Integration of the Physical and Chemical Rock Properties, Structure, and Permeability of the San Andreas Fault, San Andreas Fault Observatory at Depth Borehole, California

Utah State University, Logan UT

Investigators

Abstract

Ten years ago, about 40 m of whole rock core was collected from a depth of approximately 3 km across an actively creeping trace of the San Andreas Fault as part of Earthscope's San Andreas Fault Observatory at Depth Borehole. Every centimeter of SAFOD core sampled is extremely valuable and represents a rare view into an active plate boundary fault. The aim of the overall project is to provide more realistic constraints for earthquakes that occur along the San Andreas Fault and, more broadly, within active faults around the world through the examination of whole rock core samples and a synthesis of a variety of geological and geochemical data. Earthquakes occur as faults displace rocks in a response to stress within the Earth's crust. Before, during, and after an earthquake, the rocks record physical and chemical changes that are associated with this cycle. Earthquakes and associated hazards pose a significant risk to society that may result in loss of life, serious injuries, damage to infrastructure, and major economic impacts. This work contributes to the development of better constraints for earthquake ground-shaking and seismic safety hazard models for active fault systems. Working with undergraduate and graduate students, the project will create engaging and interactive learning modules for undergraduate non-majors and community-at-large events that contribute to the legacy of SAFOD and Earthscope data, and increase students' and the general public's awareness about the field of Earthquake Geology. This study is focused on the structure and composition of the San Andreas Fault at depth using the available range of cuttings, sidewall cores, whole-rock core, and downhole geophysical logging data from Earthscope's San Andreas Fault Observatory at Depth Borehole. Specifically, the proposed research aims to: 1) Examine the mechanisms of deformation and slip localization, the grain- to fault-scale nature of fluid-rock interactions within the fault zone, and to determine the origin, nature, and roles of carbonaceous matter in the fault zone, testing the hypothesis that carbon-bearing fault rocks influence slip weakening and localization or serve as indicators for earthquake induced fluid migration. 2) Couple new results on fault zone composition and structure with borehole-based data to determine the elastic properties of the fault zone, to examine the nature and significance of time-dependent chemical and physical fault zone processes, and to relate the material properties to key elastic parameters that affect the energy distributions in and near fault zones. 3) Synthesize and re-evaluate all published and accessible results by numerous research groups of the geology, geochemistry, and rock properties of SAFOD with our new results, to develop a predictive schematic model of fault zone structure and properties, which we will then relate to its elastic moduli. 4) Educate and mentor students to become competitive STEM Workforce members and effective science communicators through participation in research and in the development of interactive educational activities that contribute to the legacy of SAFOD and Earthscope data, and increasing students' and the general public's awareness of earthquake geology and seismic hazards. The SAFOD core provides an opportunity to rigorously examine in situ fault-rock samples in order to decipher the processes that influence seismic slip and aseismic creep. Systematic integration of microscopy and geochemistry enables us to understand structural diagenesis in dynamic, complex, and heterogeneous fault zones. Remnants of extensive alteration during fluid-rock interactions, and complex microstructural changes during deformation are difficult to resolve without diverse techniques at various scales. The proposed interdisciplinary approach using high-resolution microscopy, geochemistry, and evaluation of rock properties spans a wide range of scales and methods and will further contribute to our ability to decipher the physio-chemical processes (e.g., pressure, temperature, permeability, fluid composition and source) from processes associated with fluid migration at the microscopic scale in faults (e.g., thermal pressurization). The project will expand knowledge of the role of fluids during fault weakening, constrain the conditions associated with fluid migration at the micro-scale during the earthquake cycle, and determine the evolution of strength and slip behavior of major tectonic faults for seismic hazard assessment. The new and synthesized data sets will become accessible to the entire scientific community, as envisioned by the original Earthscope initiative and work towards answering one of Earthscope's outstanding questions identified in 2010: What is the slip distribution during earthquakes and what can we learn from heterogeneities about fault geometry and fault rheology? This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.