A method is described for quantitatively identifying ground motions containing strong velocity pulses, such as those caused by near-fault directivity. The approach uses wavelet analysis to extract the largest velocity pulse from a given ground motion. The size of the extracted pulse relative to the original ground motion is used to develop a quantitative criterion for classifying a ground motion as “pulselike.” The criterion is calibrated by using a training data set of manually classified ground motions. To identify the subset of these pulselike records of greatest engineering interest, two additional criteria are applied: the pulse arrives early in the ground motion and the absolute amplitude of the velocity pulse is large. The period of the velocity pulse (a quantity of interest to engineers) is easily determined as part of the procedure, using the pseudoperiods of the basis wavelets. This classification approach is useful for a variety of seismology and engineering topics where pulselike ground motions are of interest, such as probabilistic seismic hazard analysis, ground- motion prediction (“attenuation”) models, and nonlinear dynamic analysis of structures. The Next Generation Attenuation (nga) project ground motion library was processed using this approach, and 91 large-velocity pulses were found in the fault- normal components of the approximately 3500 strong ground motion recordings considered. It is believed that many of the identified pulses are caused by near-fault directivity effects. The procedure can be used as a stand-alone classification criterion or as a filter to identify ground motions deserving more careful study.

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Quantitative Classification of Near-Fault Ground Motions Using Wavelet Analysis

Introduced in this paper are several alternative ground-motion intensity measures (IMs) that are intended for use in assessing the seismic performance of a structure at a site susceptible to near-source and/or ordinary ground motions. A comparison of such IMs is facilitated by defining the “efficiency” and “sufficiency” of an IM, both of which are criteria necessary for ensuring the accuracy of the structural performance assessment. The efficiency and sufficiency of each alternative IM, which are quantified via (i) nonlinear dynamic analyses of the structure under a suite of earthquake records and (ii) linear regression analysis, are demonstrated for the drift response of three different moderate- to long-period buildings subjected to suites of ordinary and of near-source earthquake records. One of the alternative IMs in particular is found to be relatively efficient and sufficient for the range of buildings considered and for both the near-source and ordinary ground motions.

Structure-specific scalar intensity measures for near source and ordinary earthquake ground motions

A simple, yet effective, analytical model is proposed for the representation of near-field strong ground motions. The model adequately describes the impulsive character of near-fault ground motions both qualitatively and quantitatively. In addition, it can be used to analytically reproduce empirical observations that are based on available near-source records. The input parameters of the model have an unambiguous physical meaning. The proposed analytical model has been calibrated using a large number of actual near-field ground-motion records. It successfully simulates the entire set of available near-fault displacement, velocity, and (in many cases) acceleration time histories, as well as the corresponding deformation, velocity, and acceleration response spectra. Furthermore, a very simplified methodology for generating realistic synthetic ground motions that are adequate for engineering analysis and design is outlined and applied. Finally, it should be noted that the analytical model (along with the scaling laws of its parameters) proposed in the present work has the potential to facilitate the study of the elastic and inelastic response of conventional, nonconventional (e.g., base-isolated), and special structures (e.g., suspension bridges, fluid-storage tanks) subjected to near-source seismic excitations as a function of the model input parameters and thus, ultimately, as a function of earthquake size.

A Mathematical Representation of Near-Fault Ground Motions

................................................................................................................................. iii ACKNOWLEDGMENTS ........................................................................................................... iv CONTENTS....................................................................................................................................v LIST OF FIGURES ..................................................................................................................... xi LIST OF TABLES .....................................................................................................................xxv

Global collapse of frame structures under seismic excitations

In finite-fault modeling of earthquake ground motions, a large fault is divided into N subfaults, where each subfault is considered as a small point source The ground motions contributed by each subfault can be calculated by the stochastic point-source method and then summed at the observation point, with a proper time delay, to obtain the ground motion from the entire fault A new variation of this approach is introduced based on a "dynamic corner frequency" In this model, the corner frequency is a function of time, and the rupture history controls the frequency content of the simulated time series of each subfault The rupture begins with a high corner frequency and progresses to lower corner frequencies as the ruptured area grows Limiting the number of active subfaults in the calculation of dynamic corner frequency can control the amplitude of lower frequencies Our dynamic corner frequency approach has several advantages over previous formulations of the stochastic finite-fault method, including conservation of radiated energy at high frequencies regardless of subfault size, application to a broader mag- nitude range, and control of the relative amplitude of higher versus lower frequencies The model parameters of the new approach are calibrated by finding the best overall fit to a ground-motion database from 27 well-recorded earthquakes in California The lowest average residuals are obtained for a dynamic corner frequency model with a stress drop of 60 bars and with 25% of the fault actively slipping at any time in the rupture As an additional tool to allow the stochastic modeling to generate the impulsive long-period velocity pulses that can be caused by forward directivity of the source, the analytical approach proposed by Mavroeidis and Papageorgiou (2003) has been included in our program This novel mathematical model of near-fault ground mo- tions is based on a few additional input parameters that have an unambiguous physi- cal meaning; the method has been shown by Mavroeidis and Papageorgiou to sim- ulate the entire set of available near-fault displacement and velocity records, as well as the corresponding deformation, velocity, and acceleration response spectra The inclusion of this analytical model of long-period pulses substantially increases the power of the stochastic finite-fault simulation method to model broadband time his- tories over a wide range of distances, magnitudes, and frequencies

Stochastic Finite-Fault Modeling Based on a Dynamic Corner Frequency

Rupture directivity effects cause spatial variations in ground motion amplitude and duration around faults and cause differences between the strike-normal and strike-parallel components of horizontal ground motion amplitudes, which also have spatial variation around the fault. These variations become significant at a period of 0.6 second and generally grow in size with increasing period. We have developed modifications to empirical strong ground motion attenuation relations to account for the effects of rupture directivity on strong motion amplitudes and durations. The modifications are based on an empirical analysis of near-fault data. The ground motion parameters that are modified include the average horizontal response spectral acceleration, the duration of the acceleration time history, and the ratio of strike-normal to strike-parallel spectral acceleration. The parameters upon which the adjustments to average horizontal amplitude and duration depend are the fraction of the fault rupture that occurs on the part of the fault that lies between the hypocenter and the site, and the angle between the fault plane and the path from the hypocenter to the site. Since both of these parameters can be derived from the hypocenter location and the fault geometry, the model of rupture directivity effects on ground motions that we have developed can be directly included in probabilistic seismic hazard calculations. The spectral acceleration is larger for periods longer than 0.6 second, and the duration is smaller, when rupture propagates toward a site. For sites located close to faults, the strike-normal spectral acceleration is larger than the strike-parallel spectral acceleration at periods longer than 0.6 second in a manner that depends on magnitude, distance, and angle. To facilitate the selection of time histories that represent near-fault ground motion conditions in an appropriate manner, we provide a list of near-fault records indicating the rupture directivity parameters that each contains.

Modification of Empirical Strong Ground Motion Attenuation Relations to Include the Amplitude and Duration Effects of Rupture Directivity

INTRODUCTION 

A large amount of work has been done in recent years to estimate the distribution of slip on the fault surface during earthquakes. Generally, these slip models are derived from longer period ground motions: strong-motion velocity and displacement, and teleseismic velocity seismograms. At these longer periods, ground motions are predominantly deterministic and their waveforms can in general be accurately modeled using simple descriptions of the source and crustal structure. The opposite situation exists for the prediction of high-frequency strong ground motions. Ground motions at high frequencies are predominantly stochastic, and their waveforms in general cannot be accurately modeled using simple descriptions of the source and crustal structure. However, preliminary evidence ( e.g. , Hartzell et al. , 1996; Kamae and Irikura, 1998; Somerville, 1993; Somerville et al ., 1996; Wald et al. , 1988) suggests that variable slip models derived from longer-period ground-motion recordings are relevant for the prediction of higher-frequency ground motions. For...

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Nancy F. Smith

Papers

Modification of Empirical Strong Ground Motion Attenuation Relations to Include the Amplitude and Duration Effects of Rupture Directivity

Characterizing Crustal Earthquake Slip Models for the Prediction of Strong Ground Motion

Forward rupture directivity in the Kobe and Northridge earthquakes, and implications for structural engineering