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Most sonic logging data acquired in hydrate-bearing zones have shown increasing velocity of P and S waves accompanied by high attenuation of P and S waves. In the eastern Nankai Trough, the attenuation values of P and S wave estimated in the sonic logging frequency range (10–20 kHz for P waves and 0.5–1 kHz for S waves) also show a significant increase in hydrate-bearing sediments. However, P -wave attenuation values from zero-offset vertical seismic profiling (VSP) data (30–110 Hz) indicate a lower attenuation compared to the values in the sonic logging frequency range at the same well locations, which implies frequency-dependent P -wave attenuation in hydrate-bearing sediments. In order to elucidate the attenuation mechanisms responsible for this frequency-dependent P -wave attenuation in hydrate-bearing sediments, we apply two different rock physics models that have recently been developed to consider the squirt flow in porous/microporous hydrate and the interaction between sand and hydrate grains. The predicted attenuation values are compared with those derived from field sonic logging and VSP data. Finally, we infer that this frequency-dependent P -wave attenuation may be due to the squirt flow caused by the combined effect of the degree of hydrate saturation and two permeable systems (one is between sand grains and the other is between hydrate grains), and/or due to the squirt flow caused by fluid inclusions with different aspect ratios in a microporous hydrate. [ABSTRACT FROM AUTHOR]

In this paper we characterize the high-frequency (1.0-10 Hz) seismic wave crustal attenuation and the source excitation in the Sicily Channel and surrounding regions using background seismicity from weak-motion database. The data set includes 15 995 waveforms related to earthquakes having local magnitude ranging from 2.0 to 4.5 recorded between 2006 and 2012. The observed and predicted ground motions form the weak-motion data are evaluated in several narrow frequency bands from 0.25 to 20.0 Hz. The filtered observed peaks are regressed to specify a proper functional form for the regional attenuation, excitation and site specific term separately. The results are then used to calibrate effective theoretical attenuation and source excitation models using the random vibration theory. In the log-log domain, the regional seismic wave attenuation and the geometrical spreading coefficient are modelled together. The geometrical spreading coefficient, g(r), modelled with a bilinear piecewise functional form and given as g(r) ∝ r-1.0 for the short distances (r < 50 km) and as g(r) ∝ r-0.8 for the larger distances (r<50 km). A frequency-dependent quality factor, inverse of the seismic attenuation parameter, Q(f) = 160f/fref 0. 35 (where fref = 1.0 Hz), is combined to the geometrical spreading. The source excitation terms are defined at a selected reference distance with a magnitude independent roll-off spectral parameter, κ 0.04 s and with a Brune stress drop parameter increasing with moment magnitude, from Δσ = 2 MPa for Mw = 2.0 to Δσ = 13 MPa for Mw = 4.5. For events M ≤ 4.5 (being Mwmax = 4.5 available in the data set) the stress parameters are obtained by correlating the empirical/excitation source spectra with the Brune spectral model as function of magnitude. For the larger magnitudes (Mw>4.5) outside the range available in the calibration data set where we do not have recorded data, we extrapolate our results through the calibration of the stress parameters of the Brune source spectrum over the Bindi et al. ground-motion prediction equation selected as a reference model (hereafter also ITA10). Finally, the weak-motion-based model parameters are used through a stochastic approach in order to predict a set of region specific spectral ground-motion parameters (peak ground acceleration, peak ground velocity, and 0.3 and 1.0 Hz spectral acceleration) relative to the generic rock site as a function of distance between 10 and 250 km and magnitude between M 2.0 and M 7.0. [ABSTRACT FROM AUTHOR]

On 2013 April 13, an inland earthquake of Mw 5.8 occurred in Awaji Island, which forms the western boundary of the Osaka sedimentary basin in western Japan. The strong ground motion data were collected from more than 100 stations within the basin and it was found that in the Osaka Plain, the pseudo velocity response spectra at a period of around 6.5 s were significantly larger than at other stations of similar epicentral distance outside the basin. The ground motion lasted longer than 3 min in the Osaka Plain where its bedrock depth spatially varies from approximately 1 to 2 km. We modelled long-period (higher than 2 s) ground motions excited by this earthquake, using the finite difference method assuming a point source, to validate the present velocity structure model and to obtain better constraint of the attenuation factor of the sedimentary part of the basin. The effect of attenuation in the simulation was included in the form of Q(f) = Q0(f/f0), where Q0 at a reference frequency f0 was given by a function of the S-wave velocity, Q0 = αVS. We searched for appropriate Q0 values by changing α for a fixed value of f0 = 0.2 Hz. It was found that values of α from 0.2 to 0.5 fitted the observations reasonably well, but that the value of α = 0.3 performed best. Good agreement between the observed and simulated velocity waveforms was obtained for most stations within the Osaka Basin in terms of both amplitude and ground motion duration. However, underestimation of the pseudo velocity response spectra in the period range of 5-7 s was recognized in the central part of the Osaka Plain, which was caused by the inadequate modelling of later phases or wave packets in this period range observed approximately 2 min after the direct S-wave arrival.We analysed this observed later phase and concluded that it was a Love wave originating from the direction of the east coast of Awaji Island. [ABSTRACT FROM AUTHOR]

We present a new method for the modelling of frequency-dependent and frequency-independent Q in time-domain seismic wave propagation. Unlike previous approaches, attenuation models are constructed such that Q as a function of position in the Earth appears explicitly as a parameter in the equations of motion. This feature facilitates the derivation of Fréchet kernels for Q using adjoint techniques. Being simple products of the forward strain field and the adjoint memory variables, these kernels can be computed with no additional cost, compared to Fréchet kernels for elastic properties. The same holds for Fréchet kernels for the power-law exponent of frequency-dependent Q, that we derive as well. To illustrate our developments, we present examples from regional- and global-scale time-domain wave propagation. [ABSTRACT FROM AUTHOR]

SUMMARY We investigated the temporal changes in the seismic attenuation of a fault zone using near-source recordings of S waves from repeating microearthquakes that occurred both before and after M ∼ 2 earthquakes in the Bambanani gold mine, South Africa. Because the source locations and the mechanisms of repeating earthquakes can be regarded as identical, the attenuation change can be estimated using the spectral ratios for repeating earthquake pairs. We found an increase in the S-wave attenuation parameter on the vertical component which is positively correlated with frequency, corresponding to times before and after the M ∼ 2 earthquakes. This increase can be explained by scattering attenuation, with a typical scale of damage in the fault zone of ∼3 m. [ABSTRACT FROM AUTHOR]

The mathematical model for wave motion in a porous solid saturated by two immiscible fluids is solved for the propagation of harmonic plane waves along a general direction in 3-D space. The solution is obtained in the form of Christoffel equations, which are solved further to calculate the complex velocities and polarisations of three longitudinal waves and one transverse wave. For any of these four attenuated waves, a general inhomogeneous propagation is considered through a particular specification of complex slowness vector. Inhomogeneity of an attenuated wave is represented through a finite non-dimensional parameter. For an arbitrarily chosen value of this inhomogeneity parameter, phase velocity and attenuation of a wave are calculated from the specification of its slowness vector. This specification enables to separate the contribution from homogeneous propagation of attenuated wave to the total attenuation. The phenomenon of reflection is studied to calculate the partition of wave-induced energy incident at the plane boundary of the porous solid. A parameter is used to define the partial opening of pores at the surface of porous solid. An arbitrary value of this parameter allows to study the variations in the energy partition with the opening of surface pores from fully closed to perfectly open. Another parameter is used to vary the saturation in pores from whole liquid to whole gas. Numerical examples are considered to discuss the effects of propagation direction, inhomogeneity parameter, opening of surface pores and saturating pore-fluid on the partition of incident energy. [ABSTRACT FROM AUTHOR]

Seismic data enable imaging of the Earth, not only of velocity and density but also of attenuation contrasts. Unfortunately, the Born approximation of the constant-density visco-acoustic wave equation, which can serve as a forward modelling operator related to seismic migration, exhibits an ambiguity when attenuation is included. Different scattering models involving velocity and attenuation perturbations may provide nearly identical data. This result was obtained earlier for scatterers that did not contain a correction term for causality. Such a term leads to dispersion when considering a range of frequencies. We demonstrate that with this term, linearized inversion or iterative migration will almost, but not fully, remove the ambiguity. We also investigate if attenuation imaging suffers from the same ambiguity when using non-linear or full waveform inversion. A numerical experiment shows that non-linear inversion with causality convergences to the true model, whereas without causality, a substantial difference with the true model remains even after a very large number of iterations. For both linearized and non-linear inversion, the initial update in a gradient-based optimization scheme that minimizes the difference between modelled and observed data is still affected by the ambiguity and does not provide a good result. This first update corresponds to a classic migration operation. In our numerical experiments, the reconstructed model started to approximate the true model only after a large number of iterations. [ABSTRACT FROM AUTHOR]