Date of Award:

8-2021

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Committee Chair(s)

James Evans

Committee

James Evans

Committee

Alexis Ault

Committee

Tamara Jeppson

Abstract

Earthquakes nucleate at depth and rupture along the fault plane up to the Earth’s surface releasing seismic energy as the fault propagates. This energy creates the shaking we feel on the surface. Some faults do not rupture and create shaking but deform slowly and smoothly accommodating fault slip over extended time periods. This process is referred to as aseismic slip or creep. Whether a fault ruptures or creeps depends on the properties of the rocks through which the fault plane extends. In order to model seismic hazards correctly, we need to characterize the composition, deformation structures, and alteration materials of the fault-related rocks. In this project, we analyze rocks which have experienced deformation and alteration in the uppermost 2 km of the Earth’s crust along the San Andreas Fault at Elizabeth Lake, CA. We investigate the mechanisms which facilitate slip and energy distribution and accommodation within the fault-related damage zone in order to understand how the upper crust responds to deformation events. We identify deformation structures which indicate that slip is accommodated by both earthquake rupture and aseismic creep processes. The alteration we observe in these rocks provide evidence for fluid-assisted processes which serve to decrease the overall rock strength over multiple earthquake cycles, partially heal the damage structures between deformation events, and accommodate some aseismic fault displacement. From our data, we conclude that the rocks within the San Andreas Fault upper damage zone at Elizabeth Lake, CA are distributing deformation throughout the damage zone via a complex of multiple fault strands and microscopic-slip events within the entire volume of impacted rock. We find that most of the fault motion is likely accommodated by brittle rupture mechanisms but also has a component of aseismic slip which may function concurrently over multiple deformation events. Our characterization of the fault damage zone will help to improve seismic modelling, may help to explain the formation of fault-zone low seismic velocities, and describes the upper fault zone structure’s distribution of energy as voluminous, rather than purely localized deformation.

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