Showing papers by "David M. Boore published in 1982"

The empirical prediction of ground motion

Abstract: Recent additions to the strong-motion data set, primarily from earthquakes in California and Italy, are responsible for a large number of papers examining the prediction of ground-motion measures using regression methods. Peak acceleration is still the most common measure being considered, but increasing attention is being given to peak velocity and spectral amplitudes. Although direct comparisons among the studies are hampered by differing definitions of distance and magnitude, in general the various studies give similar answers for peak acceleration in the region of distance and magnitude space in which most of the data are concentrated. As might be expected, the differences are most pronouced for large magnitudes and distances close to the fault, where data are few. Even so, widely differing assumptions about the form of the regression equation and differences in the composition and weighting of the data set can give similar answers. This was true in recent studies by Campbell (1981b) and Joyner and Boore (1981), where the predicted accelerations for large earthquakes at close distances differed by less than 40 per cent. This seemingly large uncertainty is small compared to the scatter in the data about the regression lines. A Monte Carlo study shows that the question of whether the shape of the attenuation curves is magnitude-dependent cannot be resolved by existing data.

Analysis of the ground accelerations radiated by the 1980 Livermore Valley earthquakes for directivity and dynamic source characteristics

Abstract: The strong motion accelerograph recordings of the 24 January 1980 main shock and the 27 January 1980 aftershock of the Livermore Valley earthquake sequence are analyzed for systematic variations with azimuth or station location. The variation of the peak accelerations with epicentral azimuth is apparently reversed for the two events: the main shock accelerations are larger to the south, and the aftershock accelerations are larger to the northwest. We eliminate the site effects by forming the ratio of the peak accelerations recorded at the same station, after correcting for the epicentral distance. This analysis indicates that source directivity caused a total variation of a factor of 10 in the peak accelerations. Comparison of this variation with the spatial extent of the aftershock sequences suggests that the strong directivity in the radiated accelerations is the result of unilateral ruptures in both events. The accelerograms recorded at 10 stations within 35 km of the events were digitized to analyze the azimuthal variation of the rms acceleration, the peak velocity, and the radiated energy flux. The variation of rms acceleration correlates almost exactly with the variation of the peak accelerations. This correlation is analyzed using both deterministic and stochastic models for the acceleration waveforms. The peak velocities, corrected for epicentral distance, vary with azimuth by a factor of 5 for both events, while the radiated energy flux varies by a factor of 30 for the main shock and 15 for the aftershock. The peak velocities are strongly correlated with the radiated energy flux. The radiated seismic energies are estimated to be 2.6 ± 0.9 × 1020 dyne-cm for the main shock and 1.5 ± 0.3 × 1020 dyne-cm for the aftershock.

Anomalous shear wave delays and surface wave velocities at Yellowstone Caldera, Wyoming

Abstract: To investigate the effects of a geothermal area on the propagation of intermediate-period (1--30 s) teleseismic body waves and surface waves, a specially designed portable seismograph system was operated in Yellowstone Caldera, Wyoming. Travel time residuals, relative to a station outside the caldera, of up to 2 s for compressional phases are in agreement with short-period residuals for P phases measured by other investigators. Travel time delays for shear arrivals in the intermediate-period band range from 2 to 9 s and decrease with increasing dT/d..delta... Measured Rayleigh wave phase velocities are extremely low, ranging from 3.2 km/s at 27-s period to 2.0 km/s at 7-s period; the estimated uncertainty associated with these values is 15%. We propose a model for compressional and shear velocities and Poisson's ratio beneath the Yellowstone caldera which fits the teleseismic body and surface wave data: it consists of a highly anomalous crust with an average shear velocity of 3.0 km/s overlying an upper mantle with average velocity of 4.1 km/s. The high average value of Poisson's ratio in the crust (0.34) suggests the presence of fluids there; Poisson's ratio in the mantle between 40 and approximately 200 km is more nearly normal (0.29) than inmore » the crust. A discrepancy between normal values of Poisson's ratio in the crust calculated from short-period data and high values calculated from teleseismic data can be resolved by postulating a viscoelastic crustal model with frequency-dependent shear velocity and attenuation.« less