The only numerically tractable way yet found to incorporate attenuation into numerical time‐domain computations of seismic wave fields is to approximate the viscoelastic modulus by a low‐order rational function of frequency. The coefficients of this function can be determined by the Padé approximation. Our test computations show, however, that this approximation generally is of poor quality. Therefore, we suggest a new approach which is based on the rheological model of the generalized Maxwell body, which has a modulus of the desired rational form. We choose the relaxation frequencies logarithmically equidistant in the frequency band of interest, and determine the weight factors by simple numerical curve fitting to an arbitrary Q law. This approach is superior to the method above both in accuracy and in computational efficiency. For most practical applications, approximations of orders 2 or 3 are sufficient. The computing time and memory requirements for a finite‐difference calculation are then approximately twice those of a purely elastic calculation. As a first application of the method, we compute SH channel waves in discontinuous coal seams with Q = 50 within the coal. The results show that the high‐ frequency Airy phase is strongly attenuated. This indicates that care has to be taken in comparing the results of purely elastic model calculations of the propagation of seam waves with experimental data.

[1] Passive Image Interferometry (PII) is a seismological method employing ambient seismic noise to monitor the temporal evolution of mean shear wave velocity within the Earth. First, the elastic Green's tensor between two seismometers is computed from the cross correlation of seismic noise recorded during a certain period. Second, the constructed seismograms of different time periods are treated as earthquake multiplets, and small time shifts in their coda are used to invert a relative change in mean shear wave velocity. When applied to the source region of the 2004 M w = 6.6 mid-Niigata earthquake, Japan (centroid depth 5 km), we used noise recorded at six seismometers located at a distance of less than 25 km from the epicenter. Daily inversions during the 2 months before and after the earthquake show a sudden drop of mean shear wave velocity of some tenths of a percent at the time of the mid-Niigata earthquake. Using noise in two frequency bands, 0.1-0.5 Hz and 2-8 Hz, we find similar amplitudes for the velocity drop, which indicates that changes are not restricted to the shallow subsurface. Possible interpretations of this velocity decrease are a decrease of crustal stress after the earthquake, a nonlinear site response in the shallow subsurface layer due to strong ground motion, or structural weakening due to the creation of new fractures in the source area of the earthquake.

The AlpArray programme is a multinational, European consortium to advance our understanding of orogenesis and its relationship to mantle dynamics, plate reorganizations, surface processes and seismic hazard in the Alps-Apennines-Carpathians-Dinarides orogenic system. The AlpArray Seismic Network has been deployed with contributions from 36 institutions from 11 countries to map physical properties of the lithosphere and asthenosphere in 3D and thus to obtain new, high-resolution geophysical images of structures from the surface down to the base of the mantle transition zone. With over 600 broadband stations Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s1071 2-018-9472-4) contains supplementary material, which is available to authorized users. operated for 2 years, this seismic experiment is one of the largest simultaneously operated seismological networks in the academic domain, employing hexagonal coverage with station spacing at less than 52 km. This dense and regularly spaced experiment is made possible by the coordinated coeval deployment of temporary stations from numerous national pools, including ocean-bottom seismometers, which were funded by different national agencies. They combine with permanent networks, which also required the cooperation of many different operators. Together these stations ultimately fill coverage gaps. Following a short overview of previous large-scale seismological experiments in the Alpine region, we here present the goals, construction, deployment, characteristics and data management of the AlpArray Seismic Network, which will provide data that is expected to be unprecedented in quality to image the complex Alpine mountains at depth.

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