In situ stress at subduction zones from Borehole Breakouts: The role of sediment rheology

Abstract

Stress is a principal driver of faulting and earthquakes. Direct constraints on in situ stress at subduction zone faults would provide important and relevant insights into earthquake physics. However, in situ stress magnitudes, particularly the maximum horizontal stress, are notoriously difficult to measure directly. One method used to quantify in situ stress is the analysis of borehole breakouts, which provide information about the orientation and magnitude of stress in the crust at the time of drilling. This approach has been widely applied to data from scientific ocean drilling at subduction zones, including Cascadia, Hikurangi, the Japan Trench, and Nankai Trough. Paradoxically, this growing body of work consistently predicts lateral stresses far below thrust faulting conditions; this has been interpreted as evidence for extremely weak faults and/or large stress variations throughout the seismic cycle. However, the widely used and conventional approach assumes that shallow sediments hosting breakouts are elastic. This does not account for realistic sediment rheology, namely material weakening (plasticity or strain-softening) or complete removal of material (zero strength). Here, I propose the development and application of a new approach to quantify in situ stress from borehole breakouts using a finite-element model that accounts for these processes. Preliminary analysis of breakouts at the Japan Trench and Nankai Trough reveals that the previous elastic assumption results in a non-trivial underestimation of stress. This work is an opportunity to illuminate stress conditions that are thought to control shallow coseismic slip and tsunami genesis at subduction zones globally.

Biography

I was born and raised in Minnesota, completing my bachelor’s degree at Carleton College. During my time at Carleton, I developed an interest in structural geology and tectonic processes. After earning my degree, I pursued a career as a field engineer, working with various civil and environmental consulting firms. This role deepened my interest in the study of material properties and laboratory testing methodologies. Currently I am a graduate student at the University of Texas Institute for Geophysics, advised by Dr. Demian Saffer. My research integrates my interests in material properties and tectonics to study how stress accumulates in the subsurface, specifically around major faults. My work explores the spatial and temporal evolution of stress in subduction zones through direct measurements and numerical modeling. In addition, I conduct laboratory testing on natural subduction zone sediments to investigate their material and mechanical properties.