Fault Speedometers, Slip Localization, and Slip Complexity on Exhumed Faults

Utah State University, Logan UT

Investigators

Abstract

Rapid slip events on faults resulting in earthquakes pose serious hazards to millions of people globally. The PI and students will investigate the physics of faulting by studying highly polished, very narrow faults that may be the product of fast slip during ancient earthquakes. The goals of this project are to examine highly polished fault surfaces along the Wasatch Fault, Utah, to determine the mechanisms that created the narrow faults and to determine if these faults formed at high temperatures. If we can determine that high temperatures were the result of high-speed frictional processes, we can use the evidence from the rocks to establish the conditions and processes by which faults develop. The team will undertake both field and laboratory investigations of preserved fault surfaces. The data will help constrain theoretical models of faults that explain how earthquakes nucleate and propagate. These models make predictions about the temperatures of fault rocks, the nature of faulted rock, and the size of the faulted zone. The development of independent fault speedometers has great significance for interpreting the natural seismic fault. Establishing the rate of slip, mechanisms of localization, detailed topography at higher resolution, and kinematics of ancient faults from textures and compositions of fault-related rocks is remarkably difficult, and yet these data are important for constraining models of earthquake nucleation and rupture. One possible indicator of rapid slip is a narrow, highly polished and iridescent slip surface. The Wasatch fault zone in Utah exhumed many extremely narrow, highly polished slip surfaces that are coated with nano- to micrometer scale hematite or quartz. The team will examine the fault surfaces and the thin iridescent materials within them using optical and electron microscopy, compositional analyses, X-Ray diffraction, whole-rock geochemistry and surface measurement methods. They will use detailed micro-scale chemical analyses to evaluate the presence and/or nature of mineral transformations, hydrothermal or non-aqueous alteration processes, and the processes of slip localization. The results of this study will be combined to evaluate the likelihood that these surfaces were produced seismically. The analyses proposed here will document similarities and differences between experimental and natural fault surfaces, and in concert with recent work on the mm-scale lower limit of slip on earthquake-producing faults, will yield insights into the processes that initiate and propagate faults. They hope to use the results to characterize the distribution of asperities at the mm and smaller scale, the mechanisms of slip localization, and their relationships to so-called critical slip distances that may be required for the initiation of earthquakes.