Collaborative Research: NSFGEO-NERC: Identifying uniquely coseismic off-fault deformation preserved in fault damage zones: Can we resolve the maximum earthquake size on faults?

Scientists assess potential earthquake hazards along known faults by studying past earthquakes that have occurred along those faults. Because earthquake recurrence intervals may be hundreds to thousands of years for any given fault, the number of earthquakes directly observed on that fault in recorded history is typically one or two at most. Paleoseismologists (geologists who study past earthquakes) can dig further back in time by looking at the geologic record of ancient earthquakes: evidence of past slip events that have disturbed rocks and sediment around the fault.

This is an expensive and time-consuming process that involves excavating parts of a fault, mapping out geologic structures, and determining the absolute dates of materials disturbed by past ruptures. An alternative approach is to study indirect evidence of past earthquake ruptures preserved in the fractured and pulverized rocks surrounding faults in the so-called fault “damage zone”.

A fundamental question in this research project is whether fault damage zones contain information about the maximum earthquake size a fault can host (Mmax) in terms of type, style, extent, width and degree of damage. This project will develop criteria to distinguish damage related to earthquake rupture from damage accrued over the longer-term growth of the fault, and to use these criteria to test the hypothesis that the style and intensity of damage on faults that experience earthquake magnitudes greater than ~Mw6.6 to 6.8 can be clearly distinguished from that of faults experiencing smaller magnitude events.

This project will include a combination of field-based structural geology of crustal scale faults in southern California, cutting edge rock mechanics experiments, and theoretical rock and fracture mechanics to provide a roadmap for identifying uniquely seismic features preserved in damage zones, and to test the overarching hypothesis that the maximum earthquake size a fault can host can be estimated by examining the damage zone structure of active strike slip faults.

Mmax is a critical component of probabilistic seismic hazard assessment (PSHA) as it limits the maximum size of earthquakes considered in a seismic hazard model. Slip rates of faults are the main drivers of hazard, but Mmax controls the upper end of moment release. If Mmax is large, then a significant proportion of the long-term seismic moment release is accommodated by rare large earthquakes.

In PSHA, this decreases hazard, compared with moderate earthquakes, which also generate strong shaking but have higher recurrence rates. Hence, quantitative information on Mmax is a significant aspect of quantifying hazard to critical facilities. Current approaches for determining Mmax can be strengthened by developing independent criteria that allow for Mmax determination without knowing the full paleoseismic history.

This research will lead to the development of criteria to distinguish damage related to earthquake rupture from quasi-static damage accrued over the longer-term fault evolution, and to use these criteria to test the hypothesis that the style and intensity of damage on faults that experience earthquake magnitudes greater than ~Mw6.6 to 6.8 can be clearly distinguished from that on faults experiencing smaller magnitude events. This will be accomplished by carefully documenting the shallow expression of damage zone structure of crustal scale faults in Southern California and examining the unique characteristics of brittle damage that varies as a function of known historical and paleoseismic earthquake magnitudes.

The difference in damage state is likely to be explained by increased energy dissipation by off fault deformation above a critical moment magnitude threshold, and understanding this relation yields the potential for estimating Mmax for active faults with incomplete historical and paleoseismological records. This collaborative study will use field-based structural geology, cutting edge rock mechanics experiments, and theoretical rock and fracture mechanics to provide a roadmap for identifying uniquely seismic features preserved in damage zones, and to test the overarching hypothesis that Mmax can be estimated by examining the damage zone structure of active strike slip faults.

This project has the potential for developing an independent, deterministic criterion for Mmax on individual active faults by examining the damage zone structure.

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.