Caltech researchers have unveiled a groundbreaking method to probe the deep structure of the Earth, particularly at the Mohorovičić discontinuity, known as the Moho. This method employs the advanced technology of distributed acoustic sensing (DAS), akin to performing an ultrasound on the Earth. Their study, detailed in Science Advances, capitalizes on how seismic waves from earthquakes reflect off the Moho to gain unprecedented insights into Earth’s subsurface layers.
DAS technology utilizes fiber optic cables—commonly associated with high-speed internet—to act as extensive seismic sensors. Led by Zhongwen Zhan, Ph.D., a professor of geophysics at Caltech, and his former graduate student, now Dr. James Atterholt, the team has refined DAS to send laser beams down these cables. The technique captures how these beams scatter inside the cables when disturbed by seismic activities, such as earthquakes or even everyday traffic, effectively turning the cables into long arrays of seismometers.
This innovative approach allows researchers to interpret the reflections of laser light along the length of the cables to deduce detailed information about seismic shakes. The practical application of this method was demonstrated through the research led by Atterholt, who has since become a postdoctoral fellow at the United States Geological Survey. The team deployed DAS technology to image deep beneath the surface at the Moho boundary, achieving a resolution fine enough to change how geophysicists view this crucial geological boundary.
Traditionally, imaging the Moho, especially in seismically active regions like Southern California, where it lies approximately 45 kilometers below the surface, has been challenging. Previous methods either provided low-resolution images over large areas or were exorbitantly expensive and thus not feasible for widespread use. In contrast, the DAS method can observe the Moho’s structure across extensive regions with up to a kilometer of resolution. This capability offers a much clearer picture of the boundary that delineates the transition from Earth’s brittle crust to the more pliable mantle.
The findings from this method have significant implications. For instance, it revealed new details about the Garlock Fault, the second-largest fault in Southern California after the San Andreas Fault, showing it cuts deeper into the mantle than previously understood. Additionally, the DAS technology provided insights into the subterranean configurations beneath the Coso Volcanic Field in the northern Mojave. This region, which harbors a significant geothermal energy source and has a history of volcanic activity, displayed considerable deformation at the Moho.
Atterholt emphasized the versatility of the DAS technology, noting its ability to uncover details about tectonic interactions, historical and ongoing geological processes, and the structural integrity of the deep crust. The use of DAS in areas with sparse traditional seismic networks or in well-studied locales alike can offer invaluable data about seismic and volcanic activities.
The deployment of DAS in regions like the Mojave Desert over a two-year period underscored its potential to revolutionize our understanding of Earth’s deep structures. According to Atterholt, the creative possibilities with DAS are virtually limitless, providing a new lens through which to view the dynamic processes occurring deep within the lithosphere. This technology not only enhances our geological knowledge but also equips scientists with the tools necessary for more accurate predictions and analyses of seismic risks.
https://phys.org/news/2024-11-technique-deep-tectonic-plates.html