Towards seismic velocity monitoring using anthropogenic seismic signals

Anthropogenic seismic signals attract more and more attention in recent years. Freight trains, among different seismic sources, are of particular interest for seismic velocity monitoring due to several advantages. Trains are persistent, powerful sources that generate seismic tremors equivalent to Mw 2 earthquakes and detectable up to 100 km distance; trains move along fixed trajectories, allowing us to properly account for the source distribution and its coupling between the structure; trains generate high-frequency body-wave energy enabling us to focus on the changes at depth with high spatial resolution. We propose a fault zone monitoring framework through a case study in southern California. Freight trains running through the Coachella Valley are used to monitor changes associated with the San Jacinto Fault. The general steps include identifying sources and constructing a train catalog, extracting body waves through seismic interferometry, measuring travel-time perturbation, and mapping seismic velocity change. We analyze the seismic data from 2010 to 2020 and discover an episode of velocity changes in early 2014, manifested on all station pairs considered. The velocity perturbation shows a complicated spatial pattern, with some station pairs exhibiting positive changes, and others 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. 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 has great potentials to be applied in other settings, from wastewater injection to CO2 sequestration, using freight trains or other type of anthropogenic sources.