A quiet section of San Andreas was experiencing major earthquakes

Geological dating has helped researchers uncover evidence of large earthquakes along the San Andreas Central Fault.

Written by Jay Barber, science writer (@JayBarber77)

Quote: Barber, J., 2022, A Quiet Section of San Andreas That Once Had Great Earthquakes, Temblor, http://doi.org/10.32858/temblor.252

The northern and southern parts of the San Andreas Fault have hosted many earthquakes known in recent history. In contrast, the central portion—the 80 miles (130 kilometers) between San Juan Bautista and Parkfield—remained relatively quiet. However, new research shows that this was not always the case.

Along most of the San Andreas River, earthquakes occur during a dramatic slip between the Pacific and North American plates. On the other hand, the midsection undergoes a slow and steady motion known as “crawl,” which prevents the buildup of seismic energy.

San Andreas forms a 750-mile (1,200 km) section of the Pacific and North American tectonic plate boundaries. credit: USGS

Historically, researchers thought this made large earthquakes in the middle section rare. But rock samples along the fault reveal evidence of many similar events in the past. Therefore, the central section may pose a greater risk than previously thought. Earthquakes that originate on the northern or southern parts of the fault can rupture in central San Andreas, expanding the size and impact of some earthquakes, said Genevieve Coffey, an earthquake geologist at GNS Science and lead author of the study.

The San Andreas Fault crawls along a section between San Juan Bautista and Parkfield (yellow). Credit: Covey et al. (2022)

Earthquakes increase heat

When tectonic plates slam together during an earthquake, the friction generates a sudden burst of intense heat. When this happens, the organic matter in the sediments that make up the surrounding rock can undergo molecular changes, known as ‘biomarkers’. Biomarkers have been widely used in petroleum industry research, but are a relatively new method for locating past earthquakes. Covey and colleagues exploited this phenomenon and found hyperthermic events in sedimentary core samples excavated from fault sections.

However, earthquakes are not the only phenomenon that can generate heat, which means that Covey and her team need to verify that the events identified were caused by movement of tectonic plates rather than, for example, the movement of underground fluids. To do this, they used potassium-argon dating, which takes advantage of the temperature-dependent potassium-argon cycle in rocks.

Under normal conditions, potassium naturally present in clays and other minerals slowly turns into argon through radioactive decay. Argon accumulates inside the clay, and its concentration acts as a temporary since the mineral was formed. When the rocks are exposed to high temperatures, some of the trapped argon is released, skews the calculated age. The researchers found thin groups of rocks along the fault that were smaller than the surrounding rock, suggesting that a short burst of heat “resets” the rocks partly along the fault, while leaving those far away untouched. This localized heating can be caused by slip fault during earthquakes.

A portion of the rock core is taken from the San Andreas Fault. The green rock is serpentinite. Credit: Earthscope

Core samples reveal that more than 100 earthquakes have occurred in this region in the past 16 million years – some 3 million years ago. The largest events reached a score of 6.9. This is the first evidence of such a major earthquake in this department, Covey said.

“You have to have a big slip to make a lot of heat happen,” said Williams Ellsworth, a geophysicist at Stanford University. He said the data indicated a high temperature. “It looks like a big slip fingerprint.” But he said the core samples studied come from only one location along the fault. He said more data will be needed before we know if seismic activity has moved across the entire central section of the fault. Because the biomarkers analyzed in this study are not found everywhere along the fault, the technique may not work to identify more seismic activity, he said.

Earthquakes in central San Andreas in the future

Knowing how this creepy bug behaved in the past indicates how the bug might behave in the future. In this case, the research indicates that the center fault has the potential to cause seismic activity. The researchers said they recommend that their new findings be included in the analysis of future seismic hazards in the region. Ellsworth said he found the team’s evidence of past large earthquakes significant, but “the idea that the San Andreas Central Creeping Fault can erupt in large earthquakes is not a new idea.” He said geologists consider these past events rare but still noteworthy. Ellsworth notes that the California seismic hazard model “explicitly includes large seismic ruptures involving this part of the fault.”

Aerial view of the San Andreas Fault overlooking the Carrizo Plain. Image credit: John Wiley (CC BY3.0)

Although Covey agreed that the geology community has long assumed that the fault’s central section may produce seismic activity, she said this new research provides solid data to support these assumptions. The study could be a step forward due to its use of unique tools. Covey said the technique still needed to be “mastered” to more accurately measure the age of earthquakes. But more work is being done at the Lamont-Doherty Earth Observatory specifically to improve the potassium-argon dating technique.

references

Coffey, G.L., Savage, H.M., Polissar, P.J., Cox, SE, Hemming, S.R., Winckler, G., & Bradbury, K.K. (2022). History of earthquakes along the creeping segment of the San Andreas Fault, California, USA. Geology, 50 (4), 516-521.

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