Regional and local kinematics in SW-Germany by geodetic methods—geophysical and geological interpretation

JOURNAL OF GEODYNAMICS 9,

141-151 (1988)

141

REGIONAL A N D LOCAL KINEMATICS IN SW-GERMANY BY GEODETIC M E T H O D S - - G E O P H Y S I C A L A N D GEOLOGICAL INTERPRETATION

H. M~.LZER

Geodetic Institute, Unirersity of Karlsruhe, Karlsruhe, F.R.G. (Accepted January 5, 1988)

ABSTRACT

M~ilzer, H., 1988. Regional and local kinematics in SW-Germany by geodetic methods--geophysical and geological interpretation. In: Yu. D. Boulanger, S. Holdahl and P. Vysko~il (Editors), Recent Crustal Movements. Journal of Geodynamics, 9: 141-151. SW-Germany is one of the most interesting areas for geoscientific research in central EuroPe (Fig. 1). The active seismicity documents a living tectonic in several regions. The tectonics are controlled by a stress field related to the Alps and are also expressed by a recent vertical kinematic observed by geodetic methods. The regional and local motions of the Upper Rhinegraben and the ttohenzollerngraben area in the western Swabian Jura--the area with the highest seismic activity north of the Alps--are discussed in connection with the geological and seismotectonic pattern. Small rates of horizontal deformation detected by geodetic measurements seem to indicate the seismotectonic dislocation in the ttohenzollerngraben area.

INTRODUCTION

The basis for the estimation of recent vertical kinematics is given by the results of precise levellings which started in Germany in 1921/22, executed by the Departments of Ordnance Survey. In the different departments different numbers of 1st order levellings at different times were carried out. The average time interval between the levelling epochs varies from 15 to 25 years. The value for the a priori standard deviation for a 1 km levelling is smaller than 0.8 mm. From 1980 to 1985 a new relevelling has been carried out in the Federal Republic of Germany. Parallel to the levellings a gravity net has been observed by the Departments-of Ordnance Survey. This network is connected with the Gravity Base Net 1976 of the Federal Republic of Germany which 0264-3707/88/$3.00

O 1988 Geophysical Press Ltd.

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Fig. 1. Sketch of SW-Germany with Upper Rhinegraben, Black Forest, and Hohenzollerngraben (HZG) area. The southern levelling Rhinegraben profile (P) is marked (s. Fig. 5).

consists of 21 points including 4 absolute gravity stations (Sigl et aL, 1981). This precise relevelling and the measured gravity data give a new basis for the evaluation of geopotential cotes and in future it will be possible to estimate real height changes or real vertical movements in a more exact mathematical model.

REGIONAL AND LOCAL KINEMATICS

143

TREND ANALYSIS FOR SW-GEI~MANY

Because the temporal distribution of the levellings is very inhomogeneous only a kinematic model considering the measuring time is suited for a reliable estimation of height changes. The furthermore used single point model on the basis of the models given by Holdahl (1975) is described by

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M~ilzer et al. (1983). A" partition of the estimated height changes (detail analysis) can be done posteriori by determining long-wave variations which express an extensive trend and short-wave variations or residual values which are caused by local effects correlated to the tectonic pattern or to man-made perturbations (Zippelt and M~lzer, 1987). The result of a trend analysis using a surface model given by a function of degree 6 is represented in Figure 2. The contour-lines give a regional view on the height changes in SW-Germany. In comparison with the results of a detailed analysis (Fig. 4) the trend-analysis gives only a tendentious view of regional block motions. In general in the northern region an ESE-tilt and in the southern one (Black Forest) a S directed tilt is recognizable. The central Black Forest seems to be a stable zone. For the following investigations concerning the Upper Rhinegraben and the Hohenzollerngraben area in the western part of the Swabian Jura (Fig. 1) the results of height changes estimated by kinematic models in detail have been used.

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Fig. 3. The present-day mechanism of the Upper Rhinegraben caused by the stress field with a maximum horizontal stress of" Sit = N ~ 4 0 ~ E +10 ~ It explains the estimated recent height changes (lilies and Baumann, 1982).

REGIONAL AND LOCAL KINEMATICS

145

RHINEGRABEN AREA

It was the first stage extensional rifting which has created a rift valley with a slight zigzag configuration (Fig. 3). A nearly N-S striking northern part of the graben is followed by a nearly 35 ~ striking central segment whereas the southern part trends near 20 ~. The second stage reactivation by sinistral shear caused extensional shear and additional space for subsidence in the northern segment. The central segment has reacted by compression shear and uplift. Simple shear occurs in the southern segment (lilies and Baumann, 1982). Comparing this present-day geological model with the estimated recent height changes a remarkable agreement is evident (Fig. 4): Besides a mantriggered subsidence of more than 0,6 mm/a near the mouth of the river Neckar around Mannheim, negative height changes up to 0.6 mm/a exist in the northern part of the Rhinegraben. In the western area of the central segment and its adjacent region tendencies of uplift (0.3-0.5 mm/a) but no significant height changes in the graben itself are evaluated. The southern segment is marked by smaller negative height changes up to 0.5 mm/a, but e.g. in a local zone 30 km north of Basel (Fig. 1) subsidences of more than 1 mm/a exist (Fig. 5). Along a 1st order levelling line crossing the eastern part Of thegraben area from Breisach to the Black Forest (Fig. 5) three precise relevellings were carried out since 1939 (1959/61, 1972, 1982). For the three time intervals of 20, 11-13, and 10 years the evaluated height changes show the same tendencies up to - 2 mm/a. The recent vertical motions indicate the eastern master fault of the Rhinegraben and seem to reproduce the Quaternary base. The Upper Rhinegraben (Fig. 4) is an area of recent seismic activity (Bonjer et al., 1984). In the north the mode of seismic dislocation is normal faulting and in agreement with the rather high rates of subsidence obtained by geodetic measurements (Zippelt and M~ilzer, 1987). Concerning the central segment of the eastern Rhinegraben and the northern Black Forest there is a lower activity for the period of the last decades and also the estimated height changes are not significant (0.2 mm/a). The increasing seismic activity in the southern segment corresponds to the higher rates of subsidence but they are not of the same order as evaluated in the northern part (Zippelt and Mfilzer, i987). This fact is caused by the predominant strike-slip mode of the seismic dislocations. But in the southernmost Black Forest recent studies revealed normal faulting for the first time in this area. Therefore predominant strike-slip motions and secondary normal faulting give an explanation of these s~aller geodetic heigfit changes (Zippelt and M~ilzer, 1987).

146

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Earthquakes in SW-Germany 1970-1984

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Fig. 4. Annual height changes and epicentres of earthquakes between 1970 and 1984 in SW-Germany. The height changes (mm/a) are related to the fundamental bench mark near Freudenstadt (FDS) in the Black Forest. The magnitude of the earthquakes is given by the diameter of the signatures. A geophysical profile (GP) was measured in 1985.

REGIONAL AND LOCAL KINEMATICS

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}IOHENZOLLERNGRABEN AREA

Within the Special Research Area "Stress and Stress Release in the Lithosphere"---established at the Karlsruhe University--since 1982 precise geodetic measurements are carried out in the Hohenzollerngraben area situated in the western Swabian Jura (Fig. 6). This region is characterized by the highest seismic activity in middle Europe north of the Alps (Illies, 1982). The epicentres are directed S-N and the focal mechanism is of a strike-slip mode. The earthquake dislocations of the events after 1911 with a magnitude of more than 5 have been estimated from 10 to 25 cm. From hl-situ stress data in the Hohenzollerngraben area results a definite stress distribution (Baumann and Becker, 1986). At a longer distance south of the graben-system the measured highest horizontal principal stress SH of N 150 ~ E is identical with the mean value measured within the W-Alpine foreland (146~ At a short distance south of the graben and within the graben, SH is rotated by 20 ~ (SH = 130 ~ counterclockwise in the direction of the graben (Fig. 7a). North o'1"the graben the local stress deviation is vanishing and SH is again 150 ~ The rotation of SH in the graben direction is

148

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Fig. 6. The Hohenzollerngraben is directed N 130 ~ E. In this region three geodetic testnets were established. In the sketch the epicentres of the earthquakes with a magnitude of more than 5 between 1800 and 1979 are given [Turnovsky, 1981 ); but in the map only the epicentres of events after 1911 with a magnitude of more than 5,5 and su~'h with a magnitude of more than 3 after the 1978 event are represented. A 2nd order levelling line crosses the graben system (s. Fig. 8).

149

REGIONAL AND LOCAL KINEMATICS

also shown by the fault plane solution of the 1978 earthquake (M = 5.7; Turnovsky and Schneider, 1982). After Chinnery (1966), Illies (1982) assumed that the N-S directed strikeslip faults are the cause for the distributed local stress in the Hohenzollerngraben. He explained the local stress field by the induced stress at the end of the N-S directed faults. On the other hand, the theoretical model by Chinnery (1966) of the stress resulting from a regional 150 ~ SH direction and a 120 ~ orientated dextral strike-slip fault is compatible with the setting of the Hohenzollerngraben and the measured local stress distribution. Therefore it seems to be possible that the shear plane I is an active one (Fig. 7a). For geodynamic investigations three geodetic testnets have been established in the Hohenzollerngraben area. Besides levellings and gravity measurements precise distance measurements with the Mekometer ME 3000 are carried out since 1982 for the determination of supposed displacements. In one testnet (Fig. 7b) the first preliminary results show for some points significant displacement vectors up to 4ram; they point to a relative clockwise rotation against a stabilized field. But the time interval of 2 or 3 years is too short to give the answer, whether the horizontal displacements are in agreement with the shear plane I as it is postulated for the Hohenzollerngraben. Along a levelling line crossing the graben system and the southern

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geodetic testnet three levellings were carried out in 1963, 81 and 85 (Fig. 8). The significant negative height changes of 1 mm/a and more are remarkable and point to a recent active vertical kinematic. It cannot be excluded that this effect is caused by a recent spreading of the graben system. ACKNOWLEDGMENTS

The author is very thankful to Karl Zippelt, who has worked for many years in the field of recent crustal movements at the Geodetic Institute and has carried out all evaluations at the Computer Center of Karlsruhe University. The financial supl~ort given by the German Research Society (Deutsche Forschungsgemeinschaft) is gratefully acknowledged.

REGIONAL AND LOCAL KINEMATICS

151

REFERENCES Baumann, H. und Becker, A., 1986. Lokale Variation regionaler Spannungsfelder an zwei Beispielen aus SW-Deutschland und der NW-Schweiz. Int. Symp. Rock Stress and Rock Stress Measurement, Stockholm, Sept. 1986. Bonjer, K. P., Gelbke, C., Gilg, B., Rouland, D., Mayer-Rosa, R. and Massinou, B., 1984. Seismicity and Dynamics of the Upper Rhinegraben. Journal of Geophysics, 55: 1-12. Chinnery, M. A., 1966. Secondary Faulting. Canad. J. Earth Sciences, 3: 163-190. Holdahl, S. R., 1975. Models and strategies for computing vertical crustal movements in the United States. Int. Syrup. Recent Crustal Movements, Grenoble 1975. lilies, J. H., 1982. Der Hohenzollerngraben und Interplatten-Seismizit/it infolge Vergitterung lamell~irer Scherung mit einer Riftstruktur. Oberrhein. Geolog. Abhandlungen, 31: 47-78. lilies, J. H. and Baumann, H., 1982. Crustal dynamics and morphodynamics of the Western European Rift System. Z. Geomorphologie N.F., 48: 135-165. M/ilzer, H., Hein, G. and Zippelt, K., 1983. Height Changes in the Rhenish Massif: Determination and Analysis. In: K. Fuchs, K.v.Gehlen, H. M~ilzer, H. Murawski and H. Semmel (Editors), Plateau Uplift--The Rhenish Massif, a Case History. Springer-Verlag: 164-176. Sigl, R., Torge, W., Beetz, H. und Stuber, K., 1981. Das Schweregrundnetz der Bundesrepublik Deutschland (DSGN 76), Teil 1. Deutsche Geod~itische Kommission, 254 B, Miinchen, 199 pp. Turnovsky, J., 1981. Herdmechanismen und Herdparameter der Erdbebenserie 1978 auf der Schw~ibischen Alb. Diss. Inst. Geophysik, Stuttgart, 109 pp. Turnovsky, J. and Schneider, G., 1982. The seismotectonic character of the September 3, 1978, Swabian Jura earthquake series. Tectonophysics, 83: 151-162. Zippelt, K. and M/ilzer, H., 1987. Results of new geodetic investigations in SW-Germany. Int. Symp. Recent Crustal Movements, Budapest 1985. Journal of Geodynamics, 8: 179-191.

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