Distributed acoustic sensing, or DAS, turns ordinary fiber-optic cables into dense seismic arrays by measuring tiny changes in backscattered light. In a new study, researchers analyzed data from a single DAS deployment in a 3,000-meter borehole at the Cape Modern geothermal site in Utah. The array, with 1,600 channels spaced about 2 meters apart, captured seismic waves from thousands of microearthquakes within a few kilometers of the hole, offering a detailed look at how small quakes rupture.
The team found that seismic attenuation was strongest near the surface and generally weakened with depth, though rock type caused some variation. They also estimated spectral stress drop, a measure of shear stress released during rupture, and found it did not depend on earthquake magnitude. That result could help clarify whether small earthquakes release less stress than large ones, an open question relevant to hazard assessment.
The authors caution that two factors could bias future measurements. Cables are more sensitive to seismic phases aligned with their orientation, so directivity matters, and the gauge length over which strain is averaged affects signal-to-noise and high-frequency amplitudes. Comparing DAS source parameters with independent methods, such as empirical Green's function approaches, may help correct for these biases.