FAULT-ZONE EXPLORATION IN HIGHLY URBANIZED SETTINGS USING GUIDED WAVES: AN EXAMPLE FROM THE RAYMOND FAULT, LOS ANGELES, CALIFORNIA

Locating faults in the near-surface beneath highly urbanized settings is challenging because urbanization obscures geomorphic evidence of the faults in relatively short time periods. Yet, urban faults can represent major hazards because they directly underlie large populations. Such is the case along the western Raymond Fault in Los Angeles, where it steps over to the Hollywood Fault. It is important to identify the main and auxiliary traces that may underlie homes and buildings. Paleoseismic trenching and other invasive techniques offer the clearest evidence of faulting, but long trenches cannot be economically excavated in highly urbanized areas or zones where faulting is widely distributed, such as fault step overs. In May 2016, we used an alternative, non-invasive approach to locate faults between the Raymond and Hollywood faults. This technique involves using peak ground velocities (PGV) measured from guided-wave energy generated within a known fault trace, as recently done elsewhere (Catchings et al., 2013). From a known site (from trenching) of the Raymond Fault, we used an accelerated weight drop to input seismic energy into the fault zone. The seismic energy was recorded by seismic arrays located about 400 to 700 m from the source and aligned approximately perpendicular to the expected trend of the Raymond Fault. Although the area was bisected by a four-lane highway and numerous busy streets, our recorded seismic energy was largely noise free as a result of stacking more than 200 shots. We observed four distinct zones of high PGV values along the array, none of which coincided with roadways or other local noise sources. Instead, these zones of high PGV coincided with fault traces inferred from geologic mapping, borings, and pre-urbanization (1923; 1928) aerial photos. We find that the guided-wave PGV method is highly effective in locating faults in both urban and rural areas.