Showing papers by "David M. Boore published in 1999"

Analysis of earthquake recordings obtained from the Seafloor Earthquake Measurement System (SEMS) instruments deployed off the coast of southern California

Abstract: For more than 20 years, a program has been underway to obtain records of earthquake shaking on the seafloor at sites offshore of southern California, near oil platforms. The primary goal of the program is to obtain data that can help deter- mine if ground motions at offshore sites are significantly different than those at onshore sites; if so, caution may be necessary in using onshore motions as the basis for the seismic design of oil platforms. We analyze data from eight earthquakes recorded at six offshore sites; these are the most important data recorded on these stations to date. Seven of the earthquakes were recorded at only one offshore station; the eighth event was recorded at two sites. The earthquakes range in magnitude from 4.7 to 6.1. Because of the scarcity of multiple recordings from any one event, most of the analysis is based on the ratio of spectra from vertical and horizontal compo- nents of motion. The results clearly show that the offshore motions have very low vertical motions compared to those from an average onshore site, particularly at short periods. Theoretical calculations find that the water layer has little effect on the horizontal components of motion but that it produces a strong spectral null on the vertical component at the resonant frequency of P waves in the water layer. The vertical-to-horizontal ratios for a few selected onshore sites underlain by relatively low shear-wave velocities are similar to the ratios from offshore sites for frequencies less than about one-half the water layer P-wave resonant frequency, suggesting that the shear-wave velocities beneath a site are more important than the water layer in determining the character of the ground motions at lower frequencies.

Effect of baseline corrections on response spectra for two recordings of the 1999 Chi-Chi, Taiwan, earthquake

Abstract: 1999 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Introduction Displacements derived from the accelerogram recordings of the 1999 Chi-Chi, Taiwan earthquake at stations TCU078 and TCU129 show drifts when only a simple baseline derived 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 might include tilts in the instruments or the response of the instruments to strong shaking. This note illustrates the effect on the velocity, displacement, and response spectra of several schemes for accounting for these baseline shifts. The most important conclusion for earthquake engineering purposes is that the response spectra for periods less than about 20 sec are unaffected by the baseline correction. The results suggest, however, that static displacements estimated from the instruments should be used with caution. Although limited to the analysis of only two 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. The digital accelerograph installed and maintained by Central Weather Bureau in Taiwan produced a rich set of records from the September 20, 1999 M = 7.6 Chi-Chi, Taiwan earthquake and its aftershocks (Shin et al., 2000). This earthquake has produced the most complete set of strong-motion recordings ever obtained. The data were obtained from Teledyne Geotech Model A900 digital accelerographs, with ±2 g gain and 16-bit resolution, recorded at 200 samples per second. The transducers are flat to acceleration from dc to 50 Hz. The system uses a trigger algorithm and a buffer so that the pre-trigger part of the ground motion is preserved. At this time data from only two stations have been released (Lee, 1999), but the features found in the analysis of these records are probably not restricted 1 to those records. The stations for which data have been released are TCU078 and TCU129. These stations are within …

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

Abstract: Ground-motion recordings obtained 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 de- pendence 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 Filtra- tion Plant (JFP); the other pair was the Rinaldi Receiving Station (R/N) 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 flee- field motions. We recorded seven small earthquakes and found that the high-fre- quency, 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 mo- tions from aftershocks occurring many months after the mainshock. The weak-mo- tion 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 con- sequence of a pore pressure buildup during large-amplitude motion that was not dissipated until sometime later. The motions at (R/N) 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.

Basin Waves on a Seafloor Recording of the 1990 Upland, California, Earthquake: Implications for Ground Motions from a Larger Earthquake

Abstract: The velocity and displacement time series from a recording on the seafloor at 74 km from the 1990 Upland earthquake ( M = 5.6) are dominated by late-arriving waves with periods of 6 to 7 sec. These waves are probably surface waves traveling across the Los Angeles basin. Response spectra for the recording are in agreement with predictions from empirical regression equations and theoretical models for periods less than about 1 sec but are significantly larger than those predictions for longer periods. The longer-period spectral amplitudes are controlled by the late-arriving waves, which are not included in the theoretical models and are underrepresented in the data used in the empirical analyses. When the motions are scaled to larger magnitude, the results are in general agreement with simulations of wave propagation in the Los Angeles basin by Graves (1998).