Date of Award

Spring 2020

Project Type

Dissertation

Program or Major

Earth Sciences

Degree Name

Doctor of Philosophy

First Advisor

Margaret S Boettcher

Second Advisor

Joel E Johnson

Third Advisor

Julia G Bryce

Abstract

Key goals of earthquake science are to understand properties that control where earthquakes start, stop and spread, as well as their ground shaking potential. Stress drop, proportional to slip over the rupture length, provides insight into how variations in material properties of the fault affect the slip behavior of earthquakes. Using data from two ocean bottom seismic experiments, this dissertation explores variations in stress drop in small to moderate magnitude earthquakes in two tectonic environments where heterogeneous fault zones are inferred. On Gofar Transform Fault on the East Pacific Rise, the largest earthquakes (6.0 ≤ MW ≤ 6.2) repeatedly rupture the same portion of the fault, while intervening fault segments host swarms of microearthquakes. Using an optimized spectral analysis procedure, stress drops from 0.04 to 3.2 MPa were found for 138 earthquakes (2.3 ≤ MW ≤ 4.0) that occurred within and between the rupture areas of large earthquakes. A statistically significant higher average stress drop in fault segments where large earthquakes occur compared to fault segments that host earthquake swarms was found. An inverse correlation between stress drop and P wave velocity reduction also suggests that fault zone damage affects the ability of the fault to store strain energy that leads to spatial variations in stress drop. Additionally, lower stress drops following the MW 6.0 mainshock are consistent with increased damage and decreased fault strength after a large earthquake. At Axial Seamount on the Juan de Fuca Ridge, abundant seismicity along caldera ring-faults during inflation and deflation periods of a 2015 eruption reflect deformation induced seismicity. Stress drops from 0.6 to 43 MPa were found for 423 earthquakes (1.6 ≤ MW ≤ 3.6) at Axial, as well as a statistically higher average stress drop during inflation than deflation that suggests a temporal reduction in fault strength as a result of damage associated with the eruption. A lower average stress drop in the northern caldera is also consistent with spatially varying shear wave speed that may reflect a region of pervasive cracking due to dike injection. Lower stress drops in the northern caldera may reflect crustal damage due to diking, or may be a consequence of rupture velocity varying as a constant fraction of shear wave speed. Assuming constant rupture velocity for stress drop, the work presented in this dissertation illustrates how variations in fault zone damage affect earthquake rupture and how this behavior reflects the evolution of fault strength through seismic and volcanic cycles. With improved knowledge of how stress drop evolves through space and time, we can better understand the environments that generate earthquakes and the hazards these earthquakes present.

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