ALBERT

Beschreibung: Because of the limited number of strong-motion records that have measured ground response at large strains, any statistical analyses of seismic site-response models subject to strong ground motions are severely limited by a small number of observations. Recent earthquakes in Japan, including the M w  9.0 Tohoku earthquake of March 2011, have substantially increased the observations of strong-motion records that can be used to compare alternative site-response models at large strains and can subsequently provide insight into the accuracy and precision of site-response models. Using the Kiban-Kyoshin network (KiK-net) downhole array data in Japan, we analyze the accuracy (bias) and variability (precision) resulting from common site-response modeling assumptions, and we identify critical parameters that significantly contribute to the uncertainty in site-response analyses. We perform linear and equivalent-linear site-response analyses at 100 KiK-net sites using 3720 ground motions ranging in amplitude from weak to strong; 204 of these records have peak ground accelerations greater than at the ground surface. We find that the maximum shear strain in the soil profile, the observed peak ground acceleration at the ground surface, and the predominant spectral period of the surface ground motion are the best predictors of where the evaluated models become inaccurate and/or imprecise. The peak shear strains beyond which linear analyses become inaccurate in predicting surface pseudospectral accelerations (PSA; presumably as a result of nonlinear soil behavior) are a function of vibration period and are between 0.01% and 0.1% for periods 〈0.5 s. Equivalent-linear analyses become inaccurate at peak strains of ~0.4% over this range of periods. We find that, for the sites and ground motions considered, site-response residuals at spectral periods 〉0.5 s do not display noticeable effects of nonlinear soil behavior. Online Material: Site-specific information and model residuals at 100 KiK-net stations.

Beschreibung: The applicability of foreign ground-motion prediction equations (GMPEs) for predicting geometric-mean pseudospectral acceleration amplitudes from active shallow crustal earthquakes in New Zealand (NZ) is examined. Four different foreign GMPEs were considered, as well as the NZ-based McVerry et al. (2006) (McV06) model. It was found that the McV06 model exhibited the lowest applicability with a database of 2437 recorded ground motions, and that the Chiou et al. (2010) (C10) modification of the Chiou and Youngs (2008b) (CY08) model was the most applicable. Discrepancies between the C10 model and the NZ database, which were empirically identified and theoretically justified, were used to modify the C10 model for: (1) small magnitude scaling; (2) scaling of short period ground motion from normal faulting events in volcanic crust; (3) scaling of ground motions on very hard rock sites; (4) anelastic attenuation in the NZ crust; and (5) consideration of the increased anelastic attenuation in the Taupo Volcanic Zone. The developed NZ-specific model therefore contains features as evident from recorded ground motions in NZ and consistent scaling for parameters not well constrained by NZ data. Comparisons with ground motions from the 4 September 2010 Darfield and 22 February 2011 Christchurch earthquakes, which occurred following completion of the NZ-specific model, illustrate that it provides an empirical prediction with sufficient accuracy and precision.

Beschreibung: Continuous observational monitoring of a study site in eastern Christchurch, New Zealand, following the 2010 M w 7.1 Darfield earthquake has recorded ten distinct liquefaction episodes in the mainshock–aftershock sequence. Three nearby accelerometers allow calibration between the geological expressions of liquefaction and the intensity of earthquake-induced surface ground motion at the site. Sand blow formation was generated by M w 5.2–7.1 earthquakes with M w 7.5–normalized peak ground accelerations (PGA 7.5 ) of ≥ 0.057 g (acceleration due to gravity). Silt drapes between successive sand blow deposits provide markers for delineating distinct liquefaction-inducing earthquakes in the geologic record. However, erosion quickly modifies the surface of sand blows into alluvial and aeolian forms that complicate geologic diagnosis. The two feeder-dike generations identified in subsurface investigations significantly underrepresent the number of liquefaction-inducing earthquakes due to extensive dike reactivation. New constitutive equations enable PGA 7.5 variations to be estimated from the thickness and areal extent of sand blows.

Beschreibung: This article presents results from the consideration of epistemic uncertainties in New Zealand (NZ) probabilistic seismic-hazard analysis. Uncertainties in ground-motion prediction are accounted for via multiple ground-motion prediction equations within the logic-tree framework. Uncertainties in the fault-based seismicity of the earthquake rupture forecast due to uncertainties in fault geometry, slip parameters, and magnitude-scaling relationships are considered in a Monte Carlo simulation framework. Because of the present lack of fault-specific data quantifying uncertainties for many faults in NZ, representative values based on judgement and available data for NZ and foreign faults were utilized. Uncertainties in the modelling of background seismicity were not considered. The implications of the considered epistemic uncertainties in terms of earthquake magnitude–frequency distributions and probabilistic seismic-hazard analyses for two spectral acceleration ordinates, two soil classes, and two locations (Wellington and Christchurch) are examined. The results illustrate that, for the uncertainties considered, the variation in seismic hazard due to the adopted ground-motion prediction model is larger than that due to the uncertainties in the earthquake rupture forecast. Of the earthquake rupture forecast uncertainties considered, the magnitude-geometry scaling relationships was the most significant, followed by fault rupture length. Hence, the obtained results provide useful guidance on which modelling issues are the most critical in the reliability of seismic-hazard analyses for locations in NZ.

Beschreibung: This article presents results from the consideration of epistemic uncertainties in New Zealand (NZ) probabilistic seismic-hazard analysis. Uncertainties in ground-motion prediction are accounted for via multiple ground-motion prediction equations within the logic-tree framework. Uncertainties in the fault-based seismicity of the earthquake rupture forecast due to uncertainties in fault geometry, slip parameters, and magnitude-scaling relationships are considered in a Monte Carlo simulation framework. Because of the present lack of fault-specific data quantifying uncertainties for many faults in NZ, representative values based on judgement and available data for NZ and foreign faults were utilized. Uncertainties in the modelling of background seismicity were not considered. The implications of the considered epistemic uncertainties in terms of earthquake magnitude-frequency distributions and probabilistic seismic-hazard analyses for two spectral acceleration ordinates, two soil classes, and two locations (Wellington and Christchurch) are examined. The results illustrate that, for the uncertainties considered, the variation in seismic hazard due to the adopted ground-motion prediction model is larger than that due to the uncertainties in the earthquake rupture forecast. Of the earthquake rupture forecast uncertainties considered, the magnitude-geometry scaling relationships was the most significant, followed by fault rupture length. Hence, the obtained results provide useful guidance on which modelling issues are the most critical in the reliability of seismic-hazard analyses for locations in NZ.

Beschreibung: Acceleration spectrum intensity (ASI), defined as the integral of the pseudospectral acceleration of a ground motion from 0.1 to 0.5 sec, was originally proposed as a ground-motion intensity measure (IM) relevant for the seismic response of concrete dams over two decades ago. ASI may be a desirable IM in emerging performance-based earthquake engineering frameworks because its consideration of a range of spectral periods makes it useful for concurrent prediction of acceleration and displacement demands in individual structures and also for regional loss estimation where short-period structures are typically prevalent. This article presents a theoretical basis for predicting ASI, based on prediction equations for spectral acceleration, both for individual sites and spatially distributed regions. ASI is found to have a better predictability than conventional ground-motion IMs such as elastic pseudospectral acceleration at a specific period. Furthermore, for site-specific applications conditional response spectra are derived, which can be considered as the correct target response spectra for ground-motion selection, and the features of these conditional spectra as a function of earthquake magnitude, source-to-site distance, and epsilon are examined. For spatially distributed applications, the intraevent correlation of ASI as a function of the separation distance of two sites is derived and compared to that of other common IMs.

Beschreibung: Because of the limited number of strong-motion records that have measured ground response at large strains, any statistical analyses of seismic site-response models subject to strong ground motions are severely limited by a small number of observations. Recent earthquakes in Japan, including the M (sub w) 9.0 Tohoku earthquake of March 2011, have substantially increased the observations of strong-motion records that can be used to compare alternative site-response models at large strains and can subsequently provide insight into the accuracy and precision of site-response models. Using the Kiban-Kyoshin network (KiK-net) downhole array data in Japan, we analyze the accuracy (bias) and variability (precision) resulting from common site-response modeling assumptions, and we identify critical parameters that significantly contribute to the uncertainty in site-response analyses. We perform linear and equivalent-linear site-response analyses at 100 KiK-net sites using 3720 ground motions ranging in amplitude from weak to strong; 204 of these records have peak ground accelerations greater than Formula at the ground surface. We find that the maximum shear strain in the soil profile, the observed peak ground acceleration at the ground surface, and the predominant spectral period of the surface ground motion are the best predictors of where the evaluated models become inaccurate and/or imprecise. The peak shear strains beyond which linear analyses become inaccurate in predicting surface pseudospectral accelerations (PSA; presumably as a result of nonlinear soil behavior) are a function of vibration period and are between 0.01% and 0.1% for periods 〈0.5 s. Equivalent-linear analyses become inaccurate at peak strains of approximately 0.4% over this range of periods. We find that, for the sites and ground motions considered, site-response residuals at spectral periods 〉0.5 s do not display noticeable effects of nonlinear soil behavior. Online Material: Site-specific information and model residuals at 100 KiK-net stations.

Beschreibung: The applicability of foreign ground-motion prediction equations (GMPEs) for predicting geometric-mean pseudospectral acceleration amplitudes from active shallow crustal earthquakes in New Zealand (NZ) is examined. Four different foreign GMPEs were considered, as well as the NZ-based McVerry et al. (2006) (McV06) model. It was found that the McV06 model exhibited the lowest applicability with a database of 2437 recorded ground motions, and that the Chiou et al. (2010) (C10) modification of the Chiou and Youngs (2008b) (CY08) model was the most applicable. Discrepancies between the C10 model and the NZ database, which were empirically identified and theoretically justified, were used to modify the C10 model for: (1) small magnitude scaling; (2) scaling of short period ground motion from normal faulting events in volcanic crust; (3) scaling of ground motions on very hard rock sites; (4) anelastic attenuation in the NZ crust; and (5) consideration of the increased anelastic attenuation in the Taupo Volcanic Zone. The developed NZ-specific model therefore contains features as evident from recorded ground motions in NZ and consistent scaling for parameters not well constrained by NZ data. Comparisons with ground motions from the 4 September 2010 Darfield and 22 February 2011 Christchurch earthquakes, which occurred following completion of the NZ-specific model, illustrate that it provides an empirical prediction with sufficient accuracy and precision.