The effect of quaternary alluvium on strong ground motion in the Coyote Lake, California, earthquake of 1979

Abstract

The effect of alluvium on strong ground motion can be seen by comparing two strong-motion records of the Coyote Lake, California, earthquake of 6 August 1979 (ML = 5.9). One record at a site on Franciscan bedrock had a peak horizontal acceleration of 0.13 g and a peak horizontal velocity of 10 cm/sec. The other, at a site 2 km distant on 180 meters of Quaternary alluvium overlying Franciscan, had values of 0.26 g and 32 cm/sec, amplifications by factors of 2 and 3. Horizontal motions computed at the alluvial site for a linear plane-layered model based on measured P and S velocities show reasonably good agreement in shape with the observed motions, but the observed peak amplitudes are greater by a factor of about 1.25 in acceleration and 1.8 in velocity. About 15 per cent of the discrepancy in acceleration and 20 per cent in velocity can be attributed to the difference in source distance; the remainder may represent focusing by refraction at a bedrock surface concave upward. There is no clear evidence of nonlinear soil response. Fourier spectral ratios between motions observed on bedrock and alluvium show good agreement with ratios predicted from the linear model. In particular, the observed frequency of the fundamental peak in the amplification spectrum agrees with the computed value, indicating that no significant nonlinearity occurs in the secant shear modulus. Computations show that nonlinear models are compatible with the data if values of the coefficient of dynamic shear strength in terms of vertical effective stress are in the range of 0.5 to 1.0 or greater. The data illustrate that site amplification may be less a matter of resonance involving reinforcing multiple reflections, and more the simple effect of the low near-surface velocity. Application of traditional seismological theory leads to the conclusion that the site amplification for peak horizontal velocity is approximately proportional to the reciprocal of the square root of the product of density and shear-wave velocity.

Seismic site classification of Nakhon Nayok City, central Thailand, using the MASW technique

“…Many researchers have proposed the methods for soil classification based on their physical properties (Joyner et al, 1981;Tinsley and Fumal, 1985;Wills et al, 2000). In Borcherdt et al (1991).…”

Detection of hidden faults within the Fucino basin from single-station ambient noise measurements: The case study of the Trasacco fault system

“…The linear regression method was applied to data to find the best fitting line to plot, and then spatial coordinates (in meters) were projected on it. This kind of plot is helpful to imagine subsoil heterogeneities (Joyner et al, 1981;Famiani et al, 2020): the amplitude of spectral ratio curves is represented in a colored scale, while the XY axis shows the distance of the single noise measurement with respect to the beginning of the transect and the frequency values of HVNSR, respectively.…”

An efficient algorithm for sampling the shear-modulus reduction curve in the context of wave propagation using the elastoplastic Iwan model

“…Finally, we note that the Iwan constitutive model is not strain rate dependent, and therefore all damping comes from hysteresis. As already proposed by Joyner & Chen (1975); Joyner et al (1981), it is also possible to couple the hysteretic damping with a viscoelastic one by adding a dashpot element in parallel to take into account viscous damping at low strain (<10 −6 ).…”