Using Microtremors to Assess Potential Earthquake Site Response - a Case-Study in Flushing Meadows, New-York-City

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Bulletin of the Seismological Society of America
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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 recorded at several sites on and adjacent to the valley. While some raw spectra of valley site microtremors exhibit no predominant peaks, spectral ratios 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 interpreted as the fundamental resonance mode and the 2.4-Hz peak is interpreted as the first harmonic of the region above the base of an organic clay layer observed in borehole data. The amplification factors implied by the observed spectral ratios are unrealistically high (up to 100), which can be attributed to the close proximity of microtremor sources. An experiment involving the use of a pile-driver source located outside the valley was also performed. Signals obtained at valley sites during times when the pile-driver was in operation show a two-fold increase in the amplitude of a spectral peak when compared to signals obtained when the pile-driver was turned off. This observation gives additional evidence that observed spectral peaks represent soil layer resonances in the valley. Based on the results of this and other previous studies, we conclude that easily obtained microtremor measurements can contribute valuable microzonation and site response information to a study of regional earthquake hazard.


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