Year of Award

2020

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

Hilary Martens

Commitee Members

Hilary Martens, Rebecca Bendick, Paul Janzen, Michael Stickney

Keywords

seismology, bookshelf faulting, fault plane solutions, absolute locations, relative relocations

Publisher

University of Montana

Subject Categories

Geophysics and Seismology

Abstract

One of the most seismically active regions in the western United States, far from any major plate boundary, is the Intermountain Seismic Belt (ISB). On 6 July 2017, a M5.8 earthquake occurred 11 km southeast of Lincoln, Montana within the ISB. This was the largest earthquake to occur in the state of Montana since the 1959 M7.3 Hebgen Lake earthquake. Data from the University of Montana Seismic Network (UMSN), the Montana Regional Seismic Network (MRSN), and the United States Geological Survey (USGS) were used to investigate the aftershock sequences following the M5.8 Lincoln event. We have manually picked P- and S- wave arrival times, computed hypocenter locations and double difference re-locations, and generated focal mechanisms for hundreds of aftershocks in the two years following the main shock. We characterize the spatial and temporal evolution of the aftershock sequence, identify seismogenic faults, and characterize the local stress field. The main shock focal mechanism together with aftershock patterns indicate left-lateral slip on a NNE-trending fault plane for the main shock. Most aftershocks concentrate near the main shock but smaller clusters of hypocenters form north-south trending clusters to the west of the main shock. We suggest that faults identified oblique to the prominent WNW trending Lewis and Clark Line (LCL) faults may indicate a bookshelf faulting mechanism that accommodates differential extension rates in terrains north and south of the LCL.

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© Copyright 2020 Ellen McGough Smith