Evaluating the adequacy of horizontal-to-vertical spectral ratios to estimate site response from relatively thick (up to 1800 m) unconsolidated sedimentary sequences
Abstract
Quantifying the site response due to thick sequences of unconsolidated sediments is challenging because it is difficult to measure seismic velocities at depth for modeling the response. The horizontal-to-vertical spectral ratio (HVSR) method, using either earthquake signals or ambient noise as input, is an appealing method for estimating site responses because it uses a single seismic station rather than the two needed when using the traditional horizontal sediment-to-bedrock spectral ratio (SBSR) method. Previous studies show mixed results when comparing HVSR with SBSR methods for identifying the frequencies and amplitudes of the primary resonance peaks. Many of these studies have been carried out on sedimentary basins that can introduce substantial 3D effects such as locally-generated basin surface waves. Here we compare HVSR and SBSR methods to characterize relatively long-period (0.1 to 5 Hz), fundamental reverberations caused by the widespread Atlantic Coastal Plain (ACP) strata of the eastern U.S., which consist of nearly flat strata over a strong bedrock reflector without sloped edges to initiate basin surface waves. We compare the two methods in the 0.1 to 10 Hz frequency range at seismic stations sited on up to 1,800 m of sedimentary strata, using data from several major seismic experiments in the southeastern and mid-Atlantic U.S. Our comparisons using earthquake signals at sites on 0 to 200 m of strata show a close match in the frequencies of the fundamental resonance peaks in the 0.7 to 5 Hz range; amplitudes did not match well but were generally within a factor of 2. At sites with thicker sediment, our preliminary results show the HVSR method using either teleseismic or ambient noise signals successfully identifies the frequencies of the fundamental peaks, but underestimates their amplitudes by a consistent factor of 4 to 5. These preliminary results suggest the HVSR method could be a successful approach for estimating site response amplifications in areas with flat strata greater than about 200 m thick if appropriate amplitude adjustment factors are applied. [This publication represents the views of the authors and does not necessarily represent the views of the Defense Nuclear Facilities Safety Board.]
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.S43D0644S
- Keywords:
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- 7294 Seismic instruments and networks;
- SEISMOLOGY