Limitations of the Microtremor Method: A Case Study in the Los Angeles Basin, California

Abstract

Microtremor data from 16 broadband seismic stations across the Los Angeles basin are used to explore limitations of Nakamura's (1989) method for estimating resonant frequencies, amplification, and ground vulnerability to determine the sources of peaks in horizontal-to-vertical spectra ratios (HVSR) of microtremor data. Spectral peaks observed were analyzed in the context of stratigraphic boundaries inferred from well logs, geologic cross-sections and velocity-depth profiles generated from 3D California velocity model. The temporal stability of observed spectra from each station was tested by comparing time segments from different years. Spectral peaks and amplification values from HVSRs were compared with those from the 2008 Diamond Bar earthquake. The study suggests that microtremor data might be useful for determining resonant periods in sedimentary basins where strata are flat lying; however, a more thorough understanding of the method's limitations is necessary before it can be used to map amplification factors and ground vulnerability.