LIMITATIONS OF THE MICROTREMOR METHOD: A CASE STUDY IN THE LOS ANGELES BASIN, CALIFORNIA

We use weak motion data from 16 permanent broadband seismic stations across the Los Angeles basin to explore limitations of Nakamura's microtremor method for estimating resonant frequencies, amplification, and ground vulnerability to determine the sources of peaks in horizontal-to-vertical power spectra ratios (HVPSR) of microtremor data. Spectral peaks observed in data from each station were analyzed in the context of stratigraphic boundaries inferred from well logs, geologic cross-sections and velocity-depth profiles generated from the California 3D velocity model. Deep areas of the basin (> 6 km) show spectral peaks with periods of 6.0 to 9.0 s that correlate with the sediment-basement contact and are associated with a large impedance contrast. Intermediate periods of 4.0 to 7.0 s correspond to total basin thicknesses of 3 to 6 km, and periods less than 2.5 s correspond to areas with shallow basement. Although many peaks in the spectra can be correlated with specific stratigraphic units or velocity discontinuities, not all peaks can be explained in this way, particularly for data from stations located in structurally complex areas. Interestingly, the predominant peak was not always correlated to the sediment-basement interface. The temporal stability of observed spectra was tested for all stations by comparing time segments from different years. Although many stations exhibited stable peaks through time, spectra from some stations appeared variable. In general, results of the study agree with amplification determinations derived from other methods, such as strong motion analyses and geologic mapping. The study suggests that microtremor data can be useful for mapping ground vulnerability in sedimentary basins; however, a thorough understanding of the method's limitations is necessary.