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​​Characterizing Microearthquakes and Shallow Attenuation With Downhole Optical Fibers in the Cape Modern Geothermal Field

We use Distributed Acoustic Sensing (DAS) on the borehole optical fibers to characterize microearthquakes at the Cape Modern geothermal field in Utah. The Cape Modern geothermal site is an enhanced geothermal system. In 2024, a series of stimulations at a 2.5 km depth generated events with M<3 between 1.5 and 3.5 km depth. The downhole fibers provide a dense network that records the wavefield between 0.5 and 2.5 km depth. The body-wave event spectra contain the high-frequency (< 500 Hz) energy required to characterize these small events. We use these spectra to estimate spectral stress drop, source radius, and moment magnitude with Brune’s source spectral model. The spectral-ratio approach for source modeling is advantageous as it accounts for the wave propagation effects and instrument responses using a co-located small event as the empirical Green’s Function (eGF). However, a qualified eGF-target event pair is often not available for microearthquakes. For source modeling using individual spectra, we need the medium properties to estimate the wave propagation effects. We obtain depth-varying attenuation, which was rarely directly observed from seismic data before DAS, by deconvolving event arrivals between DAS channels. The attenuation decreases with depth in the top 1.5 km of sediments. We investigate the source parameters as well as their variabilities between events and channels to estimate the uncertainties and to understand if there is a potential difference in the source physics. We examine the conversion between DAS strain and particle motions, which might distort the shape of the spectra depending on the gauge length and the medium properties. Stress drop for small events has been difficult to measure as shallow attenuating sediments suppress the high-frequency signals and limit the bandwidth crucial for characterizing small events. The downhole DAS, which provides broadband observations and a dense network for sampling the wavefield and the depth-varying medium properties, has great potential to advance our knowledge of the physics of microearthquakes.

Session: Fiber-optic Sensing Applications in Seismology - II

Type: Oral

Room: Key Ballroom 9

Date: 4/15/2025

Presentation Time: 10:30 AM (local time)

Presenting Author: Hilary Chang

Student Presenter: Yes

Invited Presentation: Yes

Poster Number:

Additional Authors