Amplitude Of Ground Motion Research Articles

The Challenge of Defining Upper Bounds on Earthquake Ground Motions

Recent studies to assess very long-term seismic hazard in the United States and in Europe have brought the issue of upper limits on earthquake ground motions into the arena of problems requiring attention from the engineering seismological community. Few engineering projects are considered sufficiently critical to warrant the use of annual frequencies of exceedance so low that ground-motion estimates may become unphysical if limiting factors are not considered, but for nuclear waste repositories, for example, the issue is of great importance. The definition of upper bounds on earthquake ground motions also presents an exciting challenge for researchers in the area of seismic hazard assessment. This paper looks briefly at historical work on maximum values of ground-motion amplitudes before illustrating why this is an important issue for hazard assessments at very long return periods. The paper then discusses the factors that control the extreme values of motion, both in terms of generating higher-amplitude bedrock motions and of limiting the values of motion at the ground surface. Possible channels of research that could be explored in the quest to define maximum possible ground motions are also discussed. In the period between the recording of the first strong-motion accelerograms in the Long Beach earthquake of March 1933 and the end of the 1960's, a number of studies were published proposing possible upper limits on earthquake ground-motion amplitudes. Some studies were purely empirical and influenced to a large extent by the El Centro recording of the 1940 Imperial Valley earthquake: Housner (1965) proposed that peak ground acceleration (PGA) would not exceed 0.5 g; Newmark (1965) proposed a limit in the range 0.5-0.6 g on PGA and between 76 and 91 cm/s on peak ground velocity (PGV), and Newmark and Hall (1969) proposed a limit of 0.75 g on PGA and agreed with 91 cm/s …

On the Incorporation of the Effect of Crustal Structure into Empirical Strong Ground Motion Estimation

This article has two purposes. Firstly, a validation exercise of the modal summation technique for the computation of synthetic strong-motion records is performed for two regions of Europe (Umbria-Marche and south Iceland), using a variety of region specific crustal structure models, by comparing the predicted ground motion amplitudes with observed motions. It is found that the rate of decay of ground motions is well predicted by the theoretical decay curves but that the absolute size of the ground motions is underpredicted by the synthetic time-histories. This is thought to be due to the presence of low-velocity surface layers that amplify the ground motions but are not included in the crustal structure models used to compute the synthetic time-histories. Secondly, a new distance metric based on the computed theoretical decay curves is introduced which should have the ability to model the complex decay of strong ground motions. The ability of this new distance metric to reduce the associated scatter in empirically derived equations for the estimation of strong ground motions is tested. It is found that it does not lead to a reduction in the scatter but this is thought to be due to the use of crustal structure models that are not accurate or detailed enough for the regions studied.

Calibration of an ML Scale in Northwestern Turkey from 1999 Izmit Aftershocks

A local magnitude scale is derived for northwestern Turkey, using data collected by a temporary network installed by the German Task Force for Earthquakes after the 1999 Izmit earthquake ( M w 7.4) and the permanent Sapanca-Bolu network. We computed Wood–Anderson seismograms for over 5353 arrivals at 27 three-component stations, from 530 earthquakes. The hypocentral distances considered range from 5 to 140 km, with the best represented range being from 5 to 70 km. We inverted the measured amplitudes following both the nonparametric Richter's (1958) and parametric Bakun and Joyner (1984) approaches. These methods yield consistent magnitude values and station corrections. However, the calculated nonparametric distance correction, log A , implies that ground-motion attenuation is higher than what is accounted for by the equation calibrated in central California. In the range 5–62 km, the best fit is provided by -log A = log( R /17) + 0.00960( R - 17) + 2. This equation is obtained constraining the geometrical spreading parameter n to 1. The calculated value of k = 0.00960 confirms that in this range of distance seismic waves could propagate through a low- Q volume. Station corrections, which allow for a significant reduction of M L residuals, range between ±0.5 magnitude units, suggesting a strong influence of local site effects on the amplitude of ground motion. In accord with the obtained attenuation and station corrections, the magnitudes of the considered events range from 0.4 to 4.8.

The periodic impact responses and stability of a human body in a vehicle traveling on rough terrain

The periodic impact motions of passengers in a vehicle traveling on rough terrain are investigated through a linear model of vehicle and passenger systems. In this mechanical model, the human body is considered as a massless bar with a lumped mass. The linear model assumes that the motion response of vehicle is quite small compared to the passenger's pitch motion since the vehicle chassis has a quite large mass and large moment of inertia compared with each passenger. The period-1 motion pertaining to two impacts, respectively, on two walls during N-periods of the ground motion is predicted analytically and numerically. The stability and bifurcation of such a period-1 motion are determined. The impact motion becomes complicated with increasing ground motion amplitude, but the impact motion reduces with increasing torsional damping. Although the mechanical model used in this paper is an ideal one, the dynamic responses of the human body in vehicle traveling on rough terrain is useful for the rough evaluation of the human body safety in traveling vehicles.

A Screen Analysis Procedure for Seismic Slope Stability

Site-specific seismic slope stability analyses are required in California by the 1990 California Seismic Hazards Mapping Act for sites located within mapped hazard zones and scheduled for development with more than four single-family dwellings. A screen analysis is performed to distinguish sites for which only small ground deformations are likely from sites for which larger, more damaging landslide movements could occur. No additional analyses are required for sites that pass the screen, whereas relatively detailed analyses are required for sites that fail the screen. We present a screen analysis procedure that is based on a calibrated pseudo-static representation of seismic slope stability. The novel feature of the present screen procedure is that it accounts not only for the effects of ground motion amplitude on slope displacement, but also accounts for duration effects indirectly via the site seismicity. This formulation enables a more site-specific screen analysis than previous formulations that made a priori assumptions of seismicity/duration. This screen procedure has recently been adopted by the Landslide Hazard Implementation Committee for implementation by practicing engineers and engineering geologists in southern California.

Estimation of Q for Long-Period (>2 sec) Waves in the Los Angeles Basin

We simulate 0- to 0.5-Hz 3D wave propagation through the Southern California Earthquake Center seismic velocity reference model, version 2, for the 1994 Northridge earthquake in order to examine the effects of anelastic attenuation and amplification within the near-surface sediments. We use a fourth-order finite-difference staggered-grid method with the coarse-grained frequency-independent anelastic scheme of Day and Bradley (2001) and a variable slip distribution from kinematic inversion for the Northridge earthquake. We find that the near-surface material with S -wave velocity ( V s) as low as 500 m/sec significantly affects the long-period peak ground velocities, compared with simulations in which the S -wave velocity is constrained to 1 km/sec and greater. Anelastic attenuation also has a strong effect on ground-motion amplitudes, reducing the predicted peak velocity by a factor of up to 2.5, relative to lossless simulations. Our preferred Q model is Q s/ V s = 0.02 ( V s in meters per second) for V s less than 1–2 km/sec, and much larger Q s/ V s (0.1, V s in meters per second) for layers with higher velocities. The simple model reduces the standard deviation of the residuals between synthetic and observed natural log of peak velocity from 1.13 to 0.26, relative to simulations for the lossless case. The anelastic losses have their largest effect on short-period surface waves propagating in the Los Angeles basin, which are principally sensitive to Q s in the low-velocity, near-surface sediments of the basin. The low-frequency ground motion simulated here is relatively insensitive to Q p, as well as to the values of Q s at depths greater than roughly that of the 2-km/sec S -wave velocity isosurface.

Simulation of Ground Motion Using the Stochastic Method

— A simple and powerful method for simulating ground motions is to combine parametric or functional descriptions of the ground motion's amplitude spectrum with a random phase spectrum modified such that the motion is distributed over a duration related to the earthquake magnitude and to the distance from the source. This method of simulating ground motions often goes by the name “the stochastic method.” It is particularly useful for simulating the higher-frequency ground motions of most interest to engineers (generally, f>0.1 Hz), and it is widely used to predict ground motions for regions of the world in which recordings of motion from potentially damaging earthquakes are not available. This simple method has been successful in matching a variety of ground-motion measures for earthquakes with seismic moments spanning more than 12 orders of magnitude and in diverse tectonic environments. One of the essential characteristics of the method is that it distills what is known about the various factors affecting ground motions (source, path, and site) into simple functional forms. This provides a means by which the results of the rigorous studies reported in other papers in this volume can be incorporated into practical predictions of ground motion.

Earthquake Ground-Motion Scaling in Central Mexico between 0.7 and 7 Hz

We present results from a regional study of ground-motion scaling parameters in central Mexico using data from a short-period vertical-component network and two broadband stations within the valley of Mexico. A total of 1220 waveforms recorded between 1994 and 2001 are used for the analysis. A damped least-squares regression using a simple model to separate the excitation, site, and propagation effects for the peak velocities in selected narrow-bandpass frequencies was carried out. The propagation term was parameterized to define a piecewise continuous geometrical spreading function, a frequency-dependent Q ( f ), and a distance-dependent duration that are consistent with the random vibration theory. We measured the average attenuation of S and Lg waves. A final model with a quality factor of Q ( f ) = 180 f 0.66 and a geometrical spreading of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[g(r)=\begin{array}{ll}r^{-1}&0{<}r{\leq}100\\r^{-0.2}&100{<}r{\leq}150\\r^{-0.5}&150{<}r{\leq}300\end{array}\] \end{document} is used for the parameterization of the ground-motion scaling. We find that this region is characterized by a rapid decay of ground-motion amplitude with distance, similar to that in other tectonically active regions. Manuscript received 23 October 2001.

AN EXPLORATION OF THE NATURE OF THE SCATTER IN GROUND-MOTION PREDICTION EQUATIONS AND THE IMPLICATIONS FOR SEISMIC HAZARD ASSESSMENT

This paper reviews the modelling of the scatter in ground-motion prediction equations, and also explores the nature of scatter in strong ground-motion, using an extended database of European accelerograms and newly derived predictive equations. Several statistical tests are used to confirm that the distribution of the residuals is genuinely lognormal over a certain range but appears to deviate from this model significantly in the tails of the distribution. An alternative model, the upper limit lognormal (ULLN) distribution is found to provide a good fit to the data; the parameters of this model include an upper-bound on the distribution, which is effectively inferred from the statis-tics of the data itself. However, it is clear that upper bounds established in this way do not provide a reliable ceiling on ground-motion amplitudes and physical constraints are required. Investigative tools that could be used to explore the physical limits of ground-motion amplitudes are discussed. In line with the underlying focus of the work, which is to explore the sometimes almost dominant influence that the scatter can exert on PSHA, and also to provide new data to prevent the overestimation of strong-motion parameters for design, the covariance in the scatter of different strong-motion parameters is also explored.

Erratum to Empirical Ground-Motion Relations for Subduction Zone Earthquakes and Their Application to Cascadia and Other Regions

Ground-motion relations for earthquakes that occur in subduction zones are an important input to seismic-hazard analyses in many parts of the world. In the Cascadia region (Washington, Oregon, northern California, and British Columbia), for example, there is a significant hazard from megathrust earthquakes along the subduction interface and from large events within the subducting slab. These hazards are in addition to the hazard from shallow earthquakes in the overlying crust. We have compiled a response spectra database from thousands of strong-motion record- ings from events of moment magnitude (M) 5-8.3 occurring in subduction zones around the world, including both interface and in-slab events. The 2001 M 6.8 Nis- qually and 1999 M 5.9 Satsop earthquakes are included in the database, as are many records from subduction zones in Japan (Kyoshin-Net data), Mexico (Guerrero data), and Central America. The size of the database is four times larger than that available for previous empirical regressions to determine ground-motion relations for subduc- tion-zone earthquakes. The large dataset enables improved determination of attenu- ation parameters and magnitude scaling, for both interface and in-slab events. Soil response parameters are also better determined by the data. We use the database to develop global ground-motion relations for interface and in-slab earthquakes, using a maximum likelihood regression method. We analyze regional variability of ground-motion amplitudes across the global database and find that there are significant regional differences. In particular, amplitudes in Cascadia differ by more than a factor of 2 from those in Japan for the same magnitude, distance, event type, and National Earthquake Hazards Reduction Program (NEHRP) soil class. This is believed to be due to regional differences in the depth of the soil profile, which are not captured by the NEHRP site classification scheme. Regional correction factors to account for these differences are proposed for Cascadia and Japan. The results of this study differ significantly from previous analyses based on more limited data and have important implications for seismic-hazard analysis. The ground-motion relations predict that a great megathrust earthquake (M 8) at a fault distance of about 100 km would produce pseudoacceleration (PSA), 5% damped, horizontal component on soil sites of about 110 cm/sec 2 at 0.5 Hz, 660 cm/sec 2 at 2.5 Hz, and 410 cm/sec 2 at 5 Hz, with a peak ground acceleration of about 180 cm/ sec 2 . These damaging levels of motion would be experienced over a very large area, corresponding to a rectangular area about 300 km wide by 500 km long. Large in- slab events (M 7.5) would produce even higher PSA values within 100 km of the fault, but the in-slab motions attenuate much more rapidly with distance. Thus the hazard posed by moderate to large in-slab events such as the 2001 Nisqually earth- quake is modest compared to that of a Cascadia megathrust earthquake of M 8, in terms of the area that would experience damaging levels of ground motion.

Stochastic Simulation of Strong-Motion Records from the 15 April 1979 (M 7.1) Montenegro Earthquake

Acceleration time histories, recorded during the destructive 15 April 1979 ( M 7.1) Montenegro earthquake, have been simulated using a stochastic modeling technique for finite faults proposed by Beresnev and Atkinson (1997, 1998a). In this approach, the ground-motion amplitudes are simulated as a summation of stochastic point sources. The length of the fault was taken as 70 km and its width as 25 km, and the fault plane was divided into 13 × 5 elements. The applied methodology is tested against its ability to predict site-specific strong-motion records by the incorporation of mean frequency-dependent site-amplification factors, based on a gross characterization of the site class. The results show that the overall agreement between stochastic and recorded waveforms and spectra is quite satisfying. Nevertheless, significant discrepancies exist at certain stations, implying that site-amplification functions play an important role in the simulation process. A repetition of the simulations combined with the use of site-specific amplification function estimated by the horizontal-to-vertical ratios technique improved the fit to the observed time histories and spectra. Manuscript received 4 July 2001.

How Accurate Can Strong Ground Motion Attenuation Relations Be?

This article gives the results of a study using 1484 strong-motion records, which tried to find an upper limit on the accuracy that attenuation relations can achieve independently of the functional form adopted and the methods used for the construction of the equation. It is found that the current data do not allow a significant improvement in the uncertainty over what has been found for previous attenuation relations. Also, we find evidence for significant nonuniform scatter with respect to magnitude and that the scatter is not dependent on the amplitude of ground motion.

Simplified Modeling of Bridge Response on Soft Soil to Nonuniform Seismic Excitation

Simple modeling is developed for the response of bridges on soft soil to spatially nonuniform seismic excitation. The bridges are modeled as single- and multispan elastic frames founded on a homogeneous soil layer over rigid rock. Three different excitation types are considered: (1) general harmonic excitation with different amplitude and phase at each support; (2) oblique harmonic SH waves traveling within the soil layer (“wave-passage effect”); and (3) vertically propagating SH waves in conjunction with different soil conditions at the supports (“site-response effect”). Response of a single-span symmetric bridge is obtained by decomposing the excitation into symmetric and antisymmetric components. The analysis is then extended to a multispan integral bridge subjected to translational and rotational input. Harmonic and transient responses are considered and analytical expressions for various response parameters are obtained. Soil-structure interaction (SSI) effects are studied by considering long, drilled shaft foundations. To compare the effects of uniform and nonuniform input, pertinent response ratios are introduced. It is shown that when the amplitude of ground motion varies among the supports, defining a reference uniform excitation is not straightforward.

Saturation effects on horizontal and vertical motions in a layered soil–bedrock system due to inclined SV waves

A study of the effects of pore-water saturation on the horizontal and vertical components of ground motions in a multi-layered soil–bedrock system due to inclined SV waves is presented. Both the soil and the rock are modeled as a partially water-saturated porous medium which is characterized by its degree of saturation, porosity, permeability, and compressibility. An efficient formulation is developed for the computation of the two-dimensional ground motions, which are considered as functions of the angle of incidence, the degree of saturation, the frequency, and the geometry of the system. Numerical results for both the half-space model and the single-layered model indicate that the effect of saturation may be significant, and is dependent on the angle of the incidence. Even a slight decrease of full saturation of the overlying soil may cause appreciable difference in the amplitudes of ground motions in both the horizontal and vertical components and the amplitude ratios between the two components at the ground surface, implying that one may need to carefully take into account the saturation conditions in the interpretation of field observations.

Deterministic seismic hazard in Egypt

SUMMARY The regional seismic hazard in Egypt is assessed using a deterministic approach based on the computation of synthetic seismograms at a set of gridpoints located at distances of 0.2u from each other. The main input for this computation are earthquake sources and structural models. The earthquake sources are parametrized using focal mechanisms, seismogenic areas and regional seismicity. A number of deep seismic profiles have been used to define the crustal structures. Similar sets of gravity profiles have been used to define the density of the layers. The peak displacement (DMAX), peak velocity (VMAX) and design ground acceleration (DGA) are chosen and plotted to construct the seismic hazard maps. There are similarities between computed and observed amplitudes of ground motion in terms of their values and spatial distributions. The results obtained from the deterministic and probabilistic approaches are comparable. The areas of high seismic hazard level are of great socio-economic importance.

北海道内温泉井における4回のM7.5以上の地震直後の地下水位変化

M7.6 Kushiro-Oki, M7.8 Hokkaido-Nansei-Oki, M8.1 Hokkaido-Toho-Oki and M7.5 Sanriku-Haruka-Oki earthquakes occurred around Hokkaido, northern Japan during the period from January, 1993 to December, 1994. We have collected groundwater-level records in 12 wells observed by local governments in Hokkaido to investigate the relationship between coseismic changes in water levels and volumetric strain induced by the four large earthquakes. Coseismic water level changes are found in many wells. The maximum coseismic change in water level was 2.5m, induced by the Hokkaido-Nansei-Oki earthquake. The volumetric strain fields induced by the four earthquakes have been calculated by using published parameters of dislocation models, and compared with the coseismic water level changes. Generally, the volumetric strain changes at wells where water level fell coseismically are dilatation, and those at wells where water level rose are contraction. As far as the relationship between water level changes and volumetric strain induced by the earthquakes in seven wells where water level data are available for at least three earthquakes, coseismic water level changes at all the seven wells are found to be proportional to the calculated volumetric strain. Furthermore, the coseismic water level changes are found to be proportional to not only the volumetric strain but also an amplitude of ground motion caused by the earthquakes in four wells where water level are available for all the four earthquakes by applying AIC (The Akaike Information Criterion) method. The strain sensitivities of the water level at the four wells, which are between 1.6 and 19.6mm/10-8 strain when the contribution of both the volumetric strain and the ground motion are taken into account, are as large as strain sensitivities inferred from the earth tides in 17 wells observed worldwide.

Sediment non-linearity and attenuation of seismic waves: a study of accelerograms from Lefkas, western Greece

We investigate the dependence of the S-wave high-frequency spectral-decay parameter, κ (“kappa”) — a measure of wave attenuation — on ground-motion amplitude. 21 three-component accelerograms from two adjacent sediment sites in the town of Lefkas, western Greece, are used, representing 17 earthquakes with magnitudes Mw 4.7–7.0 and hypocentral distances 12–93km. Recorded peak horizontal ground accelerations (PGA) and velocities (PGV) are 22–540cm/s2 and 1.3–54.5cm/s.Fourier amplitude spectra are computed for S-wave windows, and the frequency range is visually determined where the high-frequency spectral decay can be approximated by a straight line on the linear-log plot; its slope (and hence κ) is computed by linear regression. κ is found to depend on hypocentral distance as κ=0.108+0.058R (r=0.518).As PGV increases from 1.3 to 54.5cm/s, κ0 (κ at 0km, characterising inelastic attenuation in the site's subsurface geology) varies between 0.060 and 0.160s. κ0 is found to correlate very strongly with logMGA (r=0.645) (MGA — mean horizontal acceleration in the S-wave window) but also with logPGA (r=0.447) and logPGV (r=0.627). We attribute this behaviour to sediment non-linearity (shear-modulus degradation), resulting in the decrease of the site's dominant-resonance frequency (from about 3.5 to 2.4Hz) and leading to the increase of κ0. Our results imply that at sediment sites, an important contribution to κ comes from wave attenuation (damping) in the softest sediments and show that κ0 is amplitude dependent, thus being a measure of sediment non-linearity.

Reply to Comment by R. A. W. Haddon on "Evaluation of Models for Earthquake Source Spectra in Eastern North America" by Gail M. Atkinson and David M. Boore

In his comment, Haddon attempts to refute the conclusions of our 1998 article, in which we evaluated proposed source models for eastern North America (ENA). We used a clearly defined procedure based on the well-known stochastic model to make ground-motion predictions based on each proposed source model, and then we compared the predicted motions with all recorded ENA data of M > 4. Haddon's objections to the stochastic approach, and more specifically to one of the evaluated source models (ours), are based on his theoretical interpretations. We disagree with almost all of Haddon's opinions and the manner in which he has expressed them. However we think that little will be gained by replying in detail. The technical details of our disagreement have already been discussed at some length (Atkinson et al., 1997; Haddon, 1997). So rather than providing a blow-by-blow rebuttal, we limit our reply to the following salient remarks. Seismology is an observational science, wherein the validity of proposed ground-motion models is judged empirically, based on their ability to predict observed ground-motion data. The stochastic ground-motion model is a simple tool that combines a good deal of empiricism with a little seismology, and yet has been as successful as more sophisticated methods in predicting ground-motion amplitudes over a broad range of magnitudes, distances, frequencies and tectonic environments (e.g., Atkinson and Somerville, 1994; Hartzell et al., 1999). Many find this surprising; some find it infuriating. Widespread interest in the stochastic model was aroused when McGuire and Hanks (1980), Hanks and McGuire (1981), Boore (1983), and McGuire et al. (1984) demonstrated that …

Sh-Wave Modelling of Influence of Local Geologic Condition on Strong Ground Motion

In this paper a hybrid method, based on mode summation and finite difference, is used to simulate the ground motion of SH wave in the Xiji-Langfu area induced by the 1976 Tangshan earthquake. We estimate the acceleration maximum amplitude and total energy of ground motion often used for engineering purposes to quantify seismic hazard. On the two sides of the Xiadian fault the acceleration maximum amplitude and total energy can vary by 200% and 700% respectively. Our results can explain the observed anomalous intensity in the Xiji-Langfu area induced by the 1976 Tangshan earthquake.

Nonlinear soil response in the vicinity of the Van Normal Complex following the 1994 Northridge, California, earthquake

AbstractGround-motion recording obtaineds at the Van Norman Complex from the 1994 Northridge, California, mainshock and its aftershocks constitute an excellent data set for the analysis of soil response as a function of ground-motion amplitude. We searched for nonlinear response by comparing the Fourier spectral ratios of two pairs of sites for ground motions of different levels, using data from permanent strong-motion recorders and from specially deployed portable instruments. We also compared the amplitude dependence of the observed ratios with the amplitude dependence of the theoretical ratios obtained from 1-D linear and 1-D equivalent-linear transfer functions, using recently published borehole velocity profiles at the sites to provide the low-strain material properties. One pair of sites was at the Jensen Filtration Plant (JFP); the other pair was the Rinaldi Receiving Station (RIN) and the Los Angeles Dam (LAD). Most of the analysis was concentrated on the motions at the Jensen sites. Portable seismometers were installed at the JFP to see if the motions inside the structures housing the strong-motion recorders differed from nearby free-field motions. We recorded seven small earthquakes and found that the high-frequency, low-amplitude motions in the administration building were about 0.3 of those outside the building. This means that the lack of high frequencies on the strong-motion recordings in the administration building relative to the generator building is not due solely to nonlinear soil effects. After taking into account the effects of the buildings, however, analysis of the suite of strong- and weak-motion recordings indicates that nonlinearity occurred at the JFP. As predicted by equivalent-linear analysis, the largest events (the mainshock and the 20 March 1994 aftershock) show a significant deamplification of the high-frequency motion relative to the weak motions from aftershocks occurring many months after the mainshock. The weak-motion aftershocks recorded within 12 hours of the mainshock, however, show a relative deamplification similar to that in the mainshock. The soil behavior may be a consequence of a pore pressure buildup during large-amplitude motion that was not dissipated until sometime later. The motions at (RIN) and (LAD) are from free-field sites. The comparison among spectral ratios of the mainshock, weak-motion coda waves of the mainshock, and an aftershock within ten minutes of the mainshock indicate that some nonlinearity occurred, presumably at (RIN) because it is the softer site. The spectral ratio for the mainshock is between that calculated for pure linear response and that calculated from the equivalent-linear method, using commonly used modulus reduction and damping ratio curves. In contrast to the Jensen sites, the ratio of motions soon after the high-amplitude portion of the mainshock differs from the ratio of the mainshock motions, indicating the mechanical properties of the soil returned to the low-strain values as the high-amplitude motion ended. This may indicate a type of nonlinear soil response different from that affecting motion at the Jensen administration building.