady 26 millimeters or so each year. Finally a breaking point comes the two sides lurch past each other violently, and there is an earthquake. This causes stresses to build over years, decades or centuries. In the southern and the northern sections, the plates are locked much of the time-stuck together in a dangerous, immobile embrace. In all three, the plates are trying to move past each other in opposing directions, like two hands rubbing against each other. Lesser known is the fact that the San Andreas comprises three major sections that can move independently. It’s the 800-mile-long monster that cleaves California from south to north, as two tectonic plates slowly grind against each other, threatening to produce big earthquakes. Most people have heard about the San Andreas Fault.
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