33-1 One Trough, Two Valleys, (at least) Three Faults: Strike-Slip Faulting in the Imperial-Mexicali Valley

Session: Borderless Geoscience: Linking Baja and Southern California

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Abstract:

Multiple geophysical proxies are examined to characterize faulting within the Mexicali and Imperial Valleys, focusing on the southern segment of the Imperial Fault and its surrounding structures. The observations presented extend from the U.S.–Mexico border southward across the Mexicali Valley to the region where the Imperial Fault is inferred to mechanically interact with the Cerro Prieto Fault system. Gravity surveys were conducted along and across known surface ruptures associated with the 1940 El Centro and 1979 Imperial Valley earthquakes, providing constraints on subsurface density contrasts related to fault zones and basin structure. In parallel, legacy seismic reflection profiles were reprocessed to improve imaging of subsurface discontinuities and to evaluate the relationship between mapped seismicity lineaments and buried fault structures. The combined geophysical evidence indicates the presence of multiple secondary faults, consistent with Riedel-type shear geometries commonly developed in transtensional pull-apart basin settings. These results highlight the structural complexity of faulting beneath the sedimentary cover of both valleys and underscore the need to consider distributed and interacting fault systems. Improved mapping of these buried faults is particularly relevant for regional seismic hazard and earthquake forecasting models, which should account for fault connectivity and strain partitioning across the broader California–Baja California plate boundary region.

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