Engineers use earthquake ground motions for a variety of reasons, including seismic hazard assessment, calibration of ground motion prediction equations (GMPEs), and input to nonlinear response history analysis. These analyses require a significant number of ground motions and for some scenarios, such as earthquakes with large magnitudes and short distances, it may be difficult to obtain a sufficient number of ground motion recordings. When sufficient recordings do not exist, engineers modify available recordings using scaling or spectrum matching, or they use ground motion simulations. Ground motion simulations have existed for decades, but recent advances in simulation methods due to improved source characterization and wave propagation, coupled with increased computing power, have increased potential benefits for engineers. But before simulations can be used in engineering applications, simulations must be accessible and consistent with natural observations. This dissertation contributes to the latter issue, and it investigates the application of simulations to specific engineering problems. The Southern California Earthquake Center (SCEC) Broadband Platform (BBP) is an open-source software distribution that enables third-party users to simulate ground motions using research code contributed by model developers. Because the BBP allows users to compute their own simulations with little knowledge of the underlying implementation and it ensures that all calculations are reproducible, it is extremely valuable for simulation validation and engineering applications. In this dissertation, the BBP is evaluated as a simulation generation tool from an engineering perspective. Ground motions are simulated to study parameters of engineering interest, such as high-frequency variability, near-fault ground motions, and local site response. Though some parameters need further development, such as site response (which is currently implemented using simple empirical amplification), the BBP proves to be an effective tool for facilitating these types of engineering studies. This dissertation proposes a simulation validation framework based on simple and robust proxies for the response of more complicated structures. We compile a list of proxies with robust empirical models that are insensitive to changes in earthquake scenario and do not rely on extrapolation for rarely observed events. Because predictions of these proxies are reliable under a variety of earthquake events, we can confidently compare them with simulations. The proposed proxies include correlation of epsilon across periods, ratio of maximum to median response across horizontal orientations, and ratio of inelastic to elastic displacement. The validation framework is applied to example simulations and successfully exposes some parameters that need work, such as variability and correlation of spectral acceleration. Finally, this dissertation investigates the application of simulations to response history analysis and fling-step characterization. A 3D nonlinear structural model is analyzed using recordings and simulations with similar elastic response spectra. The structural performance and resulting design decisions are similar, indicating that simulations are effective for response history analysis subject to certain conditions. To investigate fling-step, we extract fling pulses from a large set of simulations. Extracted fling properties such as amplitude and period are then compared to specially-processed recordings and relevant empirical models for surface displacement and pulse period. Reasonably good agreement is found between simulations, recordings, and empirical models. In general, ground motion simulations are found to be an effective alternative or supplement to recordings in several engineering applications. Because simulation methods are still developing, this work is not intended as an evaluation of existing methods, but rather as a development of procedures that can be used in ongoing work.

Accurate prediction of strong ground motion is central to seismic hazard analysis in order to estimate losses during major earthquakes. Ground motion simulations are essential to seismic ground motion prediction, especially for locations of infrequent observations, such as large magnitude and short distance events, where simulations can provide a viable alternative to data. Therefore, enhancement and validation of ground motion simulations, the primary goal of this dissertation, are highly desirable. In Chapter 2, we quantify the effects of four important factors on ground motions from large normal-faulting earthquakes on the Wasatch fault in the Salt Lake Basin: rupture direction, location on the hanging wall versus the footwall, deep 3D basin structure, and the distance from the rupture in the near field range. In Chapter 3, we attempt to validate the presence of several proposed waveguides in the Los Angeles area using 3D simulations and observed data from ambient noise. Here, we compare the numerical and empirical surface-to-surface Green tensors for virtual sources located on the San Andreas Fault. The regions of large peak motions caused by waveguide focusing in the simulations show generally good agreement with increases in the Green tensor amplitudes, supporting the presence of two separate waveguides in greater Los Angeles. In Chapters 4 and 5, we develop an empirical frequency-dependent spatial ground motion correlation model and methods to rectify simulation techniques that otherwise produce synthetic time histories deficient in inter-frequency and spatial correlation structure. The methods are tested using a hybrid deterministic-stochastic broadband ground motion generation module, where our method reproduces the empirical correlations well for a large number of realizations without biasing the fit of the median of the spectral accelerations to data. We find that the best fit of the inter-frequency correlation to data is obtained assuming that the horizontal components are correlated with a correlation coefficient of about 0.7.

Earthquake ground motion records are used as inputs for seismic hazard analysis, development of ground motion prediction equations and nonlinear response history analysis of structures. Real records from past earthquake events have traditionally been recognized as the best representation of seismic input to these analysis. However, our current way of implementing recorded ground motions is poorly constrained and suffers from the paucity of certain condition ground motions, such as the one with short distance and large magnitude. Meanwhile, even though the scaled ground motion is capable of matching the target spectrum, the content of frequency domain and ground motion parameters become unrealistic. With the rapid growth of computational ability and efficiency of computers, simulated ground motion can be an alternative to provide detailed and accurate prediction of earthquake effect. At the same time, simulated ground motions can provide a better representation of the whole ground motion generation process, such as fault rupture, wave propagation phenomena, and site response characterization. Hence, the aforementioned disadvantage of recorded ground motion can be overcame.Despite ground motion simulations have existed for decades, and the design code, such as ASCE/SEI 7-10 (ASCE, 2010), allow use of simulated ground motions for engineering practice, engineers still worried about the stability in ground motion simulation process and similarity between response of engineered structures to similar simulated and recorded ground motions. In order to draw simulated ground motions into engineering applications and make them practical, this dissertation is making contribution to address this issue. Simulated ground motions have to be validated and compared with recorded ground motions to prove their equivalence in engineering applications.This dissertation proposes a simulation validation framework. First step: Identify ground motion waveform parameters that well correlate with response of Multi-Degree of Freedom (MDOF) buildings and bridges. Second step: Develop goodness-of-fit measures and error functions that can describe the difference between simulated and recorded ground motion waveform characteristics and their effect on MDOF systems. Third step: Device the required update to ground motion simulation methods through which better simulations are possible. Forth step: Assess the current state of simulated ground motions for engineering applications.In general, simulated ground motions are found to be an effective surrogate and replenishment of natural records in engineering applications. However, certain drawbacks are detected, 1) Simulated ground motions are likelihood to mismatch certain ground motion parameters, for example, Arias intensity, duration and so on; 2) Structural behavior resulting from recorded ground motions and simulated ground motions are different. The difference stems from the fact that simulated motions are mostly pulse like motions. Because the simulation methods are still developing, our intent is not ranking or classifying them, but rather to provide feedback to update ground motion simulation techniques such that future simulations are more representative of recorded motions.

The study presented herein provides validation of five methodologies (EXSIM, GP, Irikura-Recipe, SDSU, and SONG) for ground motion simulation. The 1994 Northridge earthquake is selected alongside a bridge model as the test bed for this validation. This validation comprises a comparison between Engineering Demand Parameters (EDPs) obtained from Nonlinear Response History Analysis (NLRHA) of the bridge model subject to simulated and recorded motions of Northridge. The significance of the differences between the two EDPs are correlated with Intensity Measures (IMs) of the simulated and recorded motions. The IMs considered are normalized Arias intensity (Ia), Significant Duration (Td), time at mid-duration (tmin), rate of energy accumulation ( Ia/Td), rate of change in predominant frequency (o'), and predominant frequency at mid duration (omid). A regression of logged recorded parameters yielded Ia, Td, and omid as significant IMs. A regression analysis of logged recorded ground motions discovered that o' was also significant when considering multiple ground motion angles from 0 to 180 at 9° increments. A mixed effects regression was performed to establish the influence of simulation realization, ground motion station, and angle of rotation possessed on the model. Statistical distributions of the recorded and simulated IMs and EDP were compared to each other. Regardless of simulation methodology, the mean and variance of the simulated EDP were comparable to that of the recorded. It is recommended that the methods employed in this study be used with various recorded earthquakes in future research to assess the practicality of this validation approach.

In many procedures of seismic risk mitigation, ground motion simulations are needed to test systems or improve their effectiveness. For example they may be used to estimate the level of ground shaking caused by future earthquakes. Good physical models for ground motion simulation are also thought to be important for hazard assessment, as they could close gaps in the existing datasets. Since the observed ground motion in nature shows a certain variability, part of which cannot be explained by macroscopic parameters such as magnitude or position of an earthquake, it would be desirable that a good physical model is not only able to produce one single seismogram, but also to reveal this natural variability. In this thesis, I develop a method to model realistic ground motions in a way that is computationally simple to handle, permitting multiple scenario simulations. I focus on two aspects of ground motion modelling. First, I use deterministic wave propagation for the whole frequency range - from static deformation to approximately 10 Hz - but account for source variability by implementing self-similar slip distributions and rough fault interfaces. Second, I scale the source spectrum so that the modelled waveforms represent the correct radiated seismic energy. With this scaling I verify whether the energy magnitude is suitable as an explanatory variable, which characterises the amount of energy radiated at high frequencies - the advantage of the energy magnitude being that it can be deduced from observations, even in real-time. Applications of the developed method for the 2008 Wenchuan (China) earthquake, the 2003 Tokachi-Oki (Japan) earthquake and the 1994 Northridge (California, USA) earthquake show that the fine source discretisations combined with the small scale source variability ensure that high frequencies are satisfactorily introduced, justifying the deterministic wave propagation approach even at high frequencies. I demonstrate that the energy magnitude can be used to calibrate the high-frequency content in ground motion simulations. Because deterministic wave propagation is applied to the whole frequency range, the simulation method permits the quantification of the variability in ground motion due to parametric uncertainties in the source description. A large number of scenario simulations for an M=6 earthquake show that the roughness of the source as well as the distribution of fault dislocations have a minor effect on the simulated variability by diminishing directivity effects, while hypocenter location and rupture velocity more strongly influence the variability. The uncertainty in energy magnitude, however, leads to the largest differences of ground motion amplitude between different events, resulting in a variability which is larger than the one observed. For the presented approach, this dissertation shows (i) the verification of the computational correctness of the code, (ii) the ability to reproduce observed ground motions and (iii) the validation of the simulated ground motion variability. Those three steps are essential to evaluate the suitability of the method for means of seismic risk mitigation.

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