Measurement of intrinsic and scattering attenuation of shear waves in two sedimentary basins and comparison to crystalline sites in Germany

Abstract

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...

Crustal intrinsic and scattering attenuation of high‐frequency shear waves in the contiguous United States

“…The modeled energy envelopes at station i are calculated by [cf. Eulenfeld and Wegler , , equation (1)] t is the lapse time, the vector pointing from the hypocenter to the station location, and W is the spectral source energy of the event. e − b t describes the exponential intrinsic damping with time.…”

“…The energy Green's function accounts for the direct wave and the scattered wave field. The most simple approach, which is also pursued in this paper, employs the analytic approximation of the solution for three‐dimensional isotropic radiative transfer to calculate the Green's function [ Paasschens , ; Eulenfeld and Wegler , , equation (2)]. The usage of an isotropic description of scattering can be justified also for anisotropic scattering as Gaebler et al [] discussed exemplary that a scattering coefficient g 0 determined under the assumption of isotropic scattering can be directly translated to the transport scattering coefficient g ∗ in an anisotropic scattering medium ( g ∗ = g 0 ).…”

“…The usage of an isotropic description of scattering can be justified also for anisotropic scattering as Gaebler et al [] discussed exemplary that a scattering coefficient g 0 determined under the assumption of isotropic scattering can be directly translated to the transport scattering coefficient g ∗ in an anisotropic scattering medium ( g ∗ = g 0 ). Observed energy densities corrected due to the free surface [ Eulenfeld and Wegler , , equation (4)] are inverted for g ∗ , b , W , and R i with the Qopen method. g ∗ and b can be converted to Q values with equation .…”

“…Furthermore, observed and modeled envelopes are smoothed with a 2 s wide Bartlett window. Finally, the inversion for intrinsic and scattering attenuation as well as site effects and spectral source energy is performed by solving the least squares linear equation system E obs = E mod for given g ∗ and simultaneous optimization of g ∗ as described in Eulenfeld and Wegler []. The weight of the S wave window in the inversion corresponds to its width.…”

“…While scattering influences the shape of the envelope in space and time—most notably the energy level between direct S wave and scattered coda wave—intrinsic attenuation can be accounted for by an exponential decrease of the envelope with time. Basically, two methods evolved for the separation of intrinsic and scattering attenuation of shear waves using homogeneous random media: MLTWA (multiple lapse time window analysis) developed by Hoshiba et al [] and Fehler et al [] and Qopen (separation of intrinsic and scattering Q by envelope inversion) presented by Eulenfeld and Wegler [] and based upon Sens‐Schönfelder and Wegler []. Both methods invert for attenuation properties by comparing observed and modeled energy envelopes.…”