Year of Award

2022

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Geosciences

Committee Chair

Hilary Martens

Commitee Members

Joel Harper, Paul Janzen, Mike Stickney

Keywords

seismology, geophysics, seismic tomography, VELEST

Subject Categories

Geophysics and Seismology

Abstract

In seismically active areas with infrequent, large-magnitude earthquakes, high-quality seismic data are critical for determining high-resolution, accurate seismic velocity models. Here, we present a new local-scale seismic velocity model for the crust in west-central Montana as well as a new regional-scale seismic velocity model for the crust and upper mantle across broader western Montana. The new models are constrained by phase arrivals from several passive seismic networks, including the University of Montana Seismic Network (UMSN), the Montana Regional Seismic Network (MRSN), the Advanced National Seismic System (ANSS), temporary deployments by the United States Geological Survey (USGS), and the USArray Transportable Array (TA). We invert jointly for hypocenters and velocity structure using the VELEST software. The “local” seismic velocity model is the first model specific to west-central Montana, constrained primarily by P-wave arrivals from aftershocks that followed the 2017 M 5.8 Lincoln, Montana, earthquake. The Lincoln earthquake is the largest event to occur in western Montana in more than half a century. The local model consists of eight distinct layers down to 30 km depth below mean sea level and spans a region of about 40,000 km2 (200 km by 200 km). The velocity of the upper-most layer in the local model is 4.80 ± 0.12 km/s and the velocity of the lower-most layer is 7.00 ± 0.05 km/s. Additionally, we estimate station corrections for 50 stations that have operated in Montana during the period 2017-2021. 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. A larger geographical scope and deeper ray paths allow the estimation of the velocity structure of the deep crust and upper mantle. The regional model consists of thirteen distinct layers down to 45 km depth below sea level and is appropriate to an area of about 160,000 km2 (400 km by 400 km). The velocity of the upper-most layer is 4.30 ± 0.07 km/s and the velocity of the lower-most layer is 8.00 ± 0.04 km/s. The new models are similar to prior velocity models for western Montana and include enhanced depth resolution. We find that the new local model for west-central Montana revises the hypocenter locations of Lincoln aftershocks by about 0.89 km on average.

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