SCEC Award Number 23081 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Bridging nano- and macroscale observations to investigate shallow on-fault deformation of the southern San Andreas fault, Coachella Valley segment
Investigator(s)
Name Organization
Alexis Ault Utah State University Greg Hirth Brown University
Other Participants Alexandra DiMonte (PhD student, USU); Cameron Meyers (Scientific Research Engineer, Brown); Brian R. Van Devener (Lab Manager and Surface Scientist, University of Utah)
SCEC Priorities 3c, 3d, 2c SCEC Groups FARM, Geology, SDOT
Report Due Date 03/15/2024 Date Report Submitted 03/15/2024
Project Abstract
 This project is a continuation of SCEC 22082 and investigates “on-fault” and off-fault material properties of the shallowly-exhumed Coachella Valley segment of the southern San Andreas fault (SSAF). We target exposures in Mecca Hills because rocks deforming at shallow depths today are accessible at the surface and characterize multiscale textures, structures, mineralogy, frictional properties, and shear wave velocity structure from samples and locations across the SSAF near Painted Canyon. SSAF clay-rich gouge is texturally and compositionally heterogeneous at multiple scales, and comprises dominantly quartz, feldspar, and clay (illite, kaolinite, smectite). The presence of striated Mn-oxides and Fe-oxyhydroxides and undeformed salt along well-developed slip surfaces supports late-stage groundwater interaction coeval with to postdating slip surface development. Macro- to microscale structures support mechanical “mixing” between different gouge components and sandstone. Structural observations suggest clay-rich slip surfaces develop through repeated shortening with subsequent strike-slip localizing on sub-vertical and favorably oriented clay-rich layers. Active and passive source surface wave testing show low shear wave velocities (≪1 km/s) in these materials to at least 100 m and 3-D spatial heterogeneity across the SSAF. Deformation experiments on the SSAF red clay gouge highlight that even nominally dry materials have low healing rates due to the presence of clay. The presence of fluids (e.g., groundwater) further weakens the clay-rich gouge and promotes ultralow healing rates, however, additional work is required to evaluate these relationships.
Intellectual Merit As SCEC5 draws to a close, an unresolved question is how the material properties and development of on- and off-fault damage in the shallowest portions of fault zones impact earthquake behavior and radiated seismic energy. These factors can suppress or enhance earthquake rupture propagation to the surface, contribute to a shallow slip deficit, attenuate or amplify seismic waves, and modify the intensity of ground shaking. In addition, although the SCEC Community Rheology Model has characterized the ductile rheology of southern California’s lithosphere in response to variable strain rate, effective viscosity, and differential stress, an outstanding objective is to characterize the brittle-plastic rheology of the upper crust, mirroring key SCEC5 science questions (Q3: P3c, P3d; Q2: P2c). In 23081 and 22082, we address these questions by characterizing the structural, mechanical, and thermochemical properties of rocks along the mapped trace of the SSAF in Mecca Hills that have been exhumed from depths of less of <1 km.
Broader Impacts This SCEC project, a continuation of 22082, continues the collaboration between a female scientist at Utah State University (Ault) and scientist at Brown University (Hirth) and the enhanced research and educational connections between all PIs and participants. Research activities and SCEC funding support the continued field and laboratory research and training of female USU PhD student Alexandra (Alex) DiMonte. This project also provides support for Senior Research Technician Dr. Cameron Meyers (Brown), who mentored DiMonte on operation of the Instron rotary shear apparatus at Brown University. New 23081 efforts establish new partnerships through collaborations with an early career researcher, Dr. Srisharan Shreedharan, an expert in rock mechanics and frictional healing, and earthquake engineer, Dr. Brady Cox.
Exemplary Figure Figure 3. (A) Field photo looking NW along mapped trace of SSAF highlighting the width of the “red clay gouge”. In detail, gouge is structurally (B,C) and compositionally (D) heterogeneous. (E, with insets) Example of an east-west striking subsidiary gouge zone with both vertical and horizontal slickenlines expressed in different but adjacent layers. (F) Mn-oxide precipitates on clay slip surfaces observed in the field and via SEM (G). (H) SEM image showing that some clay slip surfaces comprise comminuted clay particles and clay aggregates.
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