is for the next "Big One," particularly in comparison to Japan. "The potassium-argon tool is pretty interesting, because it really gives us access to a range of faults that we haven't been able to date in the past," Coffey said.LAFAYETTE, La., J/PRNewswire-PRWeb/ - Last week's earthquakes in California illustrate how unprepared the U.S. The researchers plan to apply the potassium-argon method to other faults, including in the New Zealand bedrock, where there isn't any organic material for traditional carbon-14 dating (which only works back to about 55,000 years) and where there are no sedimentary layers to show the marks of very old quakes. Knowing that the fault has this capacity is important for understanding the earthquake hazard in central California, Coffey said. The quakes probably started on the southern portion of the fault and sped along the faultline like an unzipping zipper. "The work that we did was the first direct geologic evidence of earthquakes" in this region of the San Andreas, she said. What should you do when an earthquake hits? In photos: The great San Francisco earthquake However, the magnitude of the heating indicates that the central San Andreas can indeed undergo a lot of shaking - it's likely that the earthquakes recorded in this section of the fault ranged from magnitudes in the mid-6s to low-7s, Coffey said. Part of the ongoing work done by Coffey's collaborators involves improving the potassium-argon method for earthquake dating to narrow down that time span. But the quakes could have been far more recent, Coffey said. Their results suggested that the heating happened, at the earliest, 3 million years ago. By looking at the accumulation of argon, the researchers could determine how long it had been since the rocks were heated. When something happens to heat the rock, this gas is released, resetting the "potassium-argon clock" to zero. This method takes advantage of the fact that a naturally radioactive variation of potassium, potassium-40, slowly decays into argon gas. Next, the researchers analyzed the quake-deformed section of rock with a method called potassium-argon dating. 25 in the journal Geology (opens in new tab). This zone of the fault may have hosted more than 100 quakes, Coffey and her colleagues reported Feb. "It had lots of these small slip layers, so lots of scaley surfaces and shiny surfaces, which is what we would think of as rocks that had hosted lots of earthquakes." "That patch of the fault also consisted of these really highly deformed siltstones, mudstones," Coffey said. Deep in the core, about 1.9 miles down (3,192 to 3,196 meters), the researchers found a spot where the biomarkers showed signs of heating. The researchers analyzed a sediment core from the central San Andreas that was drilled as part of the San Andreas Fault Observatory at Depth (opens in new tab) (SAFOD) project. And those temperature changes can change the structure of organic molecules that accumulate within sediments. This heat can spike the temperature of the rocks in the fault by more than 1,800 degrees Fahrenheit (1,000 degrees Celsius). "It's like rubbing your hands together," Coffey said. To do so, the researchers took advantage of the fact that when a fault slips, it generates friction, which generates heat. She and her colleagues wanted to go back more than 2,000 years. Geoscientists have dug into the fault, looking for signs in the shape of the sediment layers of long-ago earthquakes, and they haven't found any large quakes in the last 2,000 years.īut even if the central San Andreas doesn't build up enough stress to start a large earthquake, it could act as a conduit for quakes originating on the northern or southern section of the fault, Coffey said. In between Parkfield and Hollister, though, the fault hasn't given rise to any recorded quakes larger than a magnitude 6.
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