![]() ![]() 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. By calculating the degree of heating in the rock, they say they can spot past events and estimate how far the fault moved, and from this, extrapolate roughly the sizes of resulting earthquakes. Polissar and Savage figured out how to take advantage of these so-called biomarkers, using the altered compositions to map prehistoric 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. 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. (Adapted from Coffey et al., Geology, 2022) 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. Our solution is to use geochemical proxies to identify rocks that have experienced significant earthquake heating in the past, and to estimate the degree of heating.”Ĭalifornia’s San Andreas Fault. ![]() “Paleoseismic records using techniques such as carbon dating can push us back tens of thousands of years, but are only preserved in certain settings. “One of the goals of this work is to address the problem that the largest, most dangerous earthquakes are often rare and therefore either sparse or absent from the historical records used to estimate earthquake hazard for engineering and social planning,” said Savage, an associate professor of Earth and planetary sciences at UCSC. “We should be aware that there is this potential, that it is not always just continuous creep.”Ĭoauthors Pratigya Polissar and Heather Savage, now at UC Santa Cruz, developed a new technique that enabled the team to assess the history of earthquake slip on the fault. “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, published February 25 in Geology, uses new chemical-analysis methods to gauge the heating of rocks during prehistoric quakes. 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. This is called aseismic creep.Īt least that is the story most scientists are sticking with so far. This prevents stresses from building, and thus there are no big quakes. On the other hand, 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. Then 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. But in the southern and the northern sections, the plates are locked much of the time-stuck together in an immobile, poisonous 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. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |