Fault Surface Bumps: Prevalence, Causes and Consequences

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Fault surface geometry affects all aspects of earthquake mechanics, yet until recently quantitative field data of fault shapes at the scale of earthquake slip has been scarce. We have begun to attack the problem using a combination of ground-based laser distance measurements and laboratory analyses. This initial work has already increased the available data on fault surfaces by several orders of magnitude and produced new geological observations. The data quantitatively shows that large-slip faults are smoother than small-slip ones. More strikingly, all pristine, large-scale exposures of mature faults that have been measured thus far have distinct quasi-elliptical bumps at the 10's of meter scale. These 3D bumps, known in fault mechanics as asperities, are a new field observation and the focus of the present study. Specifically, the investigators are now determining how commonly such bumps occur on a variety of fault, measuring the relationship between the bump occurrence and surround rock types, constraining the origin of bumps by measuring the total amount abraded rock powder in the bump's vicinity, mapping the occurrence of rock damage near bumps, numerically modeling the bump's stress field on the surround rocks and using seismic data to determine if the earthquakes on mature, smooth faults are different than those on immature, rough ones.