Collaborative Research: Paired paleoseismic and slip rate analysis of the central Garlock fault: Towards a true dated path of incremental slip on a major strike-slip fault

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Knowledge of historic and prehistoric recurrence intervals and slip rates for large faults is important for seismic hazard assessment as well as understanding the fundamentals of earthquake mechanics. Long-term records are essential since recurrence intervals for large earthquakes typically exceed modern instrumental records (e.g. Tohoku, Cascadia, etc.). Paleoseismic studies and slip rate measurements are used to assess recurrence intervals and earthquake potential for active faults or fault networks. These studies suggest earthquakes may cluster both spatially and temporally, slip rates may change significantly over time, and some fault networks demonstrate coordinated behavior, but definitive studies in support of these ideas are sparse. Previous paleoseismic and slip rate studies on the Garlock fault, southern California, suggest that it may have experienced fast slip events that correlate with earthquake clusters that alternated with slow slip events, or seismic lulls, that correlate with decreased rates of strain accumulation. This study would provide a definitive analysis of the prehistoric record of earthquake occurrence and fault slip in order to determine if this behavior is typical for the Garlock fault over the past 15,000 years. The results will help the scientific community better understand what controls the occurrence of large earthquakes on systems of faults so that more informed earthquake forecasts are possible. Other desired societal outcomes include full participation of women and underrepresented minorities in STEM, improved STEM educator development through a teacher summer research program, and development of a globally competitive STEM workforce through training of undergraduate and graduate students.There is mounting evidence that the occurrence of large earthquakes on both single faults and fault systems is not a random process. For example, earthquakes commonly cluster in both space and time. The growing recognition that earthquake occurrence can be highly irregular in time and space will eventually call for a new and more sophisticated method of seismic hazard analysis that takes this behavior into account. Before such new methods can be developed, however, it is necessary to understand the nature of phenomena such as earthquake clustering and the conditions under which they occur. This project will develop a detailed prehistoric record of fault slip as a function of time for the Garlock fault, a fault known to have produced earthquakes with pronounced clustering over time. For example, the Garlock fault produced four large earthquakes in the past 2,000 years, no earthquakes between 2,000 and 5,000 thousand years ago, and two earthquakes between 5,000 and 7,000 thousand years ago. This project tests whether this clustering, which has been observed at one location on the fault, is also reproducible at two other locations along the fault and extends the record of prehistoric earthquakes farther back in time to document whether the fault has been seismically quiescent for any other millennia. To do so, paleoseismic and incremental slip rate data will be acquired from the central Garlock fault through: (a) excavation of paleoseismic trenches at three sites, two of which were previously un-datable because of the dearth of datable carbon; (b) documentation of incremental fault slip rates at two sites for offsets ranging from 30-80m; (c) dating of additional small (3-18 m) offsets based on analysis of GeoEarthScope lidar data at several sites to better constrain the dated slip-path during the past few earthquakes; (d) compilation of these and previous data into a comprehensive, published record of incremental slip and paleo-earthquake ages for the Garlock fault, facilitating comparison with similar data from other major faults. Utilization of the new post-IR-IRSL225 single-grain luminescence dating method will allow the research team to determine the ages of previously un-datable strata and landforms.