The amount of ground movement during an earthquake is highly dependent on the properties of the rock and soil just below the Earth’s surface. Modeling studies suggest that ground shaking is amplified in sedimentary basins, over which are often populated urban areas. However, imaging near-surface structure around urban areas at high resolution has been challenging.
Yang et al. have developed a new approach using distributed acoustic sensing (DAS) to build a high-resolution image of the near-surface structure. DAS is an emerging technique that can turn existing fiber optic cables into seismic networks. By monitoring changes in the way light pulses scatter as they pass through the cable, scientists can calculate small stress changes in the material surrounding the fiber. In addition to recording earthquakes, DAS has proven useful in a variety of applications, such as naming the loudest marching band during the 2020 Rose Parade and discovering dramatic changes in the flow of vehicles during COVID-19 stay-at-home orders.
Previous researchers reused a 10-kilometer stretch of fiber to detect aftershocks after the 7.1 magnitude Ridgecrest earthquake in California in July 2019. Their DAS network detected about six times as many small aftershocks as conventional sensors for a period of 3 months.
In the new study, the researchers analyzed continuous seismic data produced by traffic. The DAS data allowed the team to develop a near-surface shear velocity model with subkilometer resolution two orders of magnitude better than typical models. This model revealed that along the fiber, sites where aftershocks produced more ground motion generally corresponded to those with lower shear velocity.
Such fine-scale seismic hazard mapping could improve urban seismic hazard management, especially in cities where fiber optic networks may already be present, the authors suggest.
Dark fiber seismic array discovers missed aftershocks in China earthquake
Yan Yang et al, Sub-kilometer correlation between near-surface structure and ground motion measured with distributed acoustic sensing, Geophysical Research Letters (2021). DOI: 10.1029/2021GL096503
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Quote: Fiber Optic Cables Can Produce High Resolution Underground Maps (February 15, 2022) Retrieved March 19, 2022 from https://phys.org/news/2022-02-fiber-optic-cables-high-resolution-underground .html
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