Project Abstract
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This report presents the first attempt to combine earthquake chronology and slip per- event estimates by evaluating structural relationships across a step-over at the Frazier Mountain paleoseismic site on the Southern San Andreas Fault (SAF). Transtensive step-overs, known as sags, are ubiquitous features of strike slip faults. At the Frazier Mountain site, the main trace of the southern San Andreas Fault steps to the right 40 m over 150 m along strike. Within the step are two adjoining synclines ~30 m x 70 m and ~10 m x 40 m in size. 34 paleoseismic trenches and 35 cone penetrometer tests spanning the step-overs show stratigraphic and structural relationships that demonstrate incremental coseismic fold and fault deformation. Rapid sedimentation generally buries the sag produced in an earthquake flattening the ground surface before the subsequent event. We quantify structural relief in individual ruptures across the step-overs using surveyed 3D point data for five key stratigraphic surfaces. 3D analysis suggests that three of four prehistoric events are similar in size to event FM1, the 1857 M 7.9 earthquake. Event FM4 is difficult to separate from the stratigraphic proximal FM5 but appears to be smaller than average. FM5 is slightly larger than other earthquakes, but is within typical along strike variability of displacement for surface rupture, or there is a previously unrecognized event between FM5 and 6 that is only recorded by folding in the investigation area excavated to date. Offset channels near the site suggest the 1857 earthquake (event FM1) generated ~5 m of lateral slip, and we document ~ 0.8 m of folding across the larger sag. We use the relationship between lateral slip on the fault and incremental sag deformation and find that 4 of the last 5 events produced ~ 4 – 5 m of lateral slip at the site. |
