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.
Faults may either deform by slip that occurs by repeated earthquakes, or creep without large earthquakes occurring. Earthquake slip should produce heat, due to the friction strength of fault rocks. The generation of high temperatures due to frictional heating is expected at seismic slip rates on faults, but few indicators of high-temperature, fast slip exist in the rock record. Finding evidence for this heat helps determine the strength of faults and helps us understand the processes of earthquake nucleation and propagation. We document new evidence for ancient seismicity in rocks that are related to the Wasatch fault, Utah, in the form of iridescent highly polished (mirrored) slip surfaces. High polish does not necessarily mean fast slip, and so we use a range of methods to document that these rocks experienced earthquakes. We see patchy regions and spots of iridescence on metallic highly polished slip surfaces, similar to what can be produced in the rapid cutting of metal in the machine shop. The iridescent surfaces are less-than 10 μm-thick and formed in micrometer to nanometer-scale Fe2O3 [hematite] crystals aligned parallel to the slip surface. We use a novel method, X-Ray photoelectric spectroscopy, which bombards the material with focused X-ray beams to determine the specific electron configuration of elements. For iron, we examine how much oxide iron [rust] lies in the fault surface versus high temper reduced iron occurs. We show that the iridescence is associated with high-temperature reduction at local asperities of iron at temperatures > 570°C. Similar iridescent surfaces have been documented in rotary shear experiments of metals, surfaces, in geologic materials at T> 400 ?C, and in reduced fired ceramics at T > 900° C, forming a pottery luster termed Raku or yohan. We propose that the iridescent-metallic slip surfaces in the Wasatch fault zone are the result of seismic slip. The distribution of thousands of these surfaces in the 100’s m map distance away from the surface traces of the Wasatch Fault indicates that the region experienced extensive seismic-related deformation along the main fault trace and in the footwall damage zone of the fault.
