Urban hazard reduction using dark fiber distributed sensing

Distributed fiber-optic sensing (DFOS) is an emerging technique that can turn a fiber cable into a dense seismic sensor array with meter-scale spacing. Ubiquitously existing telecom fiber cables in buildings and cities render great potential for the use of DFOS in urban hazard reduction studies. One of such aspects is to identify (hidden) fault structures beneath the populated metropolis in the Taipei basin, where the NE-SW Sanchiao Fault and several basement faults transverse. While the east-dipping Sanchiao is capable of generating M 6+ earthquakes right under the urban area, its geometrical and structural characteristics are not well understood due to the absence of outcrops covered by Quaternary alluvial deposits and the lack of high-resolution seismic data. Here, we employ an existing telecommunication cable (owned by Chunghwa Telecom Company) running across the Sanchiao Fault to investigate the fault and basin structure at unprecedented resolution. The ambient noise cross-correlation and beamforming analysis are used to measure multi-frequency Rayleigh-wave phase velocities along the cable and invert for high-resolution shallow shear velocity profile across the fault. The results show a clear velocity contrast between the Taipei Basin and the Linkou Tableland, delineating a clear east-dipping geometry of the Sanchiao Fault. The presence of thick sediments to the east in the basin is also imaged to play a key role in modulating seismic waves for strong amplification and prolonged shaking. Our study evidences that the dark-fiber DFOS is a powerful direction that can offer high-resolution mapping/monitoring of the major fault structures with least cost.