Seismologists are not known for their subtlety. So it should come as no surprise that a genre of quakes characterized by the way they wrench apart sections of Earth’s crust, leaving behind gashes or scarps, are known in the field as “ground-rupturing” earthquakes. These quakes can be devastating to society, especially if the ruptures tear through pipeline systems, railway lines, or neighborhoods.
In a new study, Scharer et al. shed light on 11 ground-rupturing earthquakes that have occurred along a section of the San Andreas Fault throughout history, using paleoseismic data from as far back as 400 CE. As it’s used here, “paleoseismic” refers to evidence of ground-rupturing earthquakes that have been preserved in the geologic record: layers upon layers of sediment and organic matter sequentially deposited and then torn apart by the fault.
The San Andreas Fault, stretching along most of California, is arguably the most famous fault in the world, as it happens to traverse two highly populated areas, Los Angeles and San Francisco. For their study the team analyzed a site called Frazier Mountain, located northeast of Los Angeles, between Bakersfield and Santa Barbara on a southern section of the fault known as the Big Bend.
The researchers conducted more than 30 excavations, compiling a complete record from 800 CE to the present. They documented how the structure and sediment composition of the area have evolved over time, particularly a small pull-apart basin (the area between two parallel strike-slip faults where the crust has been pulled apart) that has been repeatedly wrested and warped by seismic activity. A relatively high rate of sedimentation and an abundance of organic material, which can be used for radiocarbon dating, allowed them to fill in any gaps in the record from 800 CE to the present.
On average, earthquakes have ruptured the ground at this site about every 100 years for the past 1200 years, the study found, a frequency that roughly matches up with other sites nearby. According to the study, time periods between the 10 ground-rupturing earthquakes since 800 CE were as short as 20 years and as long as 200 years. The most recent earthquake in the series was a magnitude 7.7–7.9 earthquake that shook at Fort Tejon, a former U.S. Army outpost, in 1857.
Researchers compared the timing of paleoearthquakes along the San Andreas Fault to investigate the size of the ground-rupturing earthquakes that happened before the Fort Tejon quake. They did this by dividing the slip experienced by the fault over the time period, then calculating the rate of slip. Assuming that the earthquakes were responsible for all the fault’s slip over the time period, the authors could estimate the magnitudes generated by each event.
Through this method, the authors concluded that only one other earthquake similar in size to the 1857 Fort Tejon quake could have occurred since 1350 CE, around 1550 AD. This rupture would have moved along multiple sections of the fault stretching over 300 kilometers, from the Carrizo, around the Big Bend, and through the Mojave section. Minimum magnitudes for the other ruptures correlated by timing are M 7.3. The analysis, the authors conclude, emphasizes the recurrent hazards posed by this infamous fault. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2016JB013606, 2017)
—Sarah Witman, Freelance Writer