Using microtremors to assess potential earthquake site response: A case study in Flushing Meadows

We have used portable 5-sec period seismometers to record ambient noise across a glacially cut valley filled with unconsolidated Holocene sediments. The valley, located in Flushing Meadows, New York City, is surrounded by a densely populated urban environment. Microtremors were recorded at several sites on and adjacent to the valley. While some raw spectra of valley site microtremors exhibit no predominant peaks, spectral ratios taken with respect to recordings made at a reference station outside the valley show a sharp peak near 0.8 Hz. A secondary peak near 2.4 Hz is also visible in some of the spectral ratios. The frequency of the predominant peak remains constant across the valley width, while the relative amplitude decreases systematically from a maximum at the valley center to background levels near the valley edge. Resonant frequencies predicted by one-dimensional modeling of the valley center site response show excellent agreement with observations. The 0.8-Hz spectral ratio peak is inter-preted as the fundamental resonance mode, and the 2.4-Hz peak is interprete