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Roger D. Borcherdt

Publications and source records attributed to Roger D. Borcherdt.

At least 55 records · Page 3Linked to original sources
USGS1994

The Loma Prieta, California, Earthquake of October 17, 1989: Strong ground motion

Strong ground motion generated by the Loma Prieta, Calif., earthquake (MS~7.1) of October 17, 1989, resulted in at least 63 deaths, more than 3,757 injuries, and damage estimated to exceed $5.9 billion. Strong ground motion severely damaged critical lifelines (freeway overpasses, bridges, and pipelines), caused severe damage to poorly constructed buildings, and induced a significant number of ground failures associated with liquefaction and landsliding. It also caused a significant proportion of the damage and loss of life at distances as far as 100 km from the epicenter. Consequently, understanding the characteristics of the strong ground motion associated with the earthquake is fundamental to understanding the earthquake's devastating impact on society. The papers assembled in this chapter address this problem. Damage to vulnerable structures from the earthquake varied substantially with the distance from the causative fault and the type of underlying geologic deposits. Most of the damage and loss of life occurred in areas underlain by 'soft soil'. Quantifying these effects is important for understanding the tragic concentrations of damage in such areas as Santa Cruz and the Marina and Embarcadero Districts of San Francisco, and the failures of the San Francisco-Oakland Bay Bridge and the Interstate Highway 880 overpass. Most importantly, understanding these effects is a necessary prerequisite for improving mitigation measures for larger earthquakes likely to occur much closer to densely urbanized areas in the San Francisco Bay region. The earthquake generated an especially important data set for understanding variations in the severity of strong ground motion. Instrumental strong-motion recordings were obtained at 131 sites located from about 6 to 175 km from the rupture zone. This set of recordings, the largest yet collected for an event of this size, was obtained from sites on various geologic deposits, including a unique set on 'soft soil' deposits (artificial fill and bay mud). These exceptional ground-motion data are used by the authors of the papers in this chapter to infer radiation characteristics of the earthquake source, identify dominant propagation characteristics of the Earth?s crust, quantify amplification characteristics of near-surface geologic deposits, develop general amplification factors for site-dependent building-code provisions, and revise earthquake-hazard assessments for the San Francisco Bay region. Interpretations of additional data recorded in well-instrumented buildings, dams, and freeway overpasses are provided in other chapters of this report.

California↗
USGS1993

GIS mapping of earthquake ground shaking in San Francisco, California

Severe earthquakes of the last decade in Mexico, Armenia, and the United States have reemphasized the importance of local geologic site conditions in estimating damage and consequent losses. Recent advances in geographic information system (GIS) technology have created new opportunities for maintaining and integrating extensive spatial databases for applications in seismic hazard analysis. Ground motion data and damage distributions from past earthquakes are combined with GIS databases of geologic information to prepare preliminary maps for inferring input ground motion levels for earthquake resistant design. Maps of bedrock elevation, surface topography, and surface geology are used to develop maps of soil thickness and predominant period. Ground motion attenuation relationships are used to create maps estimating ground shaking intensity for a uniform ground condition. Relationships between geologic characteristics, ground motion amplification, and earthquake intensity are used to produce amplification capability maps. Estimated ground shaking for the uniform ground condition is combined with amplification capability to develop maps predicting regional ground shaking intensity. The methods of predictive GIS mapping described here are useful for estimating characteristics of input ground motion in many urbanized seismic regions of the world.

California↗
USGS1992

On the characteristics of local geology and their influence on ground motions generated by the Loma Prieta earthquake in the San Francisco Bay region, California

Strong ground motions recorded at 34 sites in the San Francisco Bay region from the Loma Prieta earthquake show marked variations in characteristics dependent on crustal structure and local geological conditions. Peak horizontal acceleration and velocity inferred for sites underlain by “rock” generally occur on the transverse component of motion. They are consistently greater with lower attenuation rates than the corresponding mean value predicted by empirical curves based on previous strong-motion data. Theoretical amplitude distributions and synthetic seismograms calculated for 10-layer models suggest that “bedrock” motions were elevated due in part to the wide-angle reflection of S energy from the base of a relatively thin (25 km) continental crust in the region. Characteristics of geologic and geotechnical units as currently mapped for the San Francisco Bay region show that average ratios of peak horizontal acceleration, velocity and displacement increase with decreasing mean shear-wave velocity. Ratios of peak acceleration for sites on “soil” (alluvium, fill/Bay mud) are statistically larger than those for sites on “hard rock” (sandstone, shale, Franciscan Complex). Spectral ratios establish the existence of predominant site periods with peak amplifications near 15 for potentially damaging levels of ground motion at some sites underlain by alluvium and fill/bay mud. Average spectral amplifications inferred for vertical and the mean horizontal motion are, respectively, (1,1) for sites on the Franciscan Complex (KJf), (1.4, 1.5) for sites on Mesozoic and Tertiary rocks (TMzs), (2.1, 2.0) for sites on the Santa Clara Formation (QTs), (2.3, 2.9) for sites on alluvium (Qal), and (2.1, 4.0) for sites on fill/Bay mud (Qaf/Qhbm). These mean values are not statistically different at the 5% significance level from those inferred from previous low-strain data. 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 km in the cities of San Francisco and Oakland.

California↗
USGS1991

On the observation, characterisation, and predictive GIS mapping of strong ground shaking for seismic zonation

Tragic earthquakes of the last decade in Mexico, Armenia, and the United States have re-emphasised the importance of local geologic site conditions in determining amounts of damage and consequent loss of life. Extensive data sets in the San Francisco Bay region on strong earthquake ground motions, the damage distributions from past earthquakes, and geologic materials provide the basis to quantify site condition effects for purposes of earthquake hazard mitigation. These observational data are reviewed and analysed to provide methodologies for the characterisation and predictive mapping of potential variations in strong ground shaking for seismic zonation. The methodologies are based on existing geologic maps. They provide a method for seismic zonation applicable to many urbanised seismic regions of the world.

California↗
USGS1989

Seismology

No abstract available.

Caucasus Mountains, Lesser Caucasus highlands↗
USGS1989

An exact anelastic model for the free-surface reflection of P and S-I waves

Exact anelastic solutions incorporating inhomogeneous waves are used to model numerically S -I and P waves incident on the free surface of a low-loss anelastic half-space. Anelastic free-surface reflection coefficients are computed for the volumetric strain and displacement components of inhomogeneous wave fields. For the problem of an incident homogeneous S -I wave in Pierre shale, the largest strain and displacement amplitudes for the reflected P wave occur at angles of incidence for which the particle motion for the reflected inhomogeneous P wave is elliptical (minor/major axis = 0.6), the specific absorption ( Q P −1 ) is greater (300 per cent) and the velocity is less (25 per cent) than those for a corresponding homogeneous P wave, the direction of phase propagation is not parallel to the free surface, and the amplitude of the wave shows a significant increase with depth (6 per cent in one wavelength). Energy reflection coefficients computed for this low-loss anelastic model show that energy flow due to interaction of the incident and reflected waves reach maxima (30 per cent of the incident energy) near large but nongrazing angles of incidence. For the problem of an incident homogeneous P wave in Pierre shale, the inhomogeneity of the reflected S wave is shown not to contribute to significant variations in wave field characteristics over those that would be expected for a homogeneous wave.

Bulletin of the Seismological Society of America↗
USGS1988

Recording strong motion studies

Instruments now permit strong‐motion signals to be recorded over broader band widths, with wider dynamic range and signal resolution and with better data accessibility via computer. These advantages are utilized by the General Earthquake Observation System (GEOS) developed by the United States Geological Survey, by which a microcomputer‐controlled system provides improved strong‐motion data sets, as described here by a member of the development team. The paper was originally given at a joint meeting of the US/Japan Panel on Wind and Seismic Effects.

Batiment International, Building Research and Prac↗
USGS1988

Vertical seismic profiling of Oroville microearthquakes: Velocity spectra and particle motion as a function of depth

Direct evidence of site distortion of P - and S -wave microearthquake source spectra at Oroville, California, is presented. The data were gathered by placing vertical and three-component seismometers at 90 m intervals in a 500 m borehole through the Cleveland Hill normal fault, on which the 1975, M L = 5.7 Oroville earthquake took place. High-pressure, hydraulic locking mechanisms were used to firmly lock the seismometer packages against the borehole wall. Digital, event-triggering GEOS recorders were used to receive the data. Some 30 seismic events, including 12 microearthquakes, were recorded during the 4 months the seismometers were deployed. By comparing the velocity spectra of microearthquake waves at the different depths, it can be seen that scattering and attenuation in the shallow crust around the borehole dominantly affect high-frequency S waves. Above 15 Hz, the value of the apparent S -wave quality factor, Q as , for the upper 500 m of crust at this site is 9. One feature of this low Q as is a gross difference between S -wave “corner” frequencies observed at ground level and 500 m downhole. For example, the uphole and downhole S -wave corner frequencies for the M coda = 0.4 microearthquake of Julian Day 259 differ by a factor of 2 or more. Low quality factors and depth-dependent corner frequencies were also observed for P waves, but these data are less definitive due to lower signal-to-noise ratios. Based on their three-component particle motions and polarizations, the direct S waves of the microearthquakes appear to be composed of two phases, which have similar amplitudes but different apparent velocities. It is possible that the velocity differences are a result of anisotropy in the underlying rock.

California↗