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The effectiveness of trenches in reducing seismic motion
Author(s) -
May Thomas W.,
Bolt Bruce A.
Publication year - 1982
Publication title -
earthquake engineering and structural dynamics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.218
H-Index - 127
eISSN - 1096-9845
pISSN - 0098-8847
DOI - 10.1002/eqe.4290100203
Subject(s) - trench , layering , attenuation , anelastic attenuation factor , geology , shear waves , seismic wave , viscoelasticity , wavelength , surface wave , coda , energy (signal processing) , seismology , shear (geology) , optics , materials science , physics , petrology , botany , layer (electronics) , quantum mechanics , composite material , biology
The effect of placing barriers in the travel path of P, SV and SH seismic waves has been studied using time‐domain solutions of plane‐strain finite element programs for two‐dimensional crustal models. The wavefields considered propagate parallel to the free surface of an elastic medium consisting of a single layer over a halfspace. Barriers take the form of open‐air trenches. Effects of damping are assessed by considering representative viscoelastic conditions. Computations are presented as the ratio of spectral energy observed at a point with the barrier system in place in the model to the spectral energy observed at the same point without the barrier system in the model. These spectral ratios are dependent upon the direction of wave propagation. The calculations brought to light the marked role of surficial layering and attenuation properties of the surface rocks or soils on the effectiveness of seismic trench barriers. Barrier models without these features cannot in general reliably predict seismic wavefields at the surface. In the range of cases studied, trench depth d rather than width is the most sensitive parameter. When the ratio d/λ , the ratio of trench depth to the wavelength of shear waves, is greater than about 0·6, power spectral ratios of 0·06 and less are found for SH waves and the vertical component of SV motion for frequencies of 4–6 Hz. By contrast, for frequencies less than 3–4 Hz, power spectral ratios from unity to about two and greater are observed, indicating amplification for the horizontal component of wave motion. Spectral ratios calculated at some locations in front of the barrier system show over two‐fold amplification. Spectral ratios also change significantly with the relative location of the free surface observation point.