Urban site characterization using DAS dark fibers on the MIT campus in Cambridge, Massachusetts

Abstract

Telecommunication dark fibers with distributed acoustic sensing (DAS) are a useful survey tool for site characterization in urban environments. In this paper, we introduce our five-day student-led DAS experiment using dark fibers at the Massachusetts Institute of Technology campus in the city of Cambridge. The campus has been identified as an area that is highly susceptible to seismic hazards due to subsurface structure and soil properties. The experiment included survey planning, data acquisition, data analysis, subsurface characterization, and site-response estimations. Rayleigh waves collected by dark fibers in the urban environment are mostly from human activities and contain abundant higher-mode energies. We invert the phase velocity dispersions to resolve the shear-wave velocity (VS) in the top 120 m of the subsurface. The VS profiles show low VS (0.1–0.3 km/s) corresponding to unconsolidated materials such as artificial fills and clays overlying a hard bedrock (1.5–1.8 km/s). The depth to bedrock is 75–95 m on the west campus. The site near the waterfront has a lower VS and deeper bedrock. The 1D site-response modeling for shear waves suggests that the fundamental resonance frequency is at 0.6 and 1 Hz, with a sediment-to-bedrock amplitude ratio of 6–7. This should be considered in building design to mitigate seismic hazards. Our results agree with previous studies and can bridge the gap between measurements at nearby sites.