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 August 6, 1979 (M. = 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 m 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 percent of the discrepancy in acceleration and 20 percent 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.

Shear-wave velocity characteristics of geological units throughout Tehran City, Iran

“…While, V s of rocks mainly depends on the type and structure of constituting minerals, degree of weathering and alteration. Joyner et al (1981) proposed that velocity to a depth equal to one-quarter wavelength of the period of interest could represent the site conditions. Fumal and Tinsley (1985) used an approach which considered the variability of V s in three dimensions.…”

Ground-motion predictions in Shillong region, northeast India

“…The velocity model given by Mitra et al (2005) is taken as a representative one for northeast India with the data adjusted according to the bedrock/basement information following Nath and Thingbaijam (2010). The representative crustal model listed in Table 2 is employed for the firstorder estimation of crustal amplification based on quarter-wavelength approximation (Joyner et al 1981;Boore and Joyner 1997); the amplification curve is depicted in Fig. 2.…”

Regional and local factors in attenuation modelling: Hong Kong case study

“…To relate the period of interest to the rock depth, the quarterwavelength approximation method (Joyner et al, 1981;Boore and Joyner, 1997;BJ97) is required. This method allows the values of r A and V A to be averaged to a depth equivalent to the quarter-wavelength of the upwardly propagating S-wave, for applying Eq.…”