UPLIFT PATTERNS ASSOCIATED WITH THE COACHELLA VALLEY SEGMENT OF THE SAN ANDREAS FAULT

Detailed three-dimensional surfaces for active faults in southern California have been compiled within the Southern California Earthquake Center Community Fault Model (CFM) based on data from geologic maps, wells seismic reflection data, and microseismicity. The CFM provides data for a variety of models, including models that simulate earthquake ruptures, models that invert geodetic data for slip, earthquake probability models, and models of fault stressing. Validation studies confirm that the CFM well represents active fault geometry in some portions of southern California, but one region of lingering mismatch is the Coachella Valley segment of the San Andreas fault (SAF). For example, prior model uplift patterns have not matched the pattern of sedimentation in the Coachella valley and rapid ongoing uplift in the Mecca Hills northeast of the SAF.

The Coachella Valley segment of the San Andreas consists of a single main strand that branches into the Mission Creek and Banning strands in the northwest. While the fault is currently modeled as vertical, seismic and structural studies suggest a steep (60-70°) northeast dip on the main strand that shallows to the northwest, where the fault merges at depth with the north-dipping San Gorgonio Pass fault zone.

Exclusion of the northeast dip on this segment of the fault may lead to overestimation of rupture extent and slip rates within models, which will consequently misrepresent the seismic hazard of the region. Incorporating fault geometry that can account for known uplift on the northeast side of the Coachella strand and localized uplift at Mecca Hill will improve the accuracy of the CFM and models that utilize it.

In order to refine the model, the geometry of the Coachella segment has been varied, in accordance with estimates of the northeast dip, to test the sensitivity of slip rates and uplift patterns to fault geometry. The results presented will compare uplift patterns from varied geometries in the 3D BEM model, based on the SCEC CFM, to a map of uplift rates constructed from geologic data in the Mecca Hills.