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Graduate Defense: Spencer Wilbur

May 18 @ 12:00 pm - 1:00 pm MDT

Thesis Information

Title: A Comparison of Three Modern Aftershock Sequences Across Southern Idaho: Characterizing the Seismogenic Zone Surrounding the Snake River Plain

Program: Master of Science in Geophysics

Advisor: Dr. Lee Liberty, Geosciences

Committee Members: Dr. Dylan Mikesell, Geosciences, and Dr. Blaine Bockholt, Geosciences

Abstract

We explore spatial and temporal aftershock patterns related to three instrumentally recorded earthquakes within Idaho. Bordering the eastern extent of the Snake River Plain, within the Intermountain Seismic Belt of Idaho, we compare aftershock sequences related to the Sulphur Peak, Challis, and Stanley earthquakes. Using machine learning we are able to locate low magnitude events; therefore providing a robust temporal distribution of the aftershock sequence for each earthquake. To assist hypocenter location we test a range of velocity models and the AK-135 model to determine the best hypocenter locations and lowest misfit. We increased the number of detected events from: 1,946 to 74,000 for Stanley, 189 to 7,079 for Challis, and 551 to 2,916 for Sulphur Peak. Early aftershock analysis for Stanley suggests that the extent of the Sawtooth Fault may extend farther than what is currently mapped, as results show a migration of events to the North-West extent of the current fault accompanied by a change from an east to west dip direction. The Challis sequence suggests similar preliminary results to the work of others, results that show a slight migration of event locations to the east and an increase in the average depth of events when compared to the USGS catalog. Preliminary results for Sulphur Peak show an increase in depth, with a mean depth of 6.4 km, as well as a migration of events to the north-west and south-east from the mainshock, intersecting the East-Bear lake normal-fault. Using machine learning picks and the localized velocity models we were able to determine the depth of the brittle-to-ductile transition zone and note that the seismogenic zone for Sulphur Peak is found approximately ~5km, less than the depth of the seismogenic zone for Stanley. Parameterizing the machine learning algorithm to work on a regional scale enhances seismic detection and aids our efforts in determining the driving mechanisms responsible for these three significant seismic events.