GPR IMAGERY AND IDENTIFICATION OF NEOTECTONIC FEATURES OF THE CHUPAMIERTOS FAULT SYSTEM, BAJA CALIFORNIA, MEXICO

Recently active faults create easily identifiable geomorphic and stratigraphic signatures in unconsolidated sediments. Identification of, and mapping of these features in trenches has been the norm for measuring offset and relating activity of faults to local geomorphic context. However; trenching is time consuming, expensive and destructive. Shallow-earth geophysical techniques may prove effective to map subsurface features related to faulting in unconsolidated sediments and certain depositional systems.

During May 2017, the research team collected 50 km of high-resolution 700, 450 and 160 MHz ground penetrating radar lines on the eastern edge of the Laguna Salada depositional system. The arid alluvial fan deposits are in fault contact with the Sierra de los Cucapah mountains. At this location, the main fault system is a low-angle normal fault. Areas to the north experienced surface rupture in 2010 and in 1954 to the south. In addition to the GPR data, a suite of trenches at 14 sites were excavated to various depths to reveal the recent activity of various splays of the main fault.

GPR lines were acquired adjacent to open trenches for ground truthing with the photographic trench log. Additional lines were acquired at various distances away from the trench and with lengths that far exceeded the trench.

The comparison of the trench logs with the GPR showed good correlation of the majority of features observable in the trench. Measured offsets from the logs and GPR data were compared. While the offset measurements were subject to the vertical error associated with the frequency of the GPR antenna, the trace of the fault was clearly identified.

The 450 MHz system provided a better correlation to the scale of features observable in the trench with the 160 MHz system was able to image the fault system to a greater depth and in places, capable of tracing splay faults into the regional detachment surface.

In this depositional context, the GPR proved effective at identifying recent faulting episodes and relating the subsurface fault trace to surface expressions. Applications of GPR to neotectonics will greatly increase the capacity to locate time intensive and expensive trenches in areas where the most useful information (measured offsets, chronological sampling) can be recovered.