Showing papers by "David M. Boore published in 2004"

Effect of Baseline Corrections on Displacements and Response Spectra for Several Recordings of the 1999 Chi-Chi, Taiwan, Earthquake

Abstract: Displacements derived from many of the accelerogram recordings of the 1999 Chi-Chi, Taiwan, earthquake show drifts when only a simple baseline de- rived from the pre-event portion of the record is removed from the records. The appearance of the velocity and displacement records suggests that changes in the zero level of the acceleration are responsible for these drifts. The source of the shifts in zero level are unknown, but in at least one case it is almost certainly due to tilting of the ground. This article illustrates the effect on the ground velocity, ground dis- placement, and response spectra of several schemes for accounting for these baseline shifts. A wide range of final displacements can be obtained for various choices of baseline correction, and comparison with nearby GPS stations (none of which are colocated with the accelerograph stations) do not help in choosing the appropriate baseline correction. The results suggest that final displacements estimated from the records should be used with caution. The most important conclusion for earthquake engineering purposes, however, is that the response spectra for periods less than about 20 sec are usually unaffected by the baseline correction. Although limited to the analysis of only a small number of recordings, the results may have more general significance both for the many other recordings of this earthquake and for data that will be obtained in the future from similar high-quality accelerograph networks now being installed or soon to be installed in many parts of the world.

Estimating V̄s(30) (or NEHRP Site Classes) from Shallow Velocity Models (Depths < 30 m)

Abstract: The average velocity to 30 m (V s (30)) is a widely used parameter for classifying sites to predict their potential to amplify seismic shaking. In many cases, however, models of shallow shear-wave velocities, from which V s(30) can be com- puted, do not extend to 30 m. If the data for these cases are to be used, some method of extrapolating the velocities must be devised. Four methods for doing this are described here and are illustrated using data from 135 boreholes in California for which the velocity model extends to at least 30 m. Methods using correlations be- tween shallow velocity and V s(30) result in significantly less bias for shallow models than the simplest method of assuming that the lowermost velocity extends to 30 m. In addition, for all methods the percent of sites misclassified is generally less than 10% and falls to negligible values for velocity models extending to at least 25 m. Although the methods using correlations do a better job on average of estimating V s(30), the simplest method will generally result in a lower value of V s(30) and thus yield a more conservative estimate of ground motion (which generally increases as V s(30) decreases).

Can site response be predicted

Abstract: Large modifications of seismic waves are produced by variations of material properties near the Earth's surface and by both surface and buried topography. These modifica-tions, usually referred to as “site response”, in general lead to larger motions on soil sites than on rock-like sites. Because the soil amplifications can be as large as a factor of ten, they are important in engineering applications that require the quantitative spec-ification of ground motions. This has been recognised for years by both seismologists and engineers, and it is hard to open an earthquake journal these days without finding an article on site response. What is often missing in these studies, however, are discussions of the uncertainty of the predicted response. A number of purely observational studies demonstrate that ground motions have large site-to-site variability for a single earthquake and large earthquake-location-dependent variability for a single site. This variability makes site-specific, earthquake-specific predi...

Comparisons of Ground Motions from the 1999 Chi-Chi Earthquake with Empirical Predictions Largely Based on Data from California

Abstract: This article has the modest goal of comparing the ground motions recorded during the 1999 Chi-Chi, Taiwan, mainshock with predictions from four empirical-based equations commonly used for western North America; these empirical predictions are largely based on data from California. Comparisons are made for peak acceleration and 5%-damped response spectra at periods between 0.1 and 4 sec. The general finding is that the Chi-Chi ground motions are smaller than those predicted from the empirically based equations for periods less than about 1 sec by factors averaging about 0.4 but as small as 0.26 (depending on period, on which equation is used, and on whether the sites are assumed to be rock or soil). There is a trend for the observed motions to approach or even exceed the predicted motions for longer periods. Motions at similar distances (30-60 km) to the east and to the west of the fault differ dramatically at periods between about 2 and 20 sec: long-duration wave trains are present on the motions to the west, and when normalized to similar amplitudes at short periods, the response spectra of the motions at the western stations are as much as five times larger than those of motions from eastern stations. The explanation for the difference is probably related to site and propagation effects; the western stations are on the Coastal Plain, whereas the eastern stations are at the foot of young and steep mountains, either in the relatively narrow Longitudinal Valley or along the eastern coast—the sediments underlying the eastern stations are probably shallower and have higher velocity than those under the western stations. Manuscript received 1 December 2000.

Ground Motion in Anchorage, Alaska, from the 2002 Denali Fault Earthquake: Site Response and Displacement Pulses

Abstract: Data from the 2002 Denali fault earthquake recorded at 26 sites in and near Anchorage, Alaska, show a number of systematic features important in studies of site response and in constructing long-period spectra for use in earthquake engi- neering. The data demonstrate that National Earthquake Hazards Reduction Program (NEHRP) site classes are a useful way of grouping stations according to site ampli- fication. In general, the sites underlain by lower shear-wave velocities have higher amplification. The amplification on NEHRP class D sites exceeds a factor of 2 relative to an average of motions on class C sites. The amplifications are period dependent. They are in rough agreement with those from previous studies, but the new data show that the amplifications extend to at least 10 sec, periods longer than considered in previous studies. At periods longer than about 14 sec, all sites have motion of similar amplitude, and the ground displacements are similar in shape, polarization, and am- plitude for all stations. The displacement ground motion is dominated by a series of four pulses, which are associated with the three subevents identified in inversion studies (the first pulse is composed of P waves from the first subevent). Most of the high-frequency ground motion is associated with the S waves from subevent 1. The pulses from subevents 1 and 2, with moment releases corresponding to M 7.1 and 7.0, are similar to the pulse of displacement radiated by the M 7.1 Hector Mine earthquake. The signature from the largest subevent (M 7.6) is more subdued than those from the first two subevents. The two largest pulses produce response spectra with peaks at a period of about 15 sec. The spectral shape at long periods is in good agreement with the recent 2003 NEHRP code spectra but is in poor agreement with the shape obtained from Eurocode 8.

Comparisons of Ground Motions from Five Aftershocks of the 1999 Chi-Chi, Taiwan, Earthquake with Empirical Predictions Largely Based on Data from California

Abstract: The observed ground motions from five large aftershocks of the 1999 Chi-Chi, Taiwan, earthquake are compared with predictions from four equations based primarily on data from California. The four equations for active tectonic re- gions are those developed by Abrahamson and Silva (1997), Boore et al. (1997), Campbell (1997, 2001), and Sadigh et al. (1997). Comparisons are made for horizontal-component peak ground accelerations and 5%-damped pseudoaccelera- tion response spectra at periods between 0.02 sec and 5 sec. The observed motions are in reasonable agreement with the predictions, particularly for distances from 10 to 30 km. This is in marked contrast to the motions from the Chi-Chi mainshock, which are much lower than the predicted motions for periods less than about 1 sec. The results indicate that the low motions in the mainshock are not due to unusual, localized absorption of seismic energy, because waves from the mainshock and the aftershocks generally traverse the same section of the crust and are recorded at the same stations. The aftershock motions at distances of 30-60 km are somewhat lower than the predictions (but not nearly by as small a factor as those for the mainshock), suggesting that the ground motion attenuates more rapidly in this region of Taiwan than it does in the areas we compare with it. We provide equations for the regional attenuation of response spectra, which show increasing decay of motion with distance for decreasing oscillator periods. This observational study also demonstrates that ground motions have large earthquake-location-dependent variability for a specific site. This variability reduces the accuracy with which an earthquake-specific predic- tion of site response can be predicted. Online Material: PGAs and PSAs from the 1999 Chi-Chi earthquake and five aftershocks.

A Study of Possible Ground-Motion Amplification at the Coyote Lake Dam, California

Abstract: The abutment site at the Coyote Lake Dam recorded an unusually large peak acceleration of 1.29g during the 1984 Morgan Hill earthquake. Following this earthquake another strong-motion station was installed about 700 m downstream from the abutment station. We study all events (seven) recorded on these stations, using ratios of peak accelerations, spectral ratios, and particle motion polarization (using holograms) to investigate the relative ground motion at the two sites. We find that in all but one case the motion at the abutment site is larger than the downstream site over a broad frequency band. The polarizations are similar for the two sites for a given event, but can vary from one event to another. This suggests that the dam itself is not strongly influencing the records. Although we can be sure that the relative motion is usually larger at the abutment site, we cannot conclude that there is anom- alous site amplification at the abutment site. The downstream site could have lower- than-usual near-surface amplifications. On the other hand, the geology near the abut- ment site is extremely complex and includes fault slivers, with rapid lateral changes in materials and presumably seismic velocities. For this reason alone, the abutment site should not be considered a normal free-field site.