Caltech researchers have developed a new method to study the earth's structure deep beneath the surface, at the boundary between Earth's brittle crust and the underlying mantle, a region called the Mohorovičić discontinuity-Moho for short. Like taking an ultrasound, the method measures how seismic waves from earthquakes are reflected off of the Moho and utilizes cutting-edge seismic technology called distributed acoustic sensing (DAS).
The research was conducted in the laboratory of Zhongwen Zhan (PhD '14), professor of geophysics. A paper describing the study appears in the journal Science Advances on November 27.
Zhan's research has long focused on DAS, a method to transform fiber optic cables like those that provide high-speed internet into makeshift seismometers. With DAS, researchers send laser beams down fiber optic cables; as the ground shakes (whether due to an earthquake or some other disturbance such as the rumblings of traffic), the laser light bounces off of the inside of the cables. Zhan and his team have developed ways to measure this reflected laser light all along the length of the cables and deduce information about the shaking. Thus, the cables act as arrays of hundreds of seismometers.
Now, led by former graduate student James Atterholt (PhD '24), the team has used the DAS technology to image deep beneath the surface at the Moho boundary.
When an earthquake occurs, waves of seismic energy radiate outward from its hypocenter, an earthquake's point of origin underground. When these waves hit the Moho, some amount of that energy is bounced back in the same way that sunlight scatters off the surface of a lake or swimming pool.
On continents, the Moho can be found at depths ranging between 20 and 70 kilometers beneath the surface; in Southern California, the Moho is around 45 kilometers belowground. Attempts had been made to image it using conventional seismometers, but these efforts either yield low-resolution results, on the scale of tens of kilometers, or were prohibitively expensive. With the DAS method, researchers can easily observe the structure of the Moho over large regions at a resolution of a kilometer, providing a far more detailed look at this geologically important region.
"The Moho is a really interesting boundary for seismologists because it tells us what's happening within and between tectonic plates at depth," says Atterholt, who is now a postdoctoral fellow at the United States Geological Survey. "It can tell us whether major faults penetrate into the mantle, how ancient and contemporary processes have left their mark on the continents, and how strong the deep crust is in specific places. Our method can be used to look at all kinds of interesting seismically active regions, including ones we know a lot about, like Southern California, and places where traditional seismic networks are sparse."
Over two years, the team used a fiber optic cable that runs through California's Mojave Desert to measure earthquakes and map the Moho in the region. They learned that the Garlock Fault, the second largest fault in Southern California after the San Andreas, cuts into the mantle and is much deeper than previously thought. They also observed that the Moho is significantly deformed beneath a volcanically active area in the northern Mojave called the Coso Volcanic Field, illuminating part of the region's "plumbing system" between its heat source in the mantle and its magma chamber in the crust. Though the Coso volcanoes last erupted around 40,000 years ago, the region still contains hot geothermal energy. Understanding the subterranean structure of regions like these is an example of the power of the new technology.
"All sorts of things are going on deep within the lithosphere-the rigid outermost part of Earth including the crust and upper mantle-and what you can do with DAS is only limited by your creativity," Atterholt says.
The paper is titled "Fine-Scale Southern California Moho Structure Uncovered with Distributed Acoustic Sensing." Atterholt and Zhan are the study's authors. Funding was provided by National Science Foundation and the Gordon and Betty Moore Foundation.
