Shallow Composition and Structure of the San Gabriel Fault, California in Drill Core and Geophysical Logs: Implications for Fault Slip and Energetics

Kaitlyn Crouch

Published 2022, SCEC Contribution #14093

Earthquakes are the sudden and intensive release of energy due to slip along faults. This energy may be felt on the Earth’s surface and may cause displacement of the Earth’s crust (seismic slip). As an earthquake ruptures, rocks in and around the fault are damaged and altered. When a fault displaces without earthquakes, it is referred to as aseismic creep. Faults may experience both seismic slip and aseismic creep throughout their cycles. In order to better model earthquake hazards and understand the cause of seismic slip versus aseismic creep in the shallow crust, we need to characterize the properties of the altered and damaged fault-related rocks.

The San Gabriel fault in California is an ancient strike-slip fault, similar to the modern San Andreas fault, that accommodated slip millions of years ago. In this project, we examine field samples and drill core drilled through the fault-related rocks that formed in the San Gabriel fault at depths of 2-2.5 km. We integrate various techniques to examine the altered and damaged fault-related rock at the mesoscopic (hand sample size) to the microscopic scale in order to characterize the properties of the fault-related rock. Synchrotron X-ray fluorescence (XRF) mapping, a new technique in the examination of shallow faults, allows us to scan surfaces of fault-related rocks and map element location and concentration in the samples. The results document hydrothermal-assisted processes that alter the fault-related rock. We identify deformation and alteration mechanisms that indicate that shallow SGF accommodates both seismic slip (earthquakes) and aseismic creep processes. We suggest that seismic slip versus aseismic creep behavior is influenced by fluid-assisted processes visible in the fault-related rocks at the mesoscopic and microscopic scales and the formation of and changes in clay minerals. This work can be used to better model earthquake hazards in active faults, such as the San Andreas fault in California.

Citation
Crouch, K. (2022). Shallow Composition and Structure of the San Gabriel Fault, California in Drill Core and Geophysical Logs: Implications for Fault Slip and Energetics (Utah State University, LOGAN). https://digitalcommons.usu.edu/etd/8451/