M. Koch and T.H. Münch. Department of Geohydraulics and Engineering Hydrology University of Kassel Kurt-Wolters-Strasse 3 D Kassel

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1 Simultaneous inversion for 3D crustal and lithospheric structure and regional hypocenters beneath Germany in the presence of an anisotropic upper mantle M. Koch and T.H. Münch Department of Geohydraulics and Engineering Hydrology University of Kassel Kurt-Wolters-Strasse 3 D Kassel kochm@uni-kassel.de J. Schlittenhardt Federal Institute for Geosciences and Natural Resources Stilleweg 2 D Hannover

2 Abstract There is ample evidence that large sections of the upper mantle underneath Europe and Germany, in particular, are anisotropic. Using regional earthquake travel times, Song et al. [2001] and Song et al. [2004] by means of a 1D time-term analysis and a full 2D Pn anisotropic inversion showed that an anisotropic velocity ellipse with the fast axis in the direction of ~N250E and an anisotropy level of ~3.5% reduces significantly the data variance, and that geologically meaningful upper mantle Pn-velocity perturbations could only be obtained if anisotropy was included. Employing a modified version of the method of simultaneous inversion for structure and hypocenters (SSH) of Koch [1993], including a priori known upper mantle anisotropy, the analyses of Song et al. [2001, 2004] are extended here to a full simultaneous inversion for 3D crustal and upper mantle structure and regional hypocenters underneath Germany. Regional travel times from local earthquakes occurring between are included in the study. First of all improved 1D P-wave velocity models are derived assuming an isotropic as well as an anisotropic upper mantle. In addition of a slightly better model fit for the anisotropic than for the isotropic model, the latter gives also a somewhat too low Pn-velocity of ~7.90 km/s, compared with ~8.0 km/s for the former. This indicates that inclusion of upper mantle anisotropy into the model is required to obtain physically reasonable Pn-velocities. Significant improvements for both the isotropic and anisotropic upper mantle cases are obtained. for full 3D SSH inversion models. Similar to the 1D Pn-velocity models there are remarkable differences in the lateral Pn-velocities, depending whether the lithosphere is corrected for anisotropy or not. Finally the manifold of structural seismological results obtained from the SSH-inversion for both crust and upper mantle will be interpreted in terms of the present-day knowledge on the geology and the tectonics of the central European Variscides.

3 Overview 1. Introduction and previous work 1.1 1D anisotropic Pn time term analysis of Song et al. (2001) 1.2 2D anisotropic Pn time term analysis of Song et al. (2004) 2. Data used in present study 2.1 Sources and selection criteria 2.2 Event distribution and ray coverage 3. Method of simultaneous inversion for structure and hypocenters 4. 1D vertically inhomogeneous P-wave velocity models 5. 3D laterally inhomogeneous P-wave velocity models 5.1 9x9 models x15 models x35 models 6. Geological and tectonical interpretation

4 1. Previous work ( Song et al., 2001, 2004) Top: Earthquakes and explosions (circles) recorded between 1975 and 1999 at seismic station (triangles) in Germany and neighbouring countries. Middle: Coverage for Pn rays Bottom: Refraction profiles of Enderle et al. (1996)

5 1.1 Previous work: 1D anisotropic Pn time term analysis of Song et al. (2001) Top: Traveltime residuals as a function of epicentral distances for the straight line fit Middle: Residuals for classical time term method Bottom: : Residuals for anisotropic time term method Note the systematic decrease of the data variance with increasing complexity of the analysis method Anisotropy-ellipsoid with major axis in N26.7 E and velocity contrast of 3,5%

6 1.2 Previous work: 2D anisotropic Pn time term analysis of Song et al. (2004) Pn velocity distribution without anisotropy Note Pn velocities too low <7.8 km/s Pn velocity distribution with anisotropy Note more realistic Pn-velocities ~8km/s

7 2. Data used in present study 2.1 Sources and selection criteria Sources Data Catalogue of Earthquakes of BGR (Henger & Leydecker) Events with P + S -Phases recorded between Preprocessing Selection of best hypocentral localisations Assume that local networks (LED, BNS etc.) have best localisations Selection criteria At least 8 recording stations phases per event Azimuthatl gap < events with P-Phases

8 2. Data used in present study 2.2 Event distribution and ray-coverage (GAP < 180 und Nobs > 7) All rays Only Pn-rays

9 3. Method of simultaneous inversion for structure and hypocenters Matrix formulation of the linear tomographic inverse problem: t = Gm with t = t t 0, the n-dimensional traveltime residual vector; m = (m H, m v ), the m-dimensional unknown model vector of the hypocentral parameters m H and velocity parameters m v Solution through minimization of following objective function T T T ( m) = ( t Gm) ( t Gm) m W Wm L + => Linear system of equations for solution vector T T T ( G G + W W) m = G t Nonlinear iterative solution by solving the forward problem through ray tracing sol (Koch, 1992, 1993)

10 4. 1D vertically inhomogeneous P-wave velocity models Variation of TSS with Azimuth of Anisotropy, IFIX = 0, I = TSS (sec²) TSS (Total Sum of residuals Squared) as a function of azimuth of anisotropy axis Azimuth (degree) Variation of TSS with Azimuth of Anisotropy, IFIX = 0, I = TSS (sec²) Azimuth (degree)

11 4. 1D vertically inhomogeneous P-wave velocity models Reduction of residuals after correction with Pn-anisotropie Only raytracing First iteration

12 4. 1D vertically inhomogeneous P-wave velocity models MKS2005 Data2003 Gap180 ifix0 iso MKS2005 Data2003 Gap180 ifix0 ani Input-model Best solution 10 Input-model Best solution Depth in km Depth in km Vp in km/sec Vp in km/sec Isotropic upper mantle Anisotropic upper mantle isotropic: av. RMS = s, TSS = s² anisotropic: av. RMS = s, TSS = s²

13 4. 1D vertically inhomogeneous P-wave velocity models Relocations of simultaneously inverted hypocenters Counts Counts delta-z [km] Events per 5km Intervall delta-z [km] Events per 5km Intervall Isotropic mantle Anisotropic mantle

14 5. 3D laterally inhomogeneous P-wave velocity models 5.1 9x9 block models 9x9 isotropic model 9x9 anisotropic model TSS = s², av. RMS = 1.22 s TSS = s², av. RMS = 1.16 s

15 5.1 9x9 models: Checkerboard resolution tests no noise: TSS = 18 s², av. RMS = s noise σ = 0.3 s, TSS = 2005 s², av. RMS = 0.30 s

16 5. 3D laterally inhomogeneous P-wave velocity models x15 block models Isotropic inversion TSS = => s² V_pn-(av) = 7.97km/s Anisotropic inversion TSS = => s² Vpn (av)= 8.00 km/s

17 5.2 15x15 models: Checkerboard resolution tests no noise, TSS = 561 s², av. RMS = s I noise σ = 0.3 s, TSS = 1856 s², av. RMS = s

18 5.3 35x35 models isotropic mantle, TSS = => s², RMS = 1.16 s I anisotropic mantle, TSS = s², RMS = 1.15 s

19 5.3 35x35 models: checkerboard resolution tests no noise, TSS = 52.0 s², av. RMS = 0.048s I noise σ = 0.1s,TSS = 250 s², av. RMS = s

20 6. Geological and tectonical implication Tectonical map of Germany Optimal 15x 15 model, first layer (0-10km)

21 7. Conclusions 1. Anisotropic models show better fit of data (smaller residuals) than isotropic ones 2. Incorporation of anisotropy is necessary for obtaining geologically meaningful upper mantle Pn-velocities ~8 km/s 3. Travel time data provide for an independent resolution of the crust and upper mantle over most of the study region. 4. Upper crust well resolved but large sections of the lower crust show poor lateral resolution, due to a paucity of earthquakes here 5. Various upper crustal tectonic (petrological) features retrieved in the 3D models: * Molasse region in the Alpine foreland * volcanic and metamorphic roots in the Black Forest, the Vosges, and parts of northern Rhinegraben 6. Future study: Analysis of anisotropy in the crust