Abstract
We determine the source parameters of three minor earthquakes in the Upper Rhine Graben (URG), a Cenozoic rift, using waveforms from permanent and temporary seismological stations. Two shallow thrust-faulting events (M L = 2.4 and 1.5) occurred on the rift shoulder just south of Heidelberg in March 2005. They indicate a possible movement along the sediment–crystalline interface due to tectonic loading from the near-by Odenwald. In February 2005, an earthquake with a normal-faulting mechanism occurred north of Speyer. This event (M L = 2.8) had an unusual depth of about 22 km and a similar deep normal-faulting event occurred there in 1972 (M L = 3.2). Other lower crustal events without fault plane solutions are known from 1981 and 1983. At such a depth, inside the lower crust, ductile behaviour instead of brittle faulting is commonly assumed and used for geodynamic modelling. Based on the newly available fault plane solutions we can confirm the brittle, extensional regime in the upper and lower crust in the central to northern URG indicated in earlier studies.
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Acknowledgments
Seismic waveform data were provided by the Landeserdbebendienst (LED) Baden-Württemberg, the Landeserdbeben-Registrierung (LER) Rheinland-Pfalz, the Hessischer Erdbebendienst (HED), the German Regional Seismic Network (GRSN) through its database at the SZGRF, RENASS (Strasbourg) and GEOFON (Potsdam). W. Scherer and H. Thomas helped with maintenance of the TIMO network which was funded by the Geophysical Institute of the Universität Karlsruhe (TH). Numerous people supported the TIMO experiment by providing safe space for the instruments. G. Peters provided digital data of faults from the northern Rhine Graben. SeismicHandler (Stammler 1993) was used for seismic data analysis and GMT (Wessel and Smith 1998) for plotting maps. We thank Prof. Dr. R. Greiling (Karlsruhe), Prof. Dr. N. Harthill (Karlsruhe) and Dr. S. Stange (Freiburg) for comments on an earlier version of the manuscript. Dr. I. Wölbern and Dr. T. Plenefisch provided useful reviews to clarify some points.
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Appendix
Appendix
In Fig. 6, the travel-time data are shown for the Heidelberg main event (May 26 2005) and Speyer event (10 February 2005). The P-wave (circles) and SH-wave (squares) first arrival times of the Heidelberg event can be extrapolated to maximum 0.3 s at the epicentre (0 km distance). Using average seismic velocities (3.0–4.0 km/s) for the sandstone in the region this results in a source depth of less than 2 km. The Wadati plot in Fig. 7 shows the consistency of the P- and SH-wave arrival times (circles), especially for short distances (P-wave arrival time less than 15 s) for which the traveltime is not yet influenced much by anisotropy and 3D heterogeneity. The traveltime data for the Speyer event contain clear offsets at the epicentre (0 km distance) of about 4 s for P-waves (triangles) and 5.5 s for SH-waves (crosses). Using average seismic velocities for the Upper Rhine Graben this results in a source depth of 20–22 km. Again the Wadati plot (Fig. 7, crosses) demonstrates the consistency of the travel-time determinations.
Traveltime curves for the P- and SH-waves of the Heidelberg main event and Speyer event. The inset shows the measured first arrival times at short distances. The arrival times differ significantly for the two events due to their different source depths
Wadati diagram for the Heidelberg main event and Speyer event
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Ritter, J.R.R., Wagner, M., Bonjer, KP. et al. The 2005 Heidelberg and Speyer earthquakes and their relationship to active tectonics in the central Upper Rhine Graben. Int J Earth Sci (Geol Rundsch) 98, 697–705 (2009). https://doi.org/10.1007/s00531-007-0284-x
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DOI: https://doi.org/10.1007/s00531-007-0284-x