Scientists imaged hidden fault structures in the Imperial Valley along a canal using techniques typically applied in lakes and oceans. What they found highlights that faults don’t care about human-defined borders.
By Jeng Hann Chong, Science Writer
Citation: Chong, J. H., 2022, First high -resolution subsurface images of faults in the Imperial Valley, Temblor, http://doi.org/10.32858/temblor.258
Coachella isn’t just a venue for the annual music and arts festival. It’s a valley through which the San Andreas Fault slices, tapering off below the waters of California’s Salton Sea, just south of the famed lowland.
In the Salton Trough, the fault branches into several strands extending south into the Imperial Valley, where the side-by-side faults also move away from each other. This “transtensional environment” is distinct from the mostly strike-slip motion of the San Andreas in which the west side of California moves approximately northwest relative to North America. South of the Salton Sea, the primary fault accommodating this motion is the Imperial Fault, but it is unclear how the fault spreads across the rock layers. Like the San Andreas, the Imperial Fault has also produced significant earthquakes, such as a magnitude-7.0 event in 1940 and a magnitude-6.5 earthquake in 1979.
Long ago, a large prehistoric lake — Lake Cahuilla — extended from Coachella Valley through the Imperial Valley, past what is now the border between the U.S. and Mexico. Today, an irrigation project called the All American Canal runs south of the Salton Sea, channeling water from the Colorado River to the Imperial Valley for agricultural purposes.
In the Imperial Valley, extensive sedimentation related to the prehistoric lake along with significant ground surface changes from farming have obscured surface evidence of past earthquakes, says Valerie Sahakian, a geophysicist at the University of Oregon. In a new study published in Geology, Sahakian and a team of researchers, for the first time, seismically imaged faults along the canal. They found new faults, traced existing ones and identified deformation that likely resulted from past earthquakes.
CHIRPing the canal
To study faults under the seafloor, seismologists sometimes use an instrument designed to detect subsurface structures in detail called an acoustic compressed high-intensity radar pulse, or CHIRP, says Sahakian. Aptly named, it also produces a chirping sound, she says.
Instead of deploying the CHIRP in the ocean or lakes, Sahakian and her team sent it into a section of the All American Canal, which is as wide as 40 meters in places, allowing sufficient space to tow the instrument. However, she says, they had to find a way to navigate around metal grates related to canal operations. To solve this problem, they towed the instrument using two trucks on either side of the canal instead of with a boat, and used a boom crane to lift it out of, and back into, the canal, when they encountered any impediments.
Sahakian and her team also struggled with hardened soil, the material with which parts of the canal was built. Most studies of lakes or oceans encounter soft sediments at the interface between water and Earth, whereas in the canal, the hardened soil reflected most of the seismic signal sent by the CHIRP, obscuring the layers underneath and making interpretations harder. Certain frequencies sent by the CHIRP more easily penetrated these barriers, though the return signals are still hard to resolve at deeper depths.
Previous studies using satellites (InSAR) and seismic surveys have proposed the existence of active faults in the Imperial Valley, aside from the Imperial Fault. With the CHIRP, Sahakian and her team imaged about 25 meters below the canal along a stretch of about 13.5 kilometers, and successfully identified several faults. For example, in the westernmost section of their survey, they identified a fault zone likely belonging to the Michoacán Fault, which is currently only mapped in Mexico. People have hypothesized that the fault extends northward into the U.S., says Sahakian.
Now, said Aron Meltzner, a geologist at the Earth Observatory of Singapore at Nanyang Technological University, this study confirms that the Michoacán Fault continues across the border.
Sahakian and her team also found deformed subsurface structures from the 1940 earthquake on the Imperial Fault, and they discovered a deformed basin east of the Michoacán Fault that suggests a slowly evolving basin bordered by faults, or a basin that deforms only when nearby earthquakes strike, she says. However, because the topsoil was removed from the canal during construction, they could only identify evidence of the most recent earthquakes.
Seismic hazards of hidden faults
Because strike-slip faults have little to no vertical offset, they can be difficult to find in regions with high sedimentation rates that quickly bury surface features or in areas with significant human modification of the ground surface, says Meltzner. In the Imperial Valley, both problems are substantial, he notes.
Hidden faults pose a problem for buildings inadvertently located atop faults because they can be badly damaged in the event of an earthquake, he says. Although much of the Imperial Valley is used for agriculture, some of the faults may pass under more densely populated communities, Meltzner says. This study provides a foundation for future studies that could explore the extent of these faults, and provides a basis for a conversation about where to avoid erecting buildings, he adds.
One way to begin would be to continue imaging along the All-American Canal, and branching out to the network of smaller irrigation canals, says Sahakian. And because at least some of these faults span the U.S. border with Mexico, she says that collaboration with scientists in Mexico will be important.
Meltzner agrees, saying that although such collaborations can be logistically challenging, scientists have a strong desire to work together. “Funding does not cross [the] border, but [the] fault does,” says Sahakian.
Sahakian, V.J., Derosier, B.J., Rockwell, T.K., and Stock, J.M., 2022, Shallow distributed faulting in the Imperial Valley, California, USA. Geology; doi: https://doi.org/10.1130/G49572.1
Lindsey, E.O., and Fialko, Y., (2016), Geodetic constraints on frictional properties and earthquake hazard in the Imperial Valley, Southern California. JGR: Solid Earth, doi.org/10.1002/2015JB012516
Persaud, P., Ma, Y., Stock, J.M., Hole, J.M., Fuis, G.S., and Han, L., (2016), Fault zone characteristics and basin complexity in the southern Salton Trough, California. Geology, doi.org/10.1130/G38033.1
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