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The Earthquake Beneath Us

Published: June 19, 2017

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PHOTO COURTESY OF JAYNE BORMANN
Jayne Bormann’s field study often finds her aboard a 170-foot research vessel out of San Diego.

The discovery of missing links between earthquake faults shows how a magnitude 7.4 temblor could rupture along the coast beneath Los Angeles, Orange and San Diego counties, says a new study co-authored by geological sciences’ newest faculty member Jayne Bormann.

The study, “Seismic Constraints on the Architecture of the Newport-Inglewood/Rose Canyon Fault: Implications for the Length and Magnitude of Future Earthquake Ruptures,” appeared online in March in the American Geophysical Union’s Journal of Geophysical Research.

Bormann joined the paper’s lead author, U.S. Geological Survey research geophysicist Valerie Sahakian and four other scholars in the discovery that the Newport-Inglewood fault and the Rose Canyon fault are actually one continuous fault system. Because much of the two faults lie underwater, in the past, it was unclear if the faults were connected, explained Bormann, who joined geological sciences at CSULB in January of 2017. However, the new study shows the gaps between fault sections are short enough to represent one continuous fault zone, making it easier for an earthquake to keep Long Beach, Seal Beach and Signal Hill shaking as it races down the longer fault.

Bormann divides her research between twin computer screens in her office and the Pacific Ocean. Her field study often finds her aboard a 170-foot research vessel out of San Diego. Her preferred investigatory tool is low-energy active source seismic reflection that studies the behavior of seismic pulses as they travel through the water and sea floor.

“We send an electrical current across spark plugs in the seawater to create a seismic impulse for the high-resolution work we do,” said Bormann. “The current across the spark plugs creates a bubble. When the bubble pops, that creates a compressional wave. The compressional waves pass through the water and layers in the sea floor, where they reflect back toward the surface. We record those waves on a ‘streamer,’ an arrangement of 48 hydrophones towed behind the boat that record the reflected signals. After 28 days on a recent cruise, we created a three-dimensional volume imaging deformation in the layers of sediment help us learn about the fault zones.”

Bormann also studies the San Diego Trough fault with an eye toward writing its earthquake biography.

“I want to know when the most recent earthquake happened, how much of the fault ruptured in the earthquake, and how much the fault slipped. Ideally, I also want to know about a few earthquakes before that, too,” she said. “I study areas where previous geophysical surveys have identified the best chance of recording sedimentary evidence of shaking during the most recent quake.

“By using sediment cores and seismic reflection profiles to date past earthquakes at Location A, we can determine there this location ruptured in three earthquakes that happened 2,000, 5,000 and 7,000 years ago,” she added. “When we look 50 kilometers down the fault zone at Location B, we might find evidence of two earthquakes, one from 2,000 years ago and another from 10,000 years ago. This tells us that the 2,000-year-old quake may have ruptured the fault zone between these two points, but that the other previous ruptures didn’t extent between the two locations. We keep working down the fault zone collecting pieces of information at different points. They help us to understand the history of earthquakes and compare. We are trying to determine if there is evidence of past an earthquake rupturing the whole fault zone.”

Her current research may upset some scientific apple carts in its tentative conclusions that some faults mapped off the coast of Southern California previously assumed to be active are not.

“I challenge that assumption,” Bormann said. “Large-scale mapping of the seafloor offshore of Southern California using higher-resolution seismic techniques began 20 years ago. The original mapping used data collected by the oil and gas industries. That was great in showing what lay deep beneath the surface. However, it does not provide the information we need about younger sediments to determine if the faults are still active today.”

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PHOTO COURTESY OF JAYNE BORMANN
Jayne Bormann

The recently published study that Bormann coauthored identified the Newport-Inglewood fault zone as active while others are not.

“What begins as the Newport-Inglewood fault in the north ends as the Rose Canyon fault in the south,” she concluded. “Our mapping shows the fault to be continuous from San Diego all the way to Newport Beach. These two faults, instead of being considered independently, should be considered as one structure. That is an important finding for seismic hazard models.”

Bormann earned her undergraduate degree from Whitman College in Walla Walla, Wash, where she joined Whitman’s varsity cross-country ski team then coached professionally for three years. She received her doctorate in 2013 from the University of Nevada, Reno.

She continues her marine geophysical surveys of the San Diego Trough this summer when she joins researchers from the University of Nevada, Reno and the Scripps Institution of Oceanography to collect higher-resolution geophysical data using a Compressed High-Intensity Radar Pulse (CHIRP) sub-bottom profiler.

When Bormann is asked when the Big One will hit, she shrugs.

“It could happen tomorrow or not for another 30 years,” she said. “One of the things I study is earthquake recurrence intervals. If we say a particular fault has a recurrence interval of 150 years, that means that the average time between earthquakes is 150 years. However, a closer look may reveal that the interval between events can be as long as 300 years or as brief as 30. We should always be prepared for earthquakes in tectonically active areas like California.”