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

Microtremor data from 16 broadband seismic stations within the Los Angeles basin are used to explore limitations of Nakamura's method for estimating resonant frequencies, amplification, and ground vulnerability. Spectral peaks observed in data from each station are 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. Although most peaks in the spectra are correlated with specific stratigraphic units or velocity discontinuities, not all peaks can be explained in this way, particularly for data from stations overlying complex basement structure. Interestingly, the predominant peak cannot always be correlated to the sediment-basement interface. The temporal stability of observed spectra is tested for all stations by comparing time segments from different years. Many stations exhibit stable peaks through time, though spectra from some stations show little consistency of spectral peaks. The spectral stability of the microtremor data is strongest for stations overlying subdued basement topography. Predominant periods and amplifications derived from the microtremor data are also compared with spectral peaks and amplifications observed in the 2008 Diamond Bar earthquake. Although predominant periods from each compare favorably for many stations, relative amplifications are consistently lower. The study suggests that the microtremor method can be used effectively to ascertain resonance periods of sites within basins at relatively low cost and ease; however, data from sites should first be investigated for temporal stability. In addition, the data are best interpreted in the context of other geological and geophysical data.