Fig. 6c: 35x35 blocs, RMS = 0.1593 sFig. 6b: I25x25 blocs, noise= 0.1 s, RMS = 0.2069 sFig. 6a, 25x25 blocs, RMS = 0.1435 s

Simultaneous Inversion for 3D crustal and lithospheric Structure and regional Hypocenters beneath Germany in the Presence of an anisotropic upper Mantle

Manfred Koch and Thomas Münch

University of Kassel, Germany

I. Introduction

II. Anisotropy, preliminary investigations

IV. 3D-seismic models for the crust and upper mantle underneath Germany

VI. Conclusions

IIIa. 3D- models, synthetic tests, random perturbations

As recognized in previous studies (Song et al., 2001, 2004), travel times of Pn-phases across Germany show anisotropic behaviour. Main task here is to study the influence of the upper mantle anisotropy onto the tomographic reconstruction of the seismic velocities in the crust and upper mantle across Germany. Dataset for the 3D SSH (Simultaneous inversion for Structure and Hypocenters, (Koch, 1993)) tomography consists of regional arrival times recorded across Germany between 1975 to 2003. Due to the large number of records (Table 1), good ray-coverage of study area is ensured (Fig. 1).

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Artificial anisotropic travel-time dataset with several anomalies in the four layers (depths=[0-10];[10-20];[20-30]; >30km) of the model (Fig. 5a) is synthesized and re-inverted. Travel times (partly with noise) are computed using the original hypocenter and station locations

Anisotropic reconstructions (Fig. 5b, Fig. 5d)) show good agreement with original model (except in layer 3), due to lack of earthquakes there. RMS of the data fit is also smaller than that of the isotropic reconstructed.

Isotropic reconstruction (Fig. 5c) has no resolution in the 1st layer, produces only artefacts in the other three layers and has a three times higher RMS than the anisotropic inversion.

Fig. 4: Determination of optimal anisotropy ellipse. For hypocenters fixed (Fig.4a) the optimal anisotropy angle of about 35° NE is obtained. For full inversion (Fig.4b) optimal angle is at 26° NE coinciding better with results of Enderle et al. (1999).

One aim of the study is to show influence of upper mantle Pn-anisotropy on the seismic inversion. Therefore, Pn- ray tracing is corrected by elliptical (azimuthal) anisotropy, quantified by the velocity contrast (%) and angle of the major axis (Fig.2)

Fig. 5d: Anisotropic inversion with noisy data, RMS = 0.1656 s

Fig 5a: Original model with random perturbations Fig. 5b: Anisotropic inversion; RMS = 0.0771 s Fig. 5c: Isotropic inversion; RMS = 0.2324 s

1. Anisotropic Pn-travel time correction reduces the observed sinusoidal residual variations

2. Anisotropic models show better fit of the data (smaller residuals) than the isotropic ones

3. Synthetic resolution tests indicate the overall appropriability of the present data set to retrieve much of the lateral seismic structure underneath Germany

4. Upper crust is 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 seismic models:* Molasse region in the Alpine foreland* Volcanic roots in the Black Forest, the Vosges, and parts of the northern Rhinegraben

6. Anisotropic Pn-correction results in more precise earthquake location, particularly for events with larger station gaps => Relocation of the Waldkirch 2004 event, (Münch et al., 2010)

7.Future work: Analysis of possible anisotropy in the crust; corrections for undulating Moho

Original RMS: 1.1184 s²

ReferencesEnderle,1996: Seismic anisotropy within the uppermost mantle of southern Germany, Geophys. J. Int., 125, 747 – 767.Koch, M., 1993:. Simultaneous inversion for 3D crustal structure and hypocenters including direct, refracted and reflected phases. I. Development, Validation and optimal regularization of the method, Geophys. J. Int., 112 ,385–412.Song, L-P., Koch, M., Koch, K., Schlittenhardt, J., 2001 : Isotropic and anisotropic Pn velocity inversion of regional earthquake traveltimes underneath Germany; Geophys. J. Int., 146, 795-800. Song et al, 2004: 2-D anisotropic Pn-velocity tomography underneath Germany using regional traveltimes, Geophys. J. Int. ,157, 645-663 Muench et al, 2010: Simultaneous inversion for 3D crustal and anisotropic lithospheric structure and regional hypocenters beneath Germany, submitted of Geophys.J. Int.Muench et al, 2010: Relocation of the December 5, 2004, Waldkirch seismic Event with regional 1D- and 3D- seismic velocity models in the presence of upper mantle anisotropy, submitted to BSSA

IIIb. 3D- models, synthetic checkerboard tests

Fig. 3: Original Pn residuals (blue) and optimally anisotropically corrected.

Fig. 3:Anisotropicall corrected with +-1% (top) and with +-5% (bottom)

Fig. 7b: Anisotropic 25x25 model; TSS = 15113 s², RMS = 0.8197 s

Fig. 7a: Isotropic 25x25 3D mode

TSS = 17458 s², RMS = 0.8810 s

Fig.8a: Isotropic 35x35 model; TSS = 16995 s², RMS = 0.8693 s Fig. 8b: Anisotropic 35x35 model; TSS = 15767 s², RMS = 0.8373 s

Checkerboard tests indicate where good lateral resolution of the model can be expected.

Even with noisy data (Fig. 6b) a relatively good resolution in the first and fourth layer for the 25x25 bloc -models is obtained. For second and third layer good resolution is obtained only in the south-western part of the model where there is a concentration of earthquakes. Fig. 6c shows how the resolved areas are reduced when a 35x35 bloc discretization is used.

V. Simultaneously relocated hypocentersSimultaneously with the isotropic and anisotropic optimal 3D velocity models relocated hypocenters show only minor differences. Isotropically computed epicenters (Fig. 9, left) appear to be more clustered in the EW- direction (effect of anisotropic bias??) than anisotropically computed ones.

No visual differences in the depth shifts of the events.

Total N-obs > 7 N-obs >7, GAP <180

Events 10058 1812 1223

Pg-Phases 46550 20279 15438

Pn-Phases 12804 9001 5880

PMP-Phases 895 873 751

Table 1: Number of events and phases used in the study

The four 3D-tomographic seismic models for the crust and upper mantle exhibit slightly different features, mainly in the upper mantle layer. Based on the objective criteria of the Total Square Sum of the residuals (TSS) and the synthetic tests, the anisotropic models are considered to be more reliable.

25x25 bloc models (Fig. 7)The first layer nearly has the same structure for both model variants, although major shallow geological features (Rhinegraben area, Vogelsberg and Nördlinger Ries) are better recognized in the anisotropic model.

Starting with the second layer, structural differences show up between isotropic and anisotropic models

Structure in the fourth layer mainly follows some tectonic features in the tectonic map (Fig. 9), i.e. may represent ancient suture zones of the variscan orogeny.

35x35 bloc models (Fig. 8)Compared with the 25x25 models, datafit is improved. But resolution in some areas of the model is not trustable, as shown by the checkerboard tests. Never-theless major structurural features are similar to the coarser model.

Fig. 9: Tectonic map of Germany with major ancient suture zones

Fig, 1: Regional seismic events and ray- coverage (P + S- phases) across Germany.

Fig. 3: Effects of anisotropic Pn-correction on the observed travel-time residuals (using a standard 1D- seismic velocity model for Germany). After anisotropic correction with -+2.5% contrast, the residuals nearly lie on a straight line.

Fig, 2: Anisotropy ellipse

Fig. 4a Fig. 4b

2010-Fall meeting

S31A-2037

Fig. 9: Epicentral and hypocentral (depth) shifts for simultaneously with 3D 25x25 optimal velocity models relocated events (left: isoptropic; right: anisotropic model

Table 2: Average shifts of epicenters and for different model variants; x= x-shift , y=y-shift; r = total horizontal shift; Φ=angle of shift.