Korn, M. (2016): Field observations of seismic velocity changes caused by shaking-induced damage and healing due to mesoscopic nonlinearity.

S U M M A R YWe developed an improved method for the separation of intrinsic and scattering attenuation of seismic shear waves by envelope inversion called Qopen. The method optimizes the fit between Green's functions for the acoustic, isotropic radiative transfer theory and observed energy densities of earthquakes. The inversion allows the determination of scattering and intrinsic attenuation, site corrections and spectral source energies for the investigated frequency bands. Source displacement spectrum and the seismic moment of the analysed events can be estimated from the obtained spectral source energies. We report intrinsic and scattering attenuation coefficients of shear waves near three geothermal reservoirs in Germany for frequencies between 1 and 70 Hz. The geothermal reservoirs are located in Insheim, Landau (both Upper Rhine Graben) and Unterhaching (Molasse basin). We compare these three sedimentary sites to two sites located in crystalline rock with respect to scattering and intrinsic attenuation. The inverse quality factor for intrinsic attenuation is constant in sediments for frequencies smaller than 10Hz and decreasing for higher frequencies. For crystalline rock, it is on a lower level and strictly monotonic decreasing with frequency. Intrinsic attenuation dominates scattering except for the Upper Rhine Graben, where scattering is dominant for frequencies below 10Hz. Observed source displacement spectra show a high-frequency fall-off greater than or equal to 3.Key words: Hydrothermal systems; Earthquake source observations; Seismic attenuation; Site effects; Wave scattering and diffraction. I N T RO D U C T I O NThe MAGS2 project (microseismic activity of geothermal systems) aims at the observation, understanding and hazard analysis of induced earthquake activity at deep geothermal systems. In the scope of the project geothermal reservoirs in the sedimentary basins of the Upper Rhine Graben and the Molasse basin are monitored for induced seismic activity. The three geothermal plants in Landau, Insheim (Upper Rhine Graben) and Unterhaching (Molasse basin) already induced earthquakes with magnitudes up to M2.7. One major goal of the project is the calculation of shake maps and its uncertainties for induced earthquakes, leading to the problem of interpolation of peak ground velocity (PGV) measurements. Therefore, it is important to understand high-frequency wave propagation and attenuation properties of the medium between the hypocentres of induced events and the surface. Furthermore, as the three geothermal sites are located in sedimentary basins and the induced earthquakes rupture near the interface between sediments and bedrock, the presented study allows for insights into scattering and absorption properties of shear waves in sediments.Attenuation of seismic waves, besides geometrical spreading, can be classified by the acting mechanism into intrinsic and scattering attenuation. Intrinsic attenuation is due to the absorption of seismic waves. Scattering attenuation is due to the redistribution...

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In this study, frequency-dependent seismic scattering and intrinsic attenuation parameters for the crustal structure beneath the W-Bohemia/Vogtland swarm earthquake region close to the border of Czech Republic and Germany are estimated. Synthetic seismogram envelopes are modelled using elastic and acoustic radiative transfer theory. Scattering and absorption parameters are determined by fitting these synthetic envelopes to observed seismogram envelopes from 14 shallow local events from the October 2008 W-Bohemia/Vogtland earthquake swarm. The two different simulation approaches yield similar results for the estimated crustal parameters and show a comparable frequency dependence of both transport mean free path and intrinsic absorption path length. Both methods suggest that intrinsic attenuation is dominant over scattering attenuation in the W-Bohemia/Vogtland region for the investigated epicentral distance range and frequency bands from 3 to 24 Hz. Elastic simulations of seismogram envelopes suggest that forward scattering is required to explain the data, however, the degree of forward scattering is not resolvable. Errors in the parameter estimation are smaller in the elastic case compared to results from the acoustic simulations. The frequency decay of the transport mean free path suggests a random medium described by a nearly exponential autocorrelation function. The fluctuation strength and correlation length of the random medium cannot be estimated independently, but only a combination of the parameters related to the transport mean free path of the medium can be computed. Furthermore, our elastic simulations show, that using our numerical method, it is not possible to resolve the value of the mean free path of the random medium.

We use 10 years of data of the USArray project to estimate the areal distribution of crustal intrinsic and scattering attenuation of shear waves for frequencies between 1 Hz and 20 Hz in the contiguous United States. Additionally, we report energy site amplification factors and estimate moment magnitudes for small earthquakes (M 1.5 to M 3.5). The Qopen method is used to invert for intrinsic and scattering attenuation for each event and nearby stations. Observations are collected for around 25,000 events, averaged at each station and interpolated between station locations. In a second inversion, energy site amplifications and moment magnitudes are corrected by assuming that site amplifications for one station and frequency are the same for different earthquakes. We observe a west‐east decline of intrinsic attenuation for high frequencies which reflects the west‐east transition from young, hot to old and cold crust. Scattering attenuation for high frequencies is stronger in the east with an extraordinary high attenuation around the southern part of the Appalachian Highlands and the Interior Low Plateaus. Results at low frequencies do not show clear trends. A large site amplification is observed at high frequencies in parts of the eastern United States. Estimated moment magnitudes show a good agreement to moment magnitudes independently derived from moment tensor inversion. Moment magnitudes in the west are higher than in the east for the same Richter magnitudes.