The central segment of the San Andreas Fault may be capable of hosting greater earthquakes than previously thought.
The famed California fault is undergoing aseismic creep between the cities of Parkfield and Hollister.
Instead of accumulating strain and then slipping in a single earth-shaking moment, the two fault portions move imperceptibly, releasing stress without triggering significant quakes. However, looking back millions of years, experts discovered that this portion of fault may have been subjected to earthquakes of magnitude 7 and greater.
That is greater than the magnitude-6.9 Loma Prieta earthquake, which killed 63 people in the San Francisco Bay Area in 1989.
The date of the major quakes on the fault is unknown, although they happened within the last 3 million years, according to Genevieve Coffey, an earthquake geologist at GNS Science in New Zealand.
“The core area should be considered as a potential source of seismic hazard,” Coffey told Live Science.
The San Andreas Fault Zone
The San Andreas Fault is divided into three portions. The southern part, which stretches from the Salton Sea to Parkfield, California, is prone to strong earthquakes. The magnitude-7.9 Fort Tejon quake, for example, displaced the land along the fault 29.5 feet in 1857. (9 meters).
The northern section of the fault stretches from Hollister, California, across the Bay Area, and up to Cape Mendocino, California. This stretch of the fault is well known for the big 1906 San Francisco earthquake, which had a magnitude of 7.9 on the Richter scale.
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However, the fault hasn’t produced any documented quakes bigger than magnitude 6 between Parkfield and Hollister. Geoscientists have dug down the fault, looking for traces of long-ago earthquakes in the shape of sediment layers, but they have found no big quakes in the last 2,000 years.
Even if the central San Andreas does not generate enough stress to cause a big earthquake, Coffey believes it could operate as a conduit for quakes originating on the northern or southern sections of the fault. She and her colleagues wished to travel back over 2,000 years.
To do so, the researchers took use of the fact that when a defect slips, it causes friction, which causes heat to be generated.
This heat has the potential to raise the temperature of the rocks in the fault by almost 1,800 degrees Fahrenheit (1,000 degrees Celsius). Temperature fluctuations can also alter the structure of organic molecules that collect in sediments.
Historical earthquakes
The scientists examined a sediment core dug in the middle of San Andreas as part of the San Andreas Fault Observatory at Depth (SAFOD) project. The researchers discovered a region deep in the core, roughly 1.9 miles below (3,192 to 3,196 meters), where the biomarkers indicated evidence of warmth.
Coffey and her colleagues revealed in the journal Geology on February 25 that this zone of the fault may have hosted more than 100 earthquakes.
The researchers next used potassium-argon dating to examine the quake-damaged portion of the rock. This approach makes use of the fact that potassium-40, a naturally radioactive variant of potassium, slowly decays into argon gas.
This gas is emitted when anything heats the rock, resetting the “potassium-argon clock” to zero. The researchers were able to calculate how long it had been since the rocks had been heated by looking at the argon buildup.
Their findings showed that the heating occurred at least 3 million years ago. However, the quakes might have been much more recent, according to Coffey. Coffey’s associates are now working on refining the potassium-argon technique for earthquake dating to narrow down that time frame.
However, the magnitude of the heating suggests that the central San Andreas can withstand significant shaking – earthquakes reported in this area of the fault are likely to have had magnitudes ranging from the mid-6s to the low-7s, according to Coffey.
“The work we accomplished was the first direct geologic proof of earthquakes” in this San Andreas region, she added.
The quakes most likely began on the southern end of the fault and accelerated along the faultline like a zipper unzipping. Knowing that the fault has this capacity is critical for comprehending the earthquake risk in central California, according to Coffey.
The researchers intend to apply the potassium-argon approach to additional faults, including those in New Zealand bedrock, where there is no organic material for standard carbon-14 dating (which only goes back around 55,000 years) and no sedimentary layers to reveal the signs of extremely old earthquakes.
“The potassium-argon approach is pretty intriguing,” Coffey said, “because it allows us to access a wide range of faults that we haven’t been able to date.”
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