A slow-motion section of the San Andreas fault might not be all that innocuous after all

A slow-motion section of the San Andreas fault might not be all that innocuous after all

Where major earthquakes are thought unlikely, deep rocks say otherwise

most people have heard San Andreas Fault. It’s an 800-mile-long monster that splits California from south to north, two tectonic plates slowly rubbing against each other, threatening to produce a major earthquake.

Lesser known is that San Andreas consists of three main sections that can move independently. In all three, the plates tried to move against each other in opposite directions, like two hands rubbing against each other. In the south and north, the plates are locked together most of the time — glued together in a dangerous, immobile hug. This can lead to stress building up over years, decades or centuries. A breaking point was finally reached; both sides swept past each other violently, and then an earthquake occurred. However, in the central section that separates the other two plates, the plates slide past each other at a pleasant, steady rate of around 26mm per year. This prevents pressure build-up and there are no major earthquakes.This is called Seismic creep.

At least that’s the story most scientists have been telling so far. Now, a study of rock drilled from nearly 2 miles below the surface shows that the Midlands have experienced many large earthquakes, some of which may have been recent. The study used new chemical analysis methods to measure the heating of rocks during prehistoric earthquakes, just appeared on the web magazine’s geology.

“This means we can have bigger earthquakes in the middle than we thought,” said the lead author. Genevieve Coffeywho conducted the research as a graduate student at Columbia University Lamont-Doherty Earth Observatory“We should be aware that there is this potential, it’s not always a constant creep.”

The threat to the San Andreas is enormous. The northern region was hit by the catastrophic 7.9 magnitude San Francisco earthquake in 1906, which killed 3,000 people and leveled much of the city. In addition, the 1989 M6.9 Loma Prieta earthquake killed more than 60 people and collapsed a major elevated highway. The southern region triggered the 1994 M6.7 Northridge earthquake near Los Angeles, which also killed about 60 people. Many scientists believe it is building up energy for an event on the scale of 1906.

By contrast, the central part seems harmless. Only a small area near its southern terminus is known to generate any real earthquakes. There, a level 6 event occurs about every 20 years — not that dangerous by most standards.Because of their regularity, scientists hoping to study clues that might herald an impending earthquake have established a major observatory On top of the fault near Parkfield City. It has a 3.2-kilometer-deep borehole from which cores were extracted, as well as above- and below-ground monitoring instruments. Coffey and her colleagues analyzed rock from near the bottom of the borehole.

The San Andreas Fault, located on the Carrizo Plain, about 100 miles from Los Angeles. (NASA Jet Propulsion Laboratory)

When a seismic fault slides, friction along moving parts can cause temperatures hundreds of degrees higher than the surrounding rock. This cooks the rock, changing the composition of organic compounds in any sedimentary formations along the fault’s path.Recently, study co-authors Pratigya Polissar and Heather Savage figured out how to harness these so-called biomarkers, using altered ingredients Prehistoric Earthquake Map. They say that by calculating how heated the rock is, they can spot past events and estimate how far the fault has moved; from this, they can roughly infer the magnitude of the earthquake. At Lamont-Doherty, they refined the method in the northeastern United States, Alaska, and near Japan.

In the new study, the researchers found many of these altered components in a belt of highly disturbed sedimentary rock lying between 3,192 and 3,196 meters below the surface. All told, they said the black, breakable things showed signs of more than 100 earthquakes. In most cases, the fault appears to have jumped more than 1.5 meters (5 feet). This would translate into an earthquake of at least magnitude 6.9, equivalent to the destructiveness of the Loma Prieta and Northridge events. But many are likely larger, the researchers say, because their methods for estimating earthquake magnitudes are still evolving. The quake in the Midlands could be similar to other large San Andreas events, including the one that devastated San Francisco, they said.

current officials California Earthquake Hazard Modelused to set building codes and insurance rates, does include The possibility of a large rupture in the central section is very small. However, this mathematical possibility is debatable due to the lack of evidence of any such prior event. The new study appears to be the first to show that such earthquakes actually occurred here. They may have originated in the middle, or more likely started in the north or south and migrated through the middle, the authors say.

Magnified microscope photo of sedimentary rock whose structure was altered during the earthquake. The slip layer (in green) is heated during fault movement. The actual field of view is only about a millimeter wide. (Kelly Bradbury/Utah State University)

So, when did these earthquakes happen? The trenches dug by paleoseismologists in the middle show no disturbed soil layers that indicate surface earthquakes over the past 2,000 years—about the limit of detection using the method in the region. But from a geological point of view, the year 2000 was just a blink of an eye. Also, digging could miss any number of earthquakes that might not necessarily disrupt the surface at a particular location.

The researchers used a second new technique to solve this problem. Biomarkers extend along very narrow bands, from microscopic to only a few centimeters wide. Just inches or feet away, the rock is only hot enough to drive out some or all of the argon that is naturally present there.Conveniently for the authors, other scientists have long used the ratio of radioactive potassium to argon, which decays slowly into argon, to measuring the age of rocks. Compared to potassium, the more argon, the older the rock. So if some or all of the argon is dissipated by the heat from the earthquake, the radioactive “clock” resets and the rock looks younger than the same rock nearby that wasn’t heated.

That’s exactly what the team found. The sediments they studied formed tens of millions of years ago in an ancient Pacific basin that subducted beneath California. However, according to the potassium-argon clock, the rocks around the thin seismic slip zone appear to be only 3.2 million years old. This sets a time frame, but only a vague one, as scientists still don’t know how to tell how much argon was vented and thus how thoroughly the clock might have been reset. That means 3.2 million years is just the upper age limit for the most recent earthquake, Coffey said. In fact, some may have happened hundreds or thousands of years ago, she said.The group is now working on a new project Perfect age explanation.

“Ultimately, our work points to the potential for larger earthquakes in central California and underscores the importance of including the central [San Andreas Fault] and other creep faults in seismic hazard analysis,” the authors wrote.

Stanford University geophysicist William Ellsworth, who led the drilling site research, noted that while the state’s official hazard assessment includes a possible large earthquake, “most earthquake scientists believe this is a rare occurrence. , because the tectonic strain is not accumulating at a significant rate, if at all, present along it,” he said.

Morgan Page, a seismologist with the U.S. Geological Survey and co-author of the hazard assessment, said the study broke new ground. “The creep section is a difficult place to do paleoseismological studies because evidence of earthquakes can easily be wiped out by creep,” she said. “If this holds true, this is the first evidence of a large earthquake rupture in this part of the fault.” She said that if a large earthquake can tear the crawling part, it means that it is possible – albeit unlikely – to recover from It starts at the southernmost end of San Andreas, goes through the middle and continues all the way to the end of the northern section – the so-called “big one” that people like to guess. “I’m excited about this new evidence and hope we can use it to better constrain this part of our model,” she said.

How much should this worry Californians? “People shouldn’t panic,” said Lamont-Doherty geologist and study co-author Stephen Cox. “California building codes are very good right now. Seismic events are inevitable. Work like this helps us figure out what the biggest events can be and helps everyone prepare.”

Other co-authors of the study are Sidney Hemming and Gisela Winckler of Lamont-Doherty, and Kelly Bradbury of Utah State University. Genevieve Coffey is now at GNS Science in New Zealand; Pratigya Polissar and Heather Savage are now at UC Santa Cruz.