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The coda quality factor of short-period S waves (Qc) excited by local earthquakes in the Pyrenees has been measured as a function of the length of the coda window (Lw) for different choices of the onset time of the coda (tw) . In the 2 - 1 6 Hz frequency band, we observe a transient regime characterized by an increase of Qc with Lw, followed by a stabilization around a plateau the value of which depends on the central frequency of the signal. Using Monte Carlo simulations of wave transport in a variety of random media (≈ 1200 models), we demonstrate that the lapsetime dependence of Qc in the Pyrenees may be modeled by multiple anisotropic scattering of seismic waves, without invoking any depth dependence of the attenuation properties in the crust. In our model, anisotropic scattering is quantified by the ratio between the transport mean free path and the mean path (l*/l). At 6 Hz, the data require an anisotropy factor l*/l ≥ 5, a transport mean free path l* ≈ 400 km, and an intrinsic quality factor Qt ≈ 800. From the frequency-dependent plateau of Qc at large lapse time, we infer an intrinsic quality factor of the form Qi ≈ 400f0.4 in the Pyrenees. We also show how the rapid increase of the lapse-time dependence of Qc with frequency may be exploited to put constraints on the power spectrum of heterogeneities in the crust. [ABSTRACT FROM AUTHOR]

Equipartition is a state in which energy of all possible wave modes becomes equal due to multiple scattering among the modes. This is important to identify the scattering regime and constitutes the basic principle underlying seismic interferometry. In this study, we calculate partitioning of total kinetic energy into the horizontal and vertical components, similar to the horizontal-to-vertical ratio, for S-wave coda of 60 local earthquake records at three borehole stations in Japan. S-wave coda in lapse times of 40-80 s and in the 2-16 Hz frequency band is used. At the surface receivers of two rock sites, the horizontal components are dominant and their contribution gradually increases with frequency. At a soft site, the energy partitioning shows stronger variations with frequency, reflecting the presence of low-velocity layers. Subsurface receivers at about 100 m depths show larger contribution of the horizontal components, irrespective of site conditions. We quantitatively explain the observations with random wave fields at equipartition in horizontally layered structures derived from well logging. Through the modeling, we test three formulations of equipartition: a mixture of (1) only body waves, (2) only surface waves, and (3) both body and surface waves. Subsurface receivers are critically important to distinguish among the three formulations. For S-wave coda at frequencies lower than about 5 Hz, equipartition holds among both body and surface waves. For higher frequencies, equipartition holds predominantly among body waves. Equipartition of coda waves enables a simple modeling of the horizontal-to-vertical ratio, which contrasts with ambient noise. [ABSTRACT FROM AUTHOR]

Physical models that can be used to obtain realistic accelerograms usually require a thorough knowledge of the source, path, and site effects. In addition, the computational resources needed might be expensive. Thus, empirical models still represent a good alternative for simulating strong ground motion. In this work, we modify and improve the model developed by Sabetta and Pugliese (1996). This new method models the time-domain accelerogram based on the assumption that the phase is random and that the time envelope can be represented by the so-called average instantaneous power. This is, in turn, described as a lognormal distribution for P and S waves combined with an algebro-exponential function representing the envelope of coda waves. In addition, the frequency content of the signal is nonstationary and follows a modified ω-square model. The method depends on four common indicators in earthquake engineering: peak ground acceleration, strong-motion duration, Arias intensity, and central frequency. These indicators are empirically connected to a given database by means of ground-motion prediction equations. In this study we calibrate the model using Japanese data recorded by the K-net array, which has high-quality digital accelerograms and station-site conditions characterized by geotechnical measurements. In addition, this technique permits the inclusion of the uncertainty of the model parameters to take into account the ground-motion natural variability in the stochastic generation of the time histories. The main goal of this work is to provide the earthquake engineering community with a flexible tool to generate realistic accelerograms for dynamic studies. [ABSTRACT FROM AUTHOR]