Knowledge of the absolute stress on faults is as essential to understanding earthquakes as it has proven elusive to measure. The goal of this project is to estimate absolute stress by measuring the response to a stress perturbation caused by fault surface geometry. Geologic features surrounding faults record this stress perturbation in outcrop and include intense cracking in the host rock (damage zones), striations on fault surfaces, and melted rock. Recently, significant progress in generalizing the interrelationships among total fault displacement, surface roughness and damage. In this project surface geometry and fault features are directly linked through detailed observations of along-strike variations. These variations provide measures of how much the surface irregularities perturbed the stress field around the fault. More specifically, the researchers will: (1) measure tensile fracture patterns surrounding rough and smooth faults or sections of faults and use this measurement to calculate the near-field off-fault stress field; (2) measure the deflection of striations away from bumps on fault surfaces and use this measurement to calculate the background stress relative to the perturbing stress; and (3) measure the along-strike variation in pseudotachylyte in relation to fault geometry and use this measurement to calculate a bound on friction.
Understanding the stress on faults is critical for understanding earthquakes at the level necessary to calculate risk from a physics-based model. This project is designed to fill in this knowledge gap by focusing on the evidence recorded in geological faults. This combined modeling and fieldwork strategy promises results that can be transferable to other fields of earthquake physics.
This project aimed to measure the stress on faults through the geological record of ancient movement. The stress on faults during slip is one of the major unknowns in earthquake physics. In particular, laboratory experiments on rock friction seem to indicate that the stress required to break rock at depth is relatively high, but drops dramatically during earthquakes. However, extrapolating these laboratory experiments to complex, natural systems is difficult. Insight that we can gain from observing rocks is helpful in producing more realistic, physically based models of earthquake rupture. We examined faults that were exposed at the surface for specific indicators of the stress during slip. We found patchy melt on some faults, which indicate that there was high enough stress initially to dissipate large amounts of heat, but only in a very small fraction of the fault. After melting, the fault stress in the patches was nearly zero. The observed fraction of the fault that is melted is consistent with typical seismological observations of the average stress drop during earthquakes and therefore the melt patches may be the geological fingerprint of stress variability during an earthquake. We also measured the deflection of grooves (striations) on a fault surface by the bumps on the fault. Surprisingly, the striations were not strongly deflected as they passed over hills and valleys on the fault surface. This indicates that either the stresses on the fault propelling motion were relatively small, or the bumps on the fault surface were relatively weak. We favor the later interpretation based on calculations of the yield stress on the faults. This observation suggests a fundamentally new picture of faults where the deformable layer near the primary slip surface accommodates significant deformation. If this result is correct, it is transformative and means that the basic formulation of the processes controlling fault motion need to be reexamined. During our studies of exposed faults, we found an amorphous silica gel on a key outcrop in San Francisco, California. Silica gel had previously been suggested as a product of earthquake rupture that can control the final stress, but it had never before been documented in nature. We also developed a new indicator of stress state based on organic geochemical markers developed originally in the petroleum industry. Certain molecules are expected to decompose at the high temperatures that would occur on a fault during slip at high stress. On an old strand of the San Andreas system, we found that the organic molecules were preserved and therefore the fault could never have reached the high temperatures that would indicate high stress. Altogether, the geological data is forming a picture of faults that are relatively weak overall with a minor fraction of the area held by much stronger patches. The strong patches are capable of generating melt locally and probably play a major role in rupture dynamics. This project had a broad impact on the human resources of the burgeoning field at the intersection of structural geology, rheology and earthquake physics. Three postdocs were supported in part by this grant, of which two have already successfully moved on to faculty positions. Five undergraduates also gained their first exposure to research through assistantships on this project.
