SCEC Award Number 21043 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Damage zones of the Elsinore and Superstition Hills Faults: Evidence of a moment-dependent bifurcation in off fault energy dissipation processes?
Name Organization
William Griffith Ohio State University Thomas Rockwell San Diego State University
Other Participants Hannah Gaston, Graduate Student at Ohio State
SCEC Priorities 3d, 3c, 3e SCEC Groups Geology, FARM, SDOT
Report Due Date 03/15/2022 Date Report Submitted 03/14/2022
Project Abstract
We proposed to conduct an initial test of the hypothesis that energy dissipation by inelastic off-fault deformation increases dramatically above Mw 6.6-6.8. We conducted field work in June 2021 and November 2021 to map fault damage zones, measure mesoscopic (outcrop-scale) fracture density and orientation data at varying distances from the fault core, and we collected oriented samples for thin section analysis of microstructure at similar positions. The bulk of the field work was conducted in November 2021 due to pandemic-related travel delays in early 2021, so we requested a one year no cost extension to complete our analysis.

In the Elsinore damage zone at Fossil Canyon, macroscopic damage is in the form of deformation band arrays. Deformation bands occur in several sets, but deformation band density does not decay with distance from the principal slip zone of the Elsinore fault. Instead, deformation bands form clusters around secondary faults within the damage zone. In contrast, at the Superstition Hills fault at Imler Road, macroscopic damage is in the form of discrete fractures, in some cases with minor normal offsets, that occur in one dominant set. Fracture density in these macroscopic damage zone fractures decays with distance from the fault.

We are currently conducting microstructural analysis of oriented damage zone samples, but preliminary inspection suggests that damage zone rocks from the Superstition Hills fault have undergone very little sub-grain fracturing at any distance from the fault, whereas subgrain fracture and shattering is prevalent in the damage zone of the Elsinore fault at Fossil Canyon.
Intellectual Merit The Intellectual Merit of this project is the potential for developing an independent, deterministic criterion for Mmax on active faults by examining the damage zone structure to link field-based observations to physics-based models of rock deformation. This research falls generally under the theme “Beyond Elasticity”, and examines the geologic record directly to answer priorities (1) P3.d. Determine how damage zones, crack healing and cementation, fault zone mineralogy, and off-fault plasticity govern the degree of strain localization, the state and stability of slip (e.g., creeping vs. locked, seismic vs aseismic), interseismic strength recovery, and rupture propagation, (2) P3.c. Assess how shear resistance and energy dissipation depend on the maturity of the fault system, and how these are expressed geologically. (3) P3.e Constrain the extent of permanent, off-fault deformation, and its contribution to geologic and geodetic fault slip-rate estimates, and (4) P2.e Describe how fault geometry and inelastic deformation interact to determine the probability of rupture propagation through structural complexities, and determine how model-based hypotheses about these interactions can be tested by the observations of accumulated slip and paleoseismic chronologies.
Broader Impacts The Broader Impacts of the project are tied closely to the intellectual merit, in that a new technique to estimate Mmax will provide an independent check on Probablistic Seismic Hazard Analysis (PSHA) based on other methods. Furthermore, the potential of using the damage zone as the primary observable allows for the determination Mmax even for faults for which little is known about the seismic history of the fault or the overall fault geometry or slip rate. THis grant has supported the MS research of one female master's student, Hannah Gaston.
Exemplary Figure Figure 3