1-D Crustal 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 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

1D Crustal Velocity Model for West-Central Montana

Western Montana is home to a significant amount of continuous, complex seismic activity due to the fact that the region lies within the Intermountain Seismic Belt (ISB). The ISB is a 100 km wide seismic belt that stretches along western Montana, reaching from Yellowstone National Park to northwest Montana, and is responsible for much of the state’s seismic activity. Like many seismically active areas, Western Montana’s earthquakes are best studied through the use of seismographs. Data from these instruments can be used to create crustal velocity models for an area within the seismic network. Velocity models are powerful tools that effectively describe seismic velocity as a function of depth and are used to enhance our understanding of an area’s crustal structure, crustal stress conditions, and earthquake hypocenter locations. In areas that are seismically active, it’s important that these models are updated regularly. However, the velocity model currently in use for Western Montana was last updated in 2003. This is because, even though Montana experiences numerous earthquakes every year, many of these earthquakes are low in magnitude, averaging from M 1.0 - M 3.5. These low magnitude earthquakes do not typically produce enough data to create or update velocity models. Oftentimes, we require much larger events (≥ M 5.0), ideally with a strong aftershock sequence. Historically, for western Montana, such events are intermittent and sometimes decades apart. We will derive a new 1-D crustal velocity model for west-central Montana by analyzing seismic-phase arrivals from the M 5.8, 6 July 2017 earthquake that occurred 11 km southeast of Lincoln, Montana, and hundreds of aftershocks that followed over a three-year period (2017-2020). The 2017 Lincoln earthquake was the largest event above M 5.5 to occur in western Montana in over half a century, the last being the 1959 M 7.3 Hebgen Lake earthquake in southwestern Montana. To determine the velocity model, we manually retrieve continuous seismic data recorded by broadband stations in the University of Montana Seismic Network, which have been strategically deployed to study the Lincoln aftershock sequence, supplemented by telemetered data from the Montana Regional Seismic Network. To constrain the model, we invert phase arrivals from several hundred well-recorded earthquakes (\u3e20 phase arrivals) using the software program VELEST. The final model will characterize the crustal velocity structure appropriate to an area in western Montana of about 5000 km2. Due to western Montana’s proclivity towards infrequent, high-magnitude earthquakes, the 2017 Lincoln event has provided a prime opportunity to collect quality seismic data that will allow us to create a much-needed crustal velocity model for this seismically active region of Montana. Not only will developing a new, regional crustal velocity model advance earthquake science in Montana, but this will also be the first model derived specifically for the west-central region of the state, as the current velocity model is most appropriate for southwestern Montana. With our model, we will be able to provide the first accurate crustal velocity structure and method to locate hypocenters for the west-central region

Deriving a 1D Seismic Velocity Model for West-Central Montana

In seismically active areas with infrequent large-magnitude earthquakes, high-quality seismic data is critical for determining regional seismic velocity models. Here, we present the first 1-D crustal seismic velocity model for west-central Montana, constrained by seismic phase arrivals from the 2017 M 5.8 earthquake that occurred near Lincoln, Montana, and hundreds of aftershocks that followed over a three-year period (2017-2020). The 2017 M 5.8 Lincoln earthquake is the only event \u3eM 5.5 to occur in western Montana in over half a century, with the previous being the 1959 M 7.3 Hebgen Lake earthquake in southwestern Montana. To derive the seismic velocity model, we analyze continuous seismic data recorded by 11 three-component, broadband stations in the University of Montana Seismic Network (UMSN), which we strategically deployed to record the Lincoln aftershock sequence. We also include seismic data from short-period, vertical-component stations in the Montana Regional Seismic Network (MRSN); three temporary three-component, broadband stations deployed by the U.S Geological Survey (USGS); and three three-component, broadband Advanced National Seismic System (ANSS) stations. We manually pick P-wave arrival times from several hundred well-recorded earthquakes using the AQMS Jiggle software and then invert these data for velocity structure using the program VELEST. To effectively constrain the structure of the deeper crust and upper mantle of western Montana as a whole, we also derive an updated velocity model for western Montana based on a regional scale dataset that also includes TA data from 2006 to 2010. This final model characterizes the velocity structure of the crust and uppermost mantle as a function of depth, appropriate to an area in western Montana of about 40,000 km2 (200 km x 200 km). Both the local and regional models improve the accuracy of hypocenter locations and advance understanding of the region’s crustal structure