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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
S31A-2037
University of Kassel, Germany
Contact: Manfred Koch, kochm@uni-kassel.de; Thomas Münch tmuench@uni-kassel.de, Department of Geotechnology and Geohydraulics, University of Kassel
2010-Fall meeting
IV. 3D-seismic models for the crust and upper mantle underneath Germany
I. Introduction
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).
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.
Table 1: Number of events and phases used in the study
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
Starting with the second layer, structural differences
show up between isotropic and anisotropic models
Fig, 1: Regional seismic events and raycoverage (P + S- phases) across Germany.
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.
II. Anisotropy, preliminary investigations
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. Nevertheless major structurural features are similar to the
coarser model.
One aim of the study is to show
influence of upper mantle Pnanisotropy 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. 7a: Isotropic 25x25 3D mode
TSS = 17458 s², RMS = 0.8810 s
Fig, 2: Anisotropy ellipse
Fig. 7b: Anisotropic 25x25 model;
TSS = 15113 s², RMS = 0.8197 s
Fig. 9: Tectonic map of Germany with major ancient suture zones
Original RMS: 1.1184 s²
V. Simultaneously relocated hypocenters
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. 3: Original Pn residuals (blue) and
optimally anisotropically corrected.
Fig. 4a
Simultaneously with the isotropic
and anisotropic optimal 3D velocity
models relocated hypocenters show
only minor differences.
Fig. 3:Anisotropicall corrected with +1% (top) and with +-5% (bottom)
Fig. 4b
Isotropically computed epicenters
(Fig. 9, left) appear to be more
clustered in the EW- direction (effect
of anisotropic bias??) than
anisotropically computed ones.
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).
No visual differences in the depth
shifts of the events.
IIIa. 3D- models, synthetic tests, random perturbations
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.
Fig.8a: Isotropic 35x35 model; TSS = 16995 s², RMS = 0.8693 s
Fig. 8b: Anisotropic 35x35 model; TSS = 15767 s², RMS = 0.8373 s
IIIb. 3D- models, synthetic checkerboard tests
Fig. 9: Epicentral and hypocentral (depth) shifts for simultaneously with 3D 25x25
optimal velocity models relocated events (left: isoptropic; right: anisotropic model
VI. Conclusions
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
Fig 5a: Original model with random perturbations
Fig. 5b: Anisotropic inversion; RMS = 0.0771 s
4. Upper crust is well resolved but large sections of the lower crust show poor lateral resolution,
due to a paucity of earthquakes here
Fig. 5c: Isotropic inversion; RMS = 0.2324 s
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
Artificial anisotropic travel-time dataset with several
anomalies in the four layers (depths=[0-10];[10-20];[2030]; >30km) of the model (Fig. 5a) is synthesized and reinverted. 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.
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
Fig. 5d: Anisotropic
inversion with noisy
data, RMS = 0.1656 s
Fig. 6a, 25x25 blocs, RMS = 0.1435 s
Fig. 6b: I25x25 blocs, noise= 0.1 s, RMS = 0.2069 s
Fig. 6c: 35x35 blocs, RMS = 0.1593 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.
References
Enderle,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