Raymond Ng, Jascha Polet

Sedimentary basins, such as the Los Angeles basin, can substantially amplify ground motion and increase its duration. To account for site response and develop better seismic hazard assessment and mitigation, it is essential to determine site characteristics across the basin at a high spatial resolution. We investigate site response within the Los Angeles Basin through the application of the microtremor Horizontal-to-Vertical Spectral Ratio (HVSR) approach on 3-component broadband waveforms from the Los Angeles Syncline Seismic Interferometry Experiment (LASSIE). LASSIE is a dense array of 73 broadband seismometers that were active for one month, transecting the Los Angeles basin at 1 km spacing from La Puente to a dense cluster in Long Beach. The spectral peak amplitudes and peak frequencies of the HVSR curves both show variation across the LASSIE network, even between stations that are spaced only 1 km apart, emphasizing the importance of micro-zonation. Our results show an average resonance period at the basin center of 6 to 10 sec. Secondary intermittent shorter period peaks are also observed near the basin edge and may be explained using basin edge resonance, the presence of small scale basins, and/or topographic effects based on their location within the array. Amplified shaking from resonance is characterized by long period HVSR peak amplitudes ranging from 2 to 5.5. The HVSR peak frequencies are higher than those predicted by a simple quarter wavelength function using a weighted velocity from 1-D profiles through the Southern California Earthquake Center Community Velocity Model – Harvard, especially near the center of the basin. Possible factors that may contribute to this discrepancy are oversimplifications used in the calculation, errors in the velocity model, resonance due to a more complex interaction between waves and physical complications introduced by the large dimensions of the LA basin.