Forward directivity effects are known to cause pulselike ground motions at near-fault sites. We propose a comprehensive framework to incorporate the effects of near-fault pulselike ground motions in probabilistic seismic hazard analysis (PSHA) computations. Also proposed is a new method to classify ground motions as pulselike or non-pulselike by rotating the ground motion and identifying pulses in all orientations. We have used this method to identify 179 recordings in the Next Generation Attenuation (NGA) database , where a pulselike ground motion is observed in at least one orientation. Information from these 179 recordings is used to fit several data-constrained models for predicting the probability of a pulselike ground motion occurring at a site, the orientations in which they are expected relative to the strike of the fault, the period of the pulselike feature, and the response spectrum amplification due to the presence of a pulselike feature in the ground motion. An algorithm describing how to use these new models in a modified PSHA computation is provided. The proposed framework is modular, which will allow for modification of one or more models as more knowledge is obtained in the future without changing other models or the overall framework. Finally, the new framework is compared with existing methods to account for similar effects in PSHA computation. Example applications are included to illustrate the use of the proposed framework, and implications for selection of ground motions for analysis of structures at near-fault sites are discussed.
Ground motions with strong velocity pulses are of special concern for structural engineers. We describe an efficient and quantitative method to identify such ground motions. Previous algorithms to classify these pulse-like ground motions considered the ground motion in a single orientation, which made classifying pulses in arbitrary orientations difficult. We propose an algorithm that can identify pulses at arbitrary orientations in multicomponent ground motions, with little extra computational cost relative to a single-orientation calculation. We use continuous wavelet transforms of two orthogonal components of the ground motion to identify the orientations most likely to contain a pulse. The wavelet transform results are then used to extract pulses from the selected orientations, and a new classification criterion based on support vector machines is proposed. Because we are mostly interested in forward directivity pulses, which are found early in the time history, a criterion to reject pulses arriving late in the time history is also proposed. The procedure was used to classify ground motions in the Next Generation Attenuation-West2 database (Ancheta et al., 2013). The list of pulse-like ground motions was then manually filtered using sourceto-site geometry and site conditions to find the pulses most likely caused by directivity effects. Lists of both pulse-like ground motions and directivity ground motions are provided, along with the periods of the pulses and the orientations in which the pulses were strongest. Using the classification results, new models to predict the probability of a pulse and pulse period for a given future earthquake scenario are developed.
The NGA-West2 project is a large multidisciplinary, multi-year research program on the Next Generation Attenuation (NGA) models for shallow crustal earthquakes in active tectonic regions. The research project has been coordinated by the Pacific Earthquake Engineering Research Center (PEER), with extensive technical interactions among many individuals and organizations. NGA-West2 addresses several key issues in ground-motion seismic hazard, including updating the NGA database for a magnitude range of 3.0–7.9; updating NGA ground-motion prediction equations (GMPEs) for the “average” horizontal component; scaling response spectra for damping values other than 5%; quantifying the effects of directivity and directionality for horizontal ground motion; resolving discrepancies between the NGA and the National Earthquake Hazards Reduction Program (NEHRP) site amplification factors; analysis of epistemic uncertainty for NGA GMPEs; and developing GMPEs for vertical ground motion. This paper presents an overview of the NGA-West2 research program and its subprojects.
The NGA-West2 research program, coordinated by the Pacific Earthquake Engineering Research Center (PEER), is a major effort to produce refined models for predicting ground motion response spectra. This study presents new models for ground motion directionality developed as part of that project. Using a database of recorded ground motions, empirical models have been developed for a variety of quantities related to direction-dependent spectra. A model is proposed for the maximum spectral acceleration observed in any orientation of horizontal ground motion shaking ( Sa RotD100), which is formulated as a multiplicative factor to be coupled with the NGA-West2 models that predict the median spectral accelerations over all orientations ( Sa RotD50). Models are also proposed for the distribution of orientations of the Sa RotD100 value, relative to the fault and the relationship between Sa RotD100 orientations at differing periods. Discussion is provided regarding how these results can be applied to perform seismic hazard analysis and compute realistic target spectra conditioned on different parameters.
Five directivity models have been developed based on data from the NGA-West2 database and based on numerical simulations of large strike-slip and reverse-slip earthquakes. All models avoid the use of normalized rupture dimension, enabling them to scale up to the largest earthquakes in a physically reasonable way. Four of the five models are explicitly “narrow-band” (in which the effect of directivity is maximum at a specific period that is a function of earthquake magnitude). Several strategies for determining the zero-level for directivity have been developed. We show comparisons of maps of the directivity amplification. This comparison suggests that the predicted geographic distributions of directivity amplification are dominated by effects of the models’ assumptions, and more than one model should be used for ruptures dipping less than about 65 degrees.
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