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Stephen H Hartzell - One of the best experts on this subject based on the ideXlab platform.
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calculation of broadband time histories of ground motion part ii kinematic and dynamic modeling using theoretical green s functions and comparison with the 1994 Northridge Earthquake
Bulletin of the Seismological Society of America, 2005Co-Authors: Stephen H Hartzell, Mariagiovanna Guatteri, Martin P Mai, Pengcheng Liu, Mark D FiskAbstract:In the evolution of methods for calculating synthetic time histories of ground motion for postulated Earthquakes, kinematic source models have dominated to date because of their ease of application. Dynamic models, however, which incorporate a physical relationship between important faulting parameters of stress drop, slip, rupture velocity, and rise time, are becoming more accessible. This article compares a class of kinematic models based on the summation of a fractal distribution of subevent sizes with a dynamic model based on the slip-weakening friction law. Kinematic modeling is done for the frequency band 0.2 to 10.0. Hz, dynamic models are calculated from 0.2 to 2.0. Hz. The strong motion data set for the 1994 Northridge Earthquake is used to evaluate and compare the synthetic time histories. Source models are propagated to the far field by convolution with 1D and 3D theoretical Green’s functions. In addition, the kinematic model is used to evaluate the importance of propagation path effects: velocity structure, scattering, and nonlinearity. At present, the kinematic model gives a better broadband fit to the Northridge ground motion than the simple slip-weakening dynamic model. In general, the dynamic model overpredicts rise times and produces insufficient shorter-period energy. Within the context of the slip-weakening model, the Northridge ground motion requires a short slip-weakening distance, on the order of 0.15 m or less. A more complex dynamic model including rate weakening or one that allows shorter rise times near the hypocenter may fit the data better.
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calculation of broadband time histories of ground motion comparison of methods and validation using strong ground motion from the 1994 Northridge Earthquake
Bulletin of the Seismological Society of America, 1999Co-Authors: Stephen H Hartzell, Stephen C Harmsen, Arthur Frankel, Shawn LarsenAbstract:This article compares techniques for calculating broadband time histories of ground motion in the near field of a finite fault by comparing synthetics with the strong-motion data set for the 1994 Northridge Earthquake. Based on this comparison, a preferred methodology is presented. Ground-motion-simulation techniques are divided into two general methods: kinematic- and composite-fault models. Green's functions of three types are evaluated: stochastic, empirical, and theoretical. A hybrid scheme is found to give the best fit to the Northridge data. Low frequencies ( 1 Hz) are calculated using a composite-fault model with a fractal subevent size distribution and stochastic, bandlimited, white-noise Green's functions. At frequencies below 1 Hz, theoretical elastic-wave-propagation synthetics introduce proper seismic-phase arrivals of body waves and surface waves. The 3D velocity structure more accurately reproduces record durations for the deep sedimentary basin structures found in the Los Angeles region. At frequencies above 1 Hz, scattering effects become important and wave propagation is more accurately represented by stochastic Green's functions. A fractal subevent size distribution for the composite fault model ensures an ω−2 spectral shape over the entire frequency band considered (0.1-20 Hz).
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site response for urban los angeles using aftershocks of the Northridge Earthquake
Bulletin of the Seismological Society of America, 1996Co-Authors: Stephen H Hartzell, Arthur Frankel, Alena L Leeds, John A MichaelAbstract:Ground-motion records from aftershocks of the 1994 Northridge Earthquake are used to estimate site response in the urban Los Angeles area. Over 1300 shear-wave records from 61 sources and 90 sites are used in a linear inversion for source and site-response spectra. The methodology makes no assumptions about the shape of the source spectrum. To obtain a stable unique inverse, a Q model and geometrical spreading factor are assumed. In addition, the site response at a hard-rock site is constrained to be approximately 1.0 with a kappa of 0.02. The site-response spectra compare favorably with the results of previous and on-going investigations in Los Angeles. A couple of first-order effects are lower site response in the surrounding mountains, dominated by Mesozoic and Tertiary rocks, and higher values in the San Fernando and Los Angeles Basins, containing surficial Pleistocene and Holocene alluvial deposits. Results show good correlation of high site-response spectral values with localized areas of severe damage (Interstate 10 collapse, Sherman Oaks, Northridge, Interstate 5/14 collapse). However, widespread trends in site response across the sedimentary basins are not obvious. The data suggest that site responses are lower near the southern margin of the San Fernando Valley for sources to the north, due to north-dipping sedimentary structures. But the general pattern of site response is characterized by high variability on length scales less than a kilometer. Variations of a factor of 2 in site response are observed over the length scale of 200 m and for the same surficial geologic unit. For some of the alluvial basin sites, surface-wave generation is a significant contributor to elevated site response at lower frequencies, below 2 Hz. The total damage pattern for the Northridge Earthquake is influenced by strong source directivity to the north and strong local site effects. The correlation of weak-motion site-response estimates with areas of significant damage demonstrates the value of these field measurements in future urban planning and in the reduction of seismic risk in urban areas.
Satish Nagarajaiah - One of the best experts on this subject based on the ideXlab platform.
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base isolated fcc building impact response in Northridge Earthquake
Journal of Structural Engineering-asce, 2001Co-Authors: Satish Nagarajaiah, Xiaohong SunAbstract:The base-isolated Fire Command and Control (FCC) building in Los Angeles experienced strong motion during the 1994 Northridge Earthquake. The California Strong Motion Instrumentation Program has instrumented the building and recorded the data during the Northridge Earthquake; these data are available for performance evaluation. Impact was observed in the base-isolated FCC building during the Northridge Earthquake. The objective of this study is to evaluate the seismic performance of the base-isolated FCC building during the 1994 Northridge Earthquake and the effect of impact. New analytical modeling techniques are developed to analyze the base-isolated FCC building with impact and are verified using system identification. The response computed, using the developed analytical modeling techniques, is verified using recorded data. The response with and without impact is presented. The effects of impact on the structural response are evaluated. The seismic performance evaluations, comparing the response of th...
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response of base isolated usc hospital building in Northridge Earthquake
Journal of Structural Engineering-asce, 2000Co-Authors: Satish Nagarajaiah, Sun XiaohongAbstract:The base-isolated University of Southern California (USC) hospital building experienced strong motion during the 1994 Northridge Earthquake. California Strong Motion Instrumentation Program data of the response are available for performance evaluation. The objective of this study is to evaluate the seismic performance of the base-isolated USC hospital building during the 1994 Northridge Earthquake. A nonlinear analytical model of the USC hospital building is developed and verified using system identification. The response computed, using the presented analytical modeling techniques, is verified using recorded data. Structural behavior during the Northridge Earthquake is evaluated in detail. The base-isolated USC hospital building performed well and reduced the response when compared to a fixed-base structure. The free-field acceleration was 0.49g and peak foundation/ground acceleration was 0.37g. The peak roof acceleration was reduced to 0.21g, nearly 50% of the peak ground acceleration. The peak drift wa...
Le Val Lund - One of the best experts on this subject based on the ideXlab platform.
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lifeline utilities performance in the 17 january 1994 Northridge california Earthquake
Bulletin of the Seismological Society of America, 1996Co-Authors: Le Val LundAbstract:Abstract Lifelines performed an important role in emergency response and restoring the community after the 17 January 1994 Northridge Earthquake. Telecommunications, radio, and electronic media played a valuable role in directing emergency response for essential services, directing the repairing of damaged lifelines and informing the public of vital information. Water supply was necessary where available for public fire protection. Highways and roads were used to move rescue, repair, fire, and medical teams and their supplies and equipment to the damaged areas. Although the Northridge Earthquake was severe, the lifeline disruption represented a relatively small percentage of inconvenience to the total population served within the total Earthquake impact area. The 1994 Northridge event showed improvement in the seismic performance of lifeline facilities and equipment installed and built, under modern seismic codes, since the 1971 San Fernando Earthquake.
Arthur Frankel - One of the best experts on this subject based on the ideXlab platform.
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calculation of broadband time histories of ground motion comparison of methods and validation using strong ground motion from the 1994 Northridge Earthquake
Bulletin of the Seismological Society of America, 1999Co-Authors: Stephen H Hartzell, Stephen C Harmsen, Arthur Frankel, Shawn LarsenAbstract:This article compares techniques for calculating broadband time histories of ground motion in the near field of a finite fault by comparing synthetics with the strong-motion data set for the 1994 Northridge Earthquake. Based on this comparison, a preferred methodology is presented. Ground-motion-simulation techniques are divided into two general methods: kinematic- and composite-fault models. Green's functions of three types are evaluated: stochastic, empirical, and theoretical. A hybrid scheme is found to give the best fit to the Northridge data. Low frequencies ( 1 Hz) are calculated using a composite-fault model with a fractal subevent size distribution and stochastic, bandlimited, white-noise Green's functions. At frequencies below 1 Hz, theoretical elastic-wave-propagation synthetics introduce proper seismic-phase arrivals of body waves and surface waves. The 3D velocity structure more accurately reproduces record durations for the deep sedimentary basin structures found in the Los Angeles region. At frequencies above 1 Hz, scattering effects become important and wave propagation is more accurately represented by stochastic Green's functions. A fractal subevent size distribution for the composite fault model ensures an ω−2 spectral shape over the entire frequency band considered (0.1-20 Hz).
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site response for urban los angeles using aftershocks of the Northridge Earthquake
Bulletin of the Seismological Society of America, 1996Co-Authors: Stephen H Hartzell, Arthur Frankel, Alena L Leeds, John A MichaelAbstract:Ground-motion records from aftershocks of the 1994 Northridge Earthquake are used to estimate site response in the urban Los Angeles area. Over 1300 shear-wave records from 61 sources and 90 sites are used in a linear inversion for source and site-response spectra. The methodology makes no assumptions about the shape of the source spectrum. To obtain a stable unique inverse, a Q model and geometrical spreading factor are assumed. In addition, the site response at a hard-rock site is constrained to be approximately 1.0 with a kappa of 0.02. The site-response spectra compare favorably with the results of previous and on-going investigations in Los Angeles. A couple of first-order effects are lower site response in the surrounding mountains, dominated by Mesozoic and Tertiary rocks, and higher values in the San Fernando and Los Angeles Basins, containing surficial Pleistocene and Holocene alluvial deposits. Results show good correlation of high site-response spectral values with localized areas of severe damage (Interstate 10 collapse, Sherman Oaks, Northridge, Interstate 5/14 collapse). However, widespread trends in site response across the sedimentary basins are not obvious. The data suggest that site responses are lower near the southern margin of the San Fernando Valley for sources to the north, due to north-dipping sedimentary structures. But the general pattern of site response is characterized by high variability on length scales less than a kilometer. Variations of a factor of 2 in site response are observed over the length scale of 200 m and for the same surficial geologic unit. For some of the alluvial basin sites, surface-wave generation is a significant contributor to elevated site response at lower frequencies, below 2 Hz. The total damage pattern for the Northridge Earthquake is influenced by strong source directivity to the north and strong local site effects. The correlation of weak-motion site-response estimates with areas of significant damage demonstrates the value of these field measurements in future urban planning and in the reduction of seismic risk in urban areas.
Robert W Graves - One of the best experts on this subject based on the ideXlab platform.
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implications of the Northridge Earthquake for strong ground motions from thrust faults
Bulletin of the Seismological Society of America, 1996Co-Authors: Paul Somerville, Chandan K Saikia, David J Wald, Robert W GravesAbstract:The peak accelerations recorded on alluvial sites during the Northridge Earthquake were about 50% larger than the median value predicted by current empirical attenuation relations at distances less than about 30 km. This raises the question of whether the ground motions from the Northridge Earthquake are anomalous for thrust events or are representative of ground motions expected in future thrust Earthquakes. Since the empirical data base contains few strong-motion records close to large-thrust Earthquakes, it is difficult to assess whether the Northridge ground motions are anomalous based on recorded data alone. For this reason, we have used a broadband strong-motion simulation procedure to help assess whether the ground motions were anomalous. The simulation procedure has been validated against a large body of strong-motion data from California Earthquakes, and so we expect it to produce accurate estimates of ground motions for any given rupture scenario, including blind-thrust events for which no good precedent existed in the strong-motion data base until the occurrence of the Northridge Earthquake. The ground motions from the Northridge Earthquake and our simulations of these ground motions have a similar pattern of departure from empirical attenuation relations for thrust Earthquakes: the peak accelerations are at about the 84th percentile level for distances within 20 to 30 km and follow the median level for larger distances. This same pattern of departure from empirical attenuation relations was obtained in our simulations of the peak accelerations of an Elysian Park blind-thrust event prior to the occurrence of the Northridge Earthquake. Since we are able to model this pattern with broadband simulations, and had done so before the Northridge Earthquake occurred, this suggests that the Northridge strong-motion records are not anomalous and are representative of ground motions close to thrust faults. Accordingly, it seems appropriate to include these recordings in strong-motion data sets that are used to develop empirical ground-motion attenuation relations for thrust faults and to use this augmented data set as the basis for evaluating the need for modifications in design coefficients in the seismic provisions of building codes.
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preliminary analysis of long period basin response in the los angeles region from the 1994 Northridge Earthquake
Geophysical Research Letters, 1995Co-Authors: Robert W GravesAbstract:Long-period (1–10 sec) ground motions recorded in the Los Angeles basin region during the Northridge Earthquake show complex waveforms, extended durations and multiple sets of arrivals which cannot be attributed solely to source processes or wave propagation within a plane-layered medium. These features suggest a strong interaction between the propagating seismic wave field and the laterally varying subsurface geologic structure of this region. Recorded motions at a hard rock site in the Santa Monica Mountains (scrs) are smaller by nearly a factor of three in peak velocity compared to recordings at more distant sites in the adjacent, northwest portion of the Los Angeles basin. In addition, although the rock site recording has a relatively simple wave shape, the basin site recordings are dominated by late arriving, large amplitude pulses of energy. We interpret these arrivals to be surface waves, which are generated by body waves interacting with the thickening margin of the basin. A preliminary modeling analysis of these data indicates that a combination of both large-scale (deep basin) and small-scale (shallow micro-basin) structures are needed to explain the observed responses.