Seismic attenuation

In this work, we map the absorption properties of the French crust by analyzing the decay properties of coda waves. Estimation of the coda quality factor Q c in five non-overlapping frequency-bands between 1 and 32 Hz is performed for more than 12,000 high-quality seismograms from about 1700 weak to moderate crustal earthquakes recorded between 1995 and 2013. Based on sensitivity analysis, Q c is subsequently approximated as an integral of the intrinsic shear wave quality factor Q i along the ray connecting the source to the station. After discretization of the medium on a 2-D Cartesian grid, this yields a linear inverse problem for the spatial distribution of Q i. The solution is approximated by redistributing Q c in the pixels connecting the source to the station and averaging over all paths. This simple procedure allows to obtain frequency-dependent maps of apparent absorption that show lateral variations of 50% at length scales ranging from 50 km to 150 km, in all the frequency bands analyzed. At low frequency, the small-scale geological features of the crust are clearly delineated: the Meso-Cenozoic basins (Aquitaine, Brabant, Southeast) appear as strong absorption regions, while crystalline massifs (Armorican, Central Massif, Alps) appear as low absorption zones. At high frequency, the correlation between the surface geological features and the absorption map disappears, except for the deepest Meso-Cenozoic basins which exhibit a strong absorption signature. Based on the tomographic results, we explore the implications of lateral variations of absorption for the analysis of both instrumental and historical seismicity. The main conclusions are as follows: (1) current local magnitude M L can be over(resp. under)-estimated when absorption is weaker(resp. stronger) than the nominal value assumed in the amplitude-distance relation ; (2) both the forward prediction of the earthquake macroseismic intensity field and the estimation of historical earthquake seismological parameters using macroseismic intensity data are significantly improved by taking into account a realistic 2-D distribution of absorption. In the future, both M L estimations and macroseismic intensity attenuation models should benefit from high-resolution models of frequency-dependent absorption such as the one produced in this study.

The seismic effective quality factor (Q C) and its frequently dependences or the frequency parameter (n) and attenuation coefficient (δ) for the Earth's crust and upper mantle of the North Tanzanian divergence zone (East African rift system) were estimated from an analysis of the earthquake coda waves recorded in the SEISMO-TANZ'07 French-Tanzanian seismic experiment. The Q C values increase and the n and δ values decrease with increasing frequency and length of the lapse time window. This behavior of the attenuation parameters may be evidence that the degree of heterogeneity of the lithosphere decreases with depth. Comparison of the depth variations in the attenuation coefficient δ and the frequency parameter n with the velocity structure of the region shows that there is a distinct change in the behavior of seismic wave attenuation at velocity discontinuities. The obtained attenuation parameters were compared with the same parameters obtained in our previous studies for other continental rift systems—the Baikal rift system (Eurasia) and the Basin and Range Province (North America).

A total number of 46 local earthquakes (2.0rMLr4.0) recorded in the period 2000-2011 by the Egyptian seismographic network (ENSN) were used to estimate the total (Q t À 1 ), intrinsic (Q i À 1 ) and scattering attenuation (Q sc À 1 ) in Cairo metropolitan area, Egypt. The multiple lapse time window analysis (MLTWA) under the assumption of multiple isotropic scattering with uniform distribution of scatters was firstly applied to estimate the pair of L e À 1 , the extinction length inverse, and B 0 , the seismic albedo, in the frequency range 3-24 Hz. To take into account the effect of a depth-dependent earth model, the obtained values of B 0 and L e À 1 were corrected for an earth structure characterized by a transparent upper mantle and a heterogeneous crust. The estimated values of Q t À 1 , Q sc À 1 and Q i À 1 exhibited frequency dependences. The average frequency-dependent relationships of attenuation characteristics estimated for the region are found to be: Q t À 1 ¼(0.01570.008)f (À 1.02 7 0.02) , Q sc À 1 ¼ (0.00670.001)f (À 1.01 7 0.02) , and Q i À 1 ¼(0.00970.008) f (À 1.03 7 0.02) ; showing a predominance of intrinsic absorption over scattering attenuation. This finding implies that the pore-fluid contents may have great effect on the attenuation mechanism in the upper crust where the River Nile is passing through the study area. The obtained results are comparable with those obtained in other tectonic regions.

Within the last decade, advances on empirical formula to determine ground acceleration is strongly involved in order to undertake seismic hazard analysis. We need to provide a means of estimating the ground shaking from an earthquake given its earthquake source type, ray-path, and local site condition called Ground Motion Prediction Equations (GMPEs). However, this study is rarely done in South and South-East of Sulawesi. This study aims to determine the GMPEs coefficient using selected 481 record of ground motion database by 18 accelerometer at 2011-2015. Local site condition of station is classified by dominant period and the type of earthquake is segregate according to tectonic regime that is shallow crustal, interface and intraslab. We made statistical regression analysis to find the relation between attenuation and distance. The result shows that attenuation relation of low value ground acceleration to increasing of distance has peak ground acceleration much lower, compare to early equation that fitted to peak ground acceleration observation in South and South-East of Sulawesi. It also show the effect of magnitude, focal depth, source distance, and local site can affected PGA predicted. This study plays vital role to understand the characteristic of ground motion attenuation.

The small scale heterogeneity of the mantle is mostly due to the mixing of petrological heterogeneities by a smooth but chaotic convection and should consist in a laminated structure (marble cake) with a power spectrum S(k) varying as 1/k, where k is the wavenumber of the anomalies. This distribution of heterogeneities during convective stirring with negligible diffusion, called Batchelor regime is documented by fluid dynamic experiments and corresponds to what can be inferred from geochemistry and seismic tomography. This laminated structure imposes density, seismic velocity and potentially, anisotropic heterogeneities with similar 1/k spectra. A seismic wave of wavenumber k 0 crossing such a medium is partly reflected by the heterogeneities and we show that the scattered energy is proportional to k 0 S(2k 0 ). The reduction of energy for the propagating wave appears therefore equivalent to a quality factor 1/Q ∝ k 0 S(2k 0 ). With the specific 1/k spectrum of the mantle, the resulting apparent attenuation should therefore be frequency independent. We show that the total contribution of 6-9% RMS density, velocity and anisotropy would explain the observed S and P attenuation of the mantle. Although these values are large, they are not unreasonable and we discuss how they depend on the range of frequencies over which the attenuation is explained. If such a level of heterogeneity were present, most of the attenuation of the Earth would be due to small scale scattering by laminations, not by intrinsic dissipation. Intrinsic dissipation must certainly exist but might correspond to a larger, yet unobserved Q . This provocative result would explain the very weak frequency dependence of the attenuation, and the fact that bulk attenuation seems negligible, two observations that have been difficult to explain for 50 years.

We apply a recently developed method based on the instantaneous frequency to analyze broadband seismic data recorded by the transportable USArray. We measure in the frequency band [0.018-0.2] Hz about 700 high-quality differential ScS-S anelastic delay times, δt ⋆ ScS-S , sampling the mantle below Central America and below Alaska that we compare to elastic delay times, δt ScS-S , obtained by cross-correlating the S and ScS signals. We confirm that the instantaneous frequency matching method is more robust than the classical spectral ratio method. By a series of careful analyses of the effects of signal-to-noise ratio, source mechanism characteristics and possible phase interferences on measurements of differential anelastic delay times, we demonstrate that in order to obtain accurate values of δt n ScS-S the seismic records must be rigorously selected. In spite of the limited number of data that satisfy our quality criteria, we recover, using an additional stacking procedure, a clear dependence of δt ⋆ ScS-S on the epicentral distance in the two regions. The absence of correlation between the obtained anelastic and elastic delay-times indicates a complex compositional-thermal origin of the attenuation structure, or effects of scattering by small scale structure, in accordance with possible presence of subducted material. The regional 1-D inversions of our measurements indicate a non-uniform lower mantle attenuation structure: a zone with high attenuation in the mid-lower mantle (Q μ ≈250) and a low attenuation layer at its base (Q μ ≈450). A comparison of our results with low-frequency normal-model Q models is consistent with frequencydependent attenuation with Q μ ∝ω α and α ¼ 0:1-0:2 (i.e. less attenuation at higher frequencies), although possible effects of lateral variations in Q in the deep mantle add some uncertainty to these values.