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Influences of local geology on strong and weak ground motions recorded in the San Francisco Bay region and their implications for site-specific building-code provisions
GEOLOGY:EARTHQUAKES
958030 Influences of local geology on strong and weak ground motions recorded in the San Francisco Bay region and their implications for site-specific bldlding-code provisions R. D. Borcherdt & G. Glassmoyer, US Geological Survey Professional Paper, 1551-A, 1994, pp 77-108. Strong ground motions recorded at 37 sites in the San Francisco Bay region during the Loma Prieta earthquake show marked variations in characteristics dependent on crustal structure and local geologic conditions. Comparative weak- and strong-motion measurements show that mean shear-wave velocity or average lowstraln spectral ratios are useful predictors of amplification at the damaging levels of ground motion observed during the earthquake. Analyses suggest that soil amplification and reflected crustal shear energy were major contributors to levels of ground motion sufficient to cause damage to vulnerable structures at distances near 100 kin. (from Authors)
958031 Ground response on Treasure Island K. M. Rollins, M. D. Mehood, R. D. Hryciw, M. Homolka & S. E. Shewbridge, US Geological Survey Professional Paper, 1551-A, 1994, pp 109-121. During the Loma Prieta earthquake, ground motions recorded on Treasure Island, a manmade fill in San Francisco Bay, were considerably greater than at the adjacent rock outcrop on Yerba Buena Island. This difference provided an opportunity to evaluate existing methods for computing ground response. To perform the analyses, soil-profile data and shear-wave-propagation velocities were measured by seismic cone-penetration testing at several sites on Treasure Island, including the ground-motion-recording station. Sensitivity studies were performed to evaluate the influence of various parameters, including base rock velocity, soil layering, and shear-wave-velocity profile at depth. Groundresponse analyses were also performed at other sites on Treasure Island, and the computed peak accelerations ranged from 0.13 to 0.20 g. The degree of damage in various places on the island correlated somewhat with the computed peak accelerations. (from Authors)
958032 Deep instrumentation array at the Treasure Island Naval Station P. De Alba, J. Benoit, D. G. Pass, J. J. Carter, T. L. Youd & A. F. Shakal, US Geological Survey Professional Paper, 1551-A, 1994, pp 155-168. High levels of rock-motion amplification by soft-soil deposits of various thicknesses were observed during the Loma Prieta earthquake. These observations raise questions about how earthquake motion at bedrock was transmitted through the overlying soil deposits, and emphasize once again the need for deep instrument arrays that would enable investigators to observe the transmission of earthquake motion from bedrock. Such an array is currently operating at the Treasure Island Naval Station in San Francisco Bay. This paper presents the results of the extensive soils-exploration program performed during instrument installation and the rationale for the instrumentation program. (from Authors)
958033 Ground-response studies at the Alameda Naval Air Station H. Carlisle & K. M. Rollins, US Geological Survey Professional Paper, 1551-A, 1994, pp 123-143. The ground motions recorded at the Alameda Naval Air Station, a soft-soil site, during the Loma Prieta earthquake were more than 3 times higher than on rock at the adjacent rock outcrop on Yerba Buena Island, even though the two sites were at similar epicentral distances. These recording enable the verification of the ability of the program SHAKE
361A
to compute ground response in soft soils. To perform these analyses, the soil profile was determined by drilling a borehole to bedrock at a depth of 470 ft (143.3 m) and obtaining samples for laboratory testing. Field testing included cone-penetration tests, standard penetration tests, and shear-wave-velocity logging, using both downbole methods and a seismic cone penetrometer. Ground-response analyses were performed by using the program SHAKE, and the computed ground motions agreed well with the measured values. (from Authors)
958034 A postearthquake reevaluation of seismic hazard in the San Francisco Bay region J. M. Murphy & S. G. Wesnousky, US Geological Survey Professional Paper, 1551-A, 1994, pp 255-272. Several years before the Loma Prieta earthquake, seismichazard maps were constructed for California on the basis of the magnitudes and locations of historical earthquakes and the slip rates of Quaternary faults, each interpreted within the framework of the concept of elastic rebound. These maps showed the estimated probability that onshore Quaternary faults would generate peak horizontal ground accelerations > 0.1 g at hard-rock sites during the subsequent 50 years. This paper uses the same method to illustrate the effect of: 1) the possible release of crustal strain along the San Andreas fault associated with the 1989 Loma Prieta earthquake and 2) the acquisition of further geologic and geodetic data bearing on the slip rates and paleoearthquake history of faults in the San Francisco Bay region. A general result is that the probability of >_ 0.1-g ground motions during the next 50 years remains high for most of the bay region. (from Authors)
958035 Aftershock observations from a dense array in Sunnyvale, California A. Frankel, S. E. Hough, P. A. Friberg & R. Busby, US Geological Survey Professional Paper, 1551-A, 1994, pp 181195. A small-aperture (~300 m) four-station array in Sunnyvale, California was deployed for 5 days to record aftershocks of the earthquake. The purpose of this deployment was to study the seismic response of the alluvium-filled Santa Clara Valley and the role of surface waves in the seismic shaking of sedimentary basins. Strong-motion records of the Loma Prieta main shock indicate that surface waves produced the peak velocities and displacements at some sites in the Santa Clara Valley. The recordings from the dense array were used to determine the apparent velocity and backazimuth (direction of propagation) for various arrivals in the seismograms of four aftershocks of M = 3.6 to 4.4. (from Authors)
958036 Site response in Oakland, California, near the failed section of the Nimitz freeway E. H. Field, S. E. Hough, K. H. Jacob & P. A. Friberg, US Geological Survey Professional Paper, 155l-A, 1994, pp 16% 179. The paper presents the results of a site-response study performed near the failed Nimitz Freeway in Oakland after the earthquake. The method of spectral ratios is used to estimate the ground-motion amplification at soft-soil sites, relative to hard-rock sites. Although individual spectral ratios of aftershocks vary widely, average spectral ratios exhibit stable peaks that agree remarkably well with the resonances predicted by one-dimensionai models whose parameters were determined independently. The predicted amplifications, however, are more than half those observed, and a conclusive explanation for this discrepancy is lacking. Given the success of one-dimensional models in predicting the resonances and overall shape of average weak-motion spectral ratios, such
958030 Influences of local geology on strong and weak ground motions recorded in the San Francisco Bay region and their implications for site-specific bldlding-code provisions R. D. Borcherdt & G. Glassmoyer, US Geological Survey Professional Paper, 1551-A, 1994, pp 77-108. Strong ground motions recorded at 37 sites in the San Francisco Bay region during the Loma Prieta earthquake show marked variations in characteristics dependent on crustal structure and local geologic conditions. Comparative weak- and strong-motion measurements show that mean shear-wave velocity or average lowstraln spectral ratios are useful predictors of amplification at the damaging levels of ground motion observed during the earthquake. Analyses suggest that soil amplification and reflected crustal shear energy were major contributors to levels of ground motion sufficient to cause damage to vulnerable structures at distances near 100 kin. (from Authors)
958031 Ground response on Treasure Island K. M. Rollins, M. D. Mehood, R. D. Hryciw, M. Homolka & S. E. Shewbridge, US Geological Survey Professional Paper, 1551-A, 1994, pp 109-121. During the Loma Prieta earthquake, ground motions recorded on Treasure Island, a manmade fill in San Francisco Bay, were considerably greater than at the adjacent rock outcrop on Yerba Buena Island. This difference provided an opportunity to evaluate existing methods for computing ground response. To perform the analyses, soil-profile data and shear-wave-propagation velocities were measured by seismic cone-penetration testing at several sites on Treasure Island, including the ground-motion-recording station. Sensitivity studies were performed to evaluate the influence of various parameters, including base rock velocity, soil layering, and shear-wave-velocity profile at depth. Groundresponse analyses were also performed at other sites on Treasure Island, and the computed peak accelerations ranged from 0.13 to 0.20 g. The degree of damage in various places on the island correlated somewhat with the computed peak accelerations. (from Authors)
958032 Deep instrumentation array at the Treasure Island Naval Station P. De Alba, J. Benoit, D. G. Pass, J. J. Carter, T. L. Youd & A. F. Shakal, US Geological Survey Professional Paper, 1551-A, 1994, pp 155-168. High levels of rock-motion amplification by soft-soil deposits of various thicknesses were observed during the Loma Prieta earthquake. These observations raise questions about how earthquake motion at bedrock was transmitted through the overlying soil deposits, and emphasize once again the need for deep instrument arrays that would enable investigators to observe the transmission of earthquake motion from bedrock. Such an array is currently operating at the Treasure Island Naval Station in San Francisco Bay. This paper presents the results of the extensive soils-exploration program performed during instrument installation and the rationale for the instrumentation program. (from Authors)
958033 Ground-response studies at the Alameda Naval Air Station H. Carlisle & K. M. Rollins, US Geological Survey Professional Paper, 1551-A, 1994, pp 123-143. The ground motions recorded at the Alameda Naval Air Station, a soft-soil site, during the Loma Prieta earthquake were more than 3 times higher than on rock at the adjacent rock outcrop on Yerba Buena Island, even though the two sites were at similar epicentral distances. These recording enable the verification of the ability of the program SHAKE
361A
to compute ground response in soft soils. To perform these analyses, the soil profile was determined by drilling a borehole to bedrock at a depth of 470 ft (143.3 m) and obtaining samples for laboratory testing. Field testing included cone-penetration tests, standard penetration tests, and shear-wave-velocity logging, using both downbole methods and a seismic cone penetrometer. Ground-response analyses were performed by using the program SHAKE, and the computed ground motions agreed well with the measured values. (from Authors)
958034 A postearthquake reevaluation of seismic hazard in the San Francisco Bay region J. M. Murphy & S. G. Wesnousky, US Geological Survey Professional Paper, 1551-A, 1994, pp 255-272. Several years before the Loma Prieta earthquake, seismichazard maps were constructed for California on the basis of the magnitudes and locations of historical earthquakes and the slip rates of Quaternary faults, each interpreted within the framework of the concept of elastic rebound. These maps showed the estimated probability that onshore Quaternary faults would generate peak horizontal ground accelerations > 0.1 g at hard-rock sites during the subsequent 50 years. This paper uses the same method to illustrate the effect of: 1) the possible release of crustal strain along the San Andreas fault associated with the 1989 Loma Prieta earthquake and 2) the acquisition of further geologic and geodetic data bearing on the slip rates and paleoearthquake history of faults in the San Francisco Bay region. A general result is that the probability of >_ 0.1-g ground motions during the next 50 years remains high for most of the bay region. (from Authors)
958035 Aftershock observations from a dense array in Sunnyvale, California A. Frankel, S. E. Hough, P. A. Friberg & R. Busby, US Geological Survey Professional Paper, 1551-A, 1994, pp 181195. A small-aperture (~300 m) four-station array in Sunnyvale, California was deployed for 5 days to record aftershocks of the earthquake. The purpose of this deployment was to study the seismic response of the alluvium-filled Santa Clara Valley and the role of surface waves in the seismic shaking of sedimentary basins. Strong-motion records of the Loma Prieta main shock indicate that surface waves produced the peak velocities and displacements at some sites in the Santa Clara Valley. The recordings from the dense array were used to determine the apparent velocity and backazimuth (direction of propagation) for various arrivals in the seismograms of four aftershocks of M = 3.6 to 4.4. (from Authors)
958036 Site response in Oakland, California, near the failed section of the Nimitz freeway E. H. Field, S. E. Hough, K. H. Jacob & P. A. Friberg, US Geological Survey Professional Paper, 155l-A, 1994, pp 16% 179. The paper presents the results of a site-response study performed near the failed Nimitz Freeway in Oakland after the earthquake. The method of spectral ratios is used to estimate the ground-motion amplification at soft-soil sites, relative to hard-rock sites. Although individual spectral ratios of aftershocks vary widely, average spectral ratios exhibit stable peaks that agree remarkably well with the resonances predicted by one-dimensionai models whose parameters were determined independently. The predicted amplifications, however, are more than half those observed, and a conclusive explanation for this discrepancy is lacking. Given the success of one-dimensional models in predicting the resonances and overall shape of average weak-motion spectral ratios, such