Seismic Fault Exploration in Urban Fault Zones, Los Angeles, California

Date: 4/24/2019

Time: 06:00 PM

Room: Fifth Avenue

The main traces of the Hollywood, Raymond, and Santa Monica faults in Los Angeles, California have been identified in several locations on the basis of geologic mapping, paleoseismic trenching, and boreholes, but there are also many locations where the main and auxiliary fault traces are not well determined. Locating fault traces is an important requirement of California’s Alquist-Priolo Fault Zoning Act, which limits building on surface fault ruptures. To more precisely locate individual traces of the Hollywood fault, in June 2018, the US Geological Survey (USGS) and the California Geological Survey (CGS) jointly conducted a series of seismic imaging investigations using V_P and V_S tomography (including V_P/V_S ratio and Poisson’s ratios), seismic reflection, surface-waves (MASW), and guided waves. Each of the seismic imaging methods identified an anomaly over a narrow zone (~20 m) identified as the main fault based on a series of borings. Using guided-wave peak ground velocities (PGV), we narrowed the location of the near-surface (upper few meters) fault trace to an ~2-m-wide zone, which was also consistent with the borings. The 2018 seismic study was one of several USGS-CGS collaborative seismic imaging studies that have successfully located fault traces along the Raymond, Hollywood, and Santa Monica faults in highly urbanized Los Angeles, and the USGS has also used these seismic methods along other major fault zones. We find that the guided-wave PGV method is effective in fault-trace mapping in both rural and urban areas and can be an excellent, less-expensive alternative to invasive exploratory methods, such as trenching and boring. However, the seismic methods do not provide critical slip information or dates, which can only be obtained from paleoseismic methods.

Presenting Author: Rufus D. Catchings

Authors