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Session: Urban Seismology [Poster]

Type: Poster

Date: 10/9/2024

Time: 07:00 AM

Room: Stanley Park Ballroom

Characterizing Near-surface Velocity Structure and Site Responses at the MIT Campus Using Telecommunication Dark Fibers with DAS

Dark fibers in the telecommunication network can be useful tools for urban site characterization with Distributed Acoustic Sensing (DAS). In 2022, we collected 5 days of continuous strain-rate data from the dark fibers on the Massachusetts Institute of Technology (MIT) campus in Cambridge, Massachusetts. The cables are buried 2 – 5 feet below ground, lying horizontally in an air-filled conduit. We performed a tap test with a hammer to locate the channels along the cable. The fiber captured environmental signals from human activities such as traffic, passing trains, and construction. We use ambient noise surface waves to investigate the shallow shear-wave velocity structure (Vs). To extract surface waves from ambient noise, we remove large transient signals and calculate cross-coherence between channel pairs using a 1-minute time window. We manage to extract coherent Rayleigh waves at two segments on the cable (40% of the total available length) after stacking in time and bin-stacking coherograms according to the inter-station distance. We use the dispersion relation of Rayleigh waves to invert for Vs structure on the top ~120 m. The resolution is constrained by (1) the length of the target cable segment and (2) twice the gauge length, which place bounds on the longest and shortest wavelength we can reliably resolve, respectively. The velocity profiles show low Vs (0.1 – 0.3 km/s) materials overlying a hard bedrock (1.5 – 1.8 km/s), which results in a strong impedance contrast at 75 m –95 m. Our results agree with previous studies on nearby sites using well logs, active and passive seismic surveys. The transfer functions simulated by the 1D Vs profile show resonance peaks at 0.6 – 1 Hz, which is important to consider for improving the seismic resistance of local buildings.

Presenting Author: Hilary

Additional Authors