Monitoring fault zones using freight trains, a case study in southern California

Trains are powerful seismic noise sources that can be used for monitoring changes in the Earth's subsurface. Unlike the low-frequency ambient seismic noise generated from oceanic activities, trains move along fixed trajectories, allowing us to properly account for the noise source distribution and its coupling between the structure. We propose a fault zone monitoring framework through a case study that focuses on the San Jacinto Fault Zone in southern California using freight trains running through the Coachella Valley. The general steps include constructing a train catalog, extracting body waves through seismic interferometry, measuring travel-time perturbation, and mapping seismic velocity change. We conduct the analysis on the seismic data recorded from 2010 to 2020 and discover an episode of velocity changes in early 2014. Such changes are manifested on all station pairs considered; moreover, some station pairs exhibit positive changes, and others present negative changes. We interpret that this velocity perturbation results from an aseismic slip near the edge of the Anza seismic gap and further validate this idea using numerical simulations. More specifically, we use the Coulomb software to simulate volumetric strain for velocity perturbation and full-waveform modeling to simulate correlation functions for estimating travel-time change. The proposed framework also has great potentials to be applied in other settings, from wastewater injection to CO2 sequestration.