Observatories and Research Facilities for EUropean Seismology
Volume 1, no 3 October 1999 Orfeus Newsletter


A 400 km long Broadband Antenna in the Eifel-Region

M. Budweg1,2, M. Weber1, G. Bock1, J. Ritter2, U. Christensen2 and the Eifel Plume Team
1 GeoForschungsZentrum Potsdam (GFZ), Telegrafenberg, D-14473 Potsdam, Germany.
2 University of Göttingen, Institute of Geophysics, Herzberger Landstr. 180, D-37075 Göttingen, Germany.

The Eifel - The BB Experiment - Configuration of the BB Array - The Aims of the BB experiment
Converted Phases - Seismic Anisotropy - Acknowledgements

The Eifel

The Eifel with its famous maars and volcanic fields is the youngest volcanic area of Central Europe. The last eruptions occurred approximately 12200 years ago at the Laacher See vulcano and 10900 years ago at the Ulmener Maar - i.e. in geological time just yesterday. Since several years numerous geological, geochemical and geophysical studies have been conducted in this area. Nevertheless little is known about the deep origin and the mechanism responsible for the volcanic activity. In the last 1000 years several destructive earthquakes hit the Lower Rhenish Embayment, which is situated immediately north of the Eifel. The most recent significant event occurred on 13. April 1992 near Roermond/Netherlands (Mw=5.3), which caused damage in excess of 50 million Euro. Other significant earthquakes occurred on 8. November 1983 near Liege/Belgium, and 1756 near Dueren/Germany. Another geodynamic feature is the uplift of the Rhenisch Massif up to 250 m in the last 800 ka. All these processes indicate that the Eifel is one of the most geodynamically active areas of Central Europe. The study of the lithospheric-asthenosheric structure of the Eifel and the processes involved is therefore the central aim of this research project.

The Broadband Experiment

A total of 32 mobile broad-band (BB) stations and 126 mobile short-period (SP) stations was operated between November 1997 and June 1998 in the area extending from western Germany to the Benelux and France (Fig. 1a). Recordings of the temporary stations were supplemented by data of permanent BB and SP stations in Germany, the Netherlands, Belgium, Luxembourg and France. The regional and teleseismic events recorded during this time are given in Fig. 1b.

Klick to enlarge
Fig. 1a.

Klick to enlarge
Fig. 1b.

Fig.1: The Eifel Plume experiment. (a) Temporary and permanent BB and SP stations of the Eifel Plume experiment. (b) 217 regional and teleseismic events recorded from November 1997 to Summer 1998. Circles are distance in degrees from the Eifel.

Configuration of the BB Array

The deployment of the 32 mobile broadband recorders was carefully planed before the field campaign. As configuration of the BB array we chose a cross with its longer arm striking SW-NE. This nearly 400 km long line with 22 stations is directed towards the active seismic regions in the NW Pacific (e.g. Japan, Kurils) and South America. The shorter, 180 km long line with 14 stations was oriented NW-SE. Together with the short-period stations this allows precise measurements of slowness and backazimuth.
As sensors 14 Streckeisen STS-2, 12 Guralp 3T, and 6 Guralp T40 were used. During the deployment these instruments were installed on small concrete basements and shielded against temperature variations. Sampling frequency 20 Hz and 50 Hz were used. The preliminary data processing show a good signal-to-noise ratio.

The Aims of the BB experiment

The aims of the seismological BB experiment are
  • Investigate upper mantle structure using converted phases and array methods
  • Determine the seismic anisotropy using splitting of SKS and other shear waves.
The joint interpretation of these results with teleseismic tomography and geodynamic modelling will help in shedding light on the deep processes responsible for the Eifel volcanism and the ongoing uplift.

Converted Phases

S -> P
Teleseismic S-to-P converted waves underneath a seismic station arrive before the direct S-wave and are best observed on the vertical components (Fig. 3a). The advance relative to S can be used to determine the depth of the discontinuity at which the conversion occurred. Fig. 3b shows a data example recorded at the stations in the Eifel. Fig 3c shows the slowness stack (vespagram) of events with an backazimuth of 286°.

SPConversion.gif
Fig. 3a
Klick to enlarge
Fig. 3b
Klick to enlarge
Fig. 3c
Fig.3.: (a) Sketch of the method and phases used (b) Vertical component recordings of stations of the Eifel experiment. Events are sorted by distance (c) Slowness stack (vespagram) of events with backazimuth 286°. The onset X ahead of S is possibly a S-to-P converted wave at the 410 km discontinuity (thanks to Y.-F. Temme for initial processing).

Seismic Anisotropy

Anisotropy frozen into the lithosphere during the geological past and present-day flow in the asthenosphere are frequently discussed causes to explain splitting of teleseismic shear waves. We will investigate splitting of SKS waves because it can be unambiguously related to anisotropy in the crust and/or mantle beneath the receiver. As a first example we analyzed data from the deep earthquake of November 28, 1997, which occurred in the Peru-Bolivia border region at a depth of 586 km. Good SKS phases were recorded from this event at the Eifel Plume stations. Standard splitting analysis of broad-band and some short-period waveforms (the latter restituted to 20 seconds by deconvolution of the seismometer response) gave the preliminary results shown in Fig. 4. Delay times between the slow and fast split waves are relatively small in this case (less than or equal to 0.5 s), particularly in the area of the Eifel volcanic fields. The fast polarization directions seem to be quite different to those observed elsewhere in central Europe. This suggests that the regional anisotropy pattern is considerably disturbed locally in the immediate area of the Eifel volcanic fields, but more events have to be analyzed before firm conclusions can be drawn.

Klick to enlarge
Fig. 4.: Preliminary plot of the inferred directions of the fast split wave for a deep event in the Peru-Bolivia border region. Crosses indicate "Nulls", i.e. no splitting observed. The crosses are aligned parallel to the back azimuth and perpendicular to it, representing possible directions of the fast and slow split wave. The surface of the Eifel volcanic area is indicated by the grey-filled areas

Acknowledgements

The field equipment were provided by the Geophysical Instrument of the Geo-Forschungs-Zentrum Potsdam, universities of Potsdam, Karlsruhe, and the Free University of Berlin, Germany; the Lithoscope Pool, France; the Royal Observatory of Belgium; and the University of Utrecht; Netherlands. Data from permanent stations were supplied by the Bundesanstalt fuer Geowissenschaften und Rohstoffe, the Erdbebenwarte Bensberg, the GeoForschungsZentrum Potsdam, the Geologisches Landesamt Nordrhein Westfalen, the Landesamt fuer Geologie, Rohstoffe und Bergbau Baden-Wuerttemberg, Germany; the ORFEUS Data Center, the Seismological Division of KNMI, Holland; and the Royal Observatoire of Belgium. Special thanks to Yorck-Fabian Temme for the collection and preparation of the data, the county administration of Daun/Vulkaneifel and the GeoZentrum Vulkaneifel (Dr. Eschghi and Mrs. Rudolf) for providing important logistical help, and numerous offical and private people for providing sites for installationof the instruments. The project is financed by the Deutsche Forschungsgemeinschaft, the institutions involved, the Royal Observatory of Belgium and the European Center for Geodynamics and Seismology - Musee National d'Histoire Naturelle, Luxembourg.

References

Granet, M., Wislon, M. and Achauer, U., 1995. Imaging a mantle plume beneath the French Massif Central, Earth Planet, Sci. Lett., 136, 281-296
Raikes, S., and Bonjer, K.-P., 1983; Large-scale mantle heterogeneity beneath the Rhenish Massif and its vicinity from teleseismic P-residuals measurements, in: Fuchs at al. (eds.), Plateau uplift, Springer Verlag, Berlin, 315-331.
Stammler, K., 1993. Seismic Handler - Programmable multichannel data handler for interactive and automatic processing of seismological analyses. Computer and Geosciences, 2, 135-140.


For more information visit the Eifel homepage: www.geo.physik.uni-goettingen.de/~eifel
Eifellogo
A geoscientific program to study the Earth's mantle under the Eifel and adjoining regions
The broadband seismic experiment


page 23
copyright © 1999. Orfeus. All rights reserved.
a/html>