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We have taken advantage of the recent increase in strong-motion data at close distances to derive new attenuation relations for peak horizontal acceleration and velocity. This new analysis uses a magnitude-independent shape, based on geometrical spreading and anelastic attenuation, for the attenuation curve. An innovation in technique is introduced that decouples the determination of the distance dependence of the data from the magnitude dependence. The resulting equations are log A =-1.02 + 0.249M-log r-0.00255r + 0.26P r_-(d2-1-7.32) 1/2 5.0_

We have derived equations for predicting the larger horizontal and the random horizontal component of peak acceleration and of 2-, 5-, 10-, and 20-percent-damped pseudovelocity response spectra for 46 periods ranging from 0.1 to 2.0 sec. The equations were obtained by fitting a functional form to empirical data using a two-stage regression method. 271 two-component recordings from 20 earthquakes were used to develop the equations for peak acceleration, and 112 two-component recordings from 14 earthquakes were used for the response spectral equations. The data included a subset of those used in earlier studies

Recent code provisions for buildings and other structures (1994 and 1997 NEHRP Provisions, 1997 UBC) have adopted new site amplification factors and a new procedure for site classification. Two amplitude-dependent site amplification factors are specified: Fa for short periods and Fv for longer periods. Previous codes included only a long period factor S and did not provide for a short period amplification factor. The new site classification system is based on definitions of five site classes in terms of a representative average shear wave velocity to a depth of 30 m (V¯s). This definition permits sites to be classified unambiguously. When the shear wave velocity is not available, other soil properties such as standard penetration resistance or undrained shear strength can be used. The new site classes denoted by letters A - E, replace site classes in previous codes denoted by S1 - S4. Site classes A and B correspond to hard rock and rock, Site Class C corresponds to soft rock and very stiff / very dense soil, and Site Classes D and E correspond to stiff soil and soft soil. A sixth site class, F, is defined for soils requiring site-specific evaluations. Both Fa and Fv are functions of the site class, and also of the level of seismic hazard on rock, defined by parameters such as Aa and Av ( 1994 NEHRP Provisions), Ss and Sl ( 1997 NEHRP Provisions) or Z ( 1997 UBC). The values of Fa and Fv decrease as the seismic hazard on rock increases due to soil nonlinearity. The greatest impact of the new factors Fa and Fv as compared with the old S factors occurs in areas of low-to-medium seismic hazard. This paper summarizes the new site provisions, explains the basis for them, and discusses ongoing studies of site amplification in recent earthquakes that may influence future code developments.

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Residual displacements for large earthquakes can sometimes be determined from recordings on modern digital instruments, but baseline offsets of unknown origin make it difficult in many cases to do so. To recover the residual displacement, we suggest tailoring a correction scheme by studying the character of the velocity obtained by integration of zeroth-order-corrected acceleration and then seeing if the residual displacements are stable when the various parameters in the particular correction scheme are varied. For many seismological and engineering purposes, however, the residual displacements are of lesser importance than ground motions at periods less than about 20 sec. These ground motions are often recoverable with simple baseline correction and low-cut filtering. In this largely empirical study, we illustrate the consequences of various correction schemes, drawing primarily from digital recordings of the 1999 Hector Mine, California, earthquake. We show that with simple processing the displacement waveforms for this event are very similar for stations separated by as much as 20 km. We also show that a strong pulse on the transverse component was radiated from the Hector Mine earthquake and propagated with little distortion to distances exceeding 170 km; this pulse leads to large response spectral amplitudes around 10 sec.

We have estimated ground motions at the site of a bridge collapse during the 1994 Northridge, California, earthquake. The estimated motions are based on correcting motions recorded during the mainshock 2.3 km from the collapse site for the relative site response of the two sites. Shear-wave slownesses and damping based on analysis of borehole measurements at the two sites were used in the site response analysis. We estimate that the motions at the collapse site were probably larger, by factors ranging from 1.2 to 1.6, than at the site at which the ground motion was recorded, for periods less than about 1 sec.

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.