1D Seismic Velocity Models for West-Central and Western Montana

In seismically active areas with infrequent large-magnitude earthquakes, high-quality seismic data are critical for determining accurate seismic velocity models. Here, we present a new local-scale crustal seismic velocity model in west-central Montana as well as a new regional-scale crustal and upper mantle seismic velocity model for broader western Montana. These models are constrained by phase arrivals from several seismic networks, including the University of Montana Seismic Network, the Montana Regional Seismic Network, the Advanced National Seismic System, temporary deployments by the U.S. Geological Survey, and the USArray Transportable Array. We jointly invert for hypocenters and velocity structure using the VELEST software. The local seismic velocity model is the first model specific to west-central Montana, using P-wave arrivals from aftershocks of the 2017 M 5.8 Lincoln, Montana, earthquake. The local model consists of eight layers down to 30 km depth below the mean sea level and spans a region of approximately 40,000 km2 (200 km x 200 km). The velocity of the uppermost layer in the local model is 4.80 +/- 0.11 km/s, and the velocity of the lowermost layer resolved from the data is 7.00 +/- 0.05 km/s. Using an expanded dataset across a broader geographical area, we develop a "regional" seismic velocity model that represents spatially averaged velocity structure across western Montana. The regional model consists of thirteen layers down to 45 km depth below the sea level and is appropriate to an area of 160,000 km2 (400 km x 400 km). The velocity of the uppermost layer is 4.30 +/- 0.15 km/ s, and the velocity of the lowermost layer is 8.00 +/- 0.04 km/s. The new models are similar to prior velocity models for western Montana, but feature improved vertical resolution of layer velocities.