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J Geophys (1984) 55: 1-12 Journal of Geophysics

Seismicity and dynamics of the Upper Rhinegraben

K.-P. Bonjer1 , C. Gelbke 1 *, B. Gilg 1 **, D. Rouland 2 , D. Mayer-Rosa 3 and B. Massinon 4 1 Geophysical Institute, University of , Hertzstr. 16, D-7500 Karlsruhe, FRG 2 Institut du Physique de Globe, F-67000 Strasbourg, France . 3 Institute of Geophysics, ETH-Hoenggerberg, CH-8093 Zurich, Switzerland 4 LDG, B.P. 136, F-92124 Montrouge, France

Abstract. In this paper we present the results of a 10-year Introduction period (1971-1980) of research on the seismicity of the Up­ The Upper Rhinegraben was initially formed as a tensional per Rhinegraben. Our investigations are exclusively based feature in mid-Eocene to early Miocene times, apparently on instrumentally recorded earthquakes with local magni­ following pre-existing zones of weakness in the basement tudes between 0.5

Key words: Seismicity - Continental rift - Stress in the crust - Seismogenic zone - Upper Rhinegraben

* Present address: Institut fiir Geophysik, Westfiilische Bergge­ werkschaftskasse, Herner Str. 45, D-4630 Bochum, FRG Present address: Prakla-Seismos GmbH, D-3000 Hannover 51, ** Fig. 1. General map of the Upper Rhinegraben _area (OD=Oden­ FRG. wald, BF=Black Forest, VM=Vosges mountams, KG=Kra1ch­ Offprint request to: K.-P. Bonjer gau) 2

50 60 70 80 53° 90 100 110 120 130 14 0 150 160 170530

52° 52°

. .. .. ·. 51 ° . \. 51° ." .·. ..> ·_, ..~ :·.· .

500 50°

49° 49°

48° . 48°

47° 47°

...., ~ ...:. :.·.r~ ;f. ~ Fig. 2. Seismicity of central Europe 46° ... ) '';.I 46° (1000--1970). (References: Sieberg, 1940a, b; Sponheuer, 1952 ; Hiller et al., 1967; Ahorner, 1970; Sponheuer et al ., 1968; Caputo and Postpischl, 1972 ; Prochazkova, 1974; Gutdeutsch and Arie, 1976; Pavoni, 1977; and others

gest event with a maximum intensity (MSK) of at least

60 so / 10° 0 = IX (Mayer-Rosa and Cadiot, 1979) occurred in 1356 A.D. near Basel, in the southern end of the Rhinegraben (Fig. 1). Knowledge of the seismicity up to 1960 is based mainly on macroseismic information. Installation of seismic stations started around the turn of the century. Due to poorly known crustal and upper mantle structure, low mag­ nification and great station separations, the intrumentally determined focal coordinates were of scanty precision and often less accurate than the macroseismic determinations (e.g. Sponheuer, 1960). However, these data have already allowed the Upper Rhinegraben to be recognized as the seismic tie between the Alps in the south and the Rhenish Massif in the north. The Rhinegraben also separates the South German block in the east from the Lorrainese block in the west (Fig. 2), which both show little to negligible seismic and orogenic activity. On the other hand, the pre­ cision of the "historical" data in general is insufficient to study the seismic dislocations in detail. The installation of a dense network of high-gain seismic stations in the Rhine­ graben area (Fig. 3) and the progress of crustal structure studies in the past 20 years (e.g. Prodehl et al., 1976) pro­ vided a sufficient basis for a thorough investigation of the seismicity in this region for the first time. ~r.-,-,-,.-l'-r-...--r-,-,.-4--~-,-,.~1-.-~-..--,.-+-47° so 10° Data Fig. 3. The seismic stations in the Rhinegraben area. Three charac­ ters: seismic stations. Black circles : stations of the seismic network Magnitude-frequency relationships operated by the Geophysical Institute, University of Karlsruhe. Triangles and squares : stations operated by other Institutes. Lines Figure 3 reproduces the short-period seismic network in the with solid arrows : telemetry lines Rhinegraben area. The majority of these 37 stations have |00000009||

3

observations. The proper discrimination of individual focal regions is not only important for the assessment of seismic hazard but may also serve as a tool in understanding the present-day dynamics of the Upper Rhinegraben.

The average P-wave velocity model In parallel with the increase in the number of high-gain seismic stations, the structure of the crust and upper mantle has been studied in detail during the past 2 decades (e.g. Prodehl et al., 1976; Prodehl, 1981). On the basis of the crustal models deduced by Edel et al. (1975) from refraction profiles of different regions in the Rhinegraben area, an 0.1 average model for the southern graben was constructed by Gelbke (1978). From the results of Pg-time term analysis, Gelbke decreased the basement velocity of the model by Edel et al. down to 6.0 km/s and the velocity gradient, in the depth range between 15 and 20 km, was slightly lower~d to explain crustal phases of deeper events recorded at dis­ 0 2 3 tances of up to about 140 km. This average model was Fig. 4. Cumulative magnitude-frequency distribution for Upper again refined by Gilg (1980), who replaced the remaining Rhinegraben earthquakes (circles: instrumental data 1971-1979; velocity gradient zones between 15 and 20 km, as well as squares: macroseismic and instrumental data 1900-1970) between 20 and 26 km, by layers with constant average velocities in order to match crustal phases which often dom­ high magnifications of about 100,000 (1 Hz) and were set inate the records up to distances of about 500 km. In this up by French, German and Swiss institutes between 1~65 way, the middle crust, regarded as characteristic for a conti­ and 1980. A considerable lowering of the mean detect10n nental graben by Mueller (1977), disappeared as a separate threshold, to at least a magnitude of ML= 1.2, was achieve_d crustal unit. by this network. Lippert (1979) demonstrated that this In the construction of an average crustal model for the threshold holds for the entire Upper Rhinegraben area, us­ northern part of the Upper Rhinegraben, Gilg's model was ing the events recorded between 1971 and 1978. The local modified slightly by increasing the depth of the crust-mantle magnitudes are determined from coda length scaling laws boundary by 1.5 km to a depth of 27.5 km. The uppermost deduced by Lippert (1979) for the Rhinegraben area. As mantle velocities were taken from Ansorge et al. (1979). it has been shown recently by digital data of the station A vP/vs-ratio of was used throug_hout this ~tudy. In FEL (Gruber, 1983), the coda-magnitudes are in sur­ (3 Fig. 5, the velocity model, correspondmg travel times and prisingly good agreement with simulated Wood-Anderson ?-arrival times of the Sierentz mainshock of July 15, 1980 magnitudes when applying Richter's (1935) procedure. are shown to demonstrate the agreement between observed Nevertheless, because of the different geological settings and computed travel times. For distances greater than in California and the Upper Rhinegraben, a bias with re­ 260 km (i.e. outside the Rhinegraben area) the upper mantle spect to the original local California magnitudes cannot velocity model is no longer valid, but the average crustal be excluded (Bonjer et al., 1982). For the graben proper, model may still be used. Fig. 4 (Bonjer, 1979b) demonstrates that the magnitude­ The focal coordinates are calculated with the computer frequency distribution of the historical events, labelled as program HYPGRAD (Gelbke, 1978), a modified version macroseismic data (e.g. Ahorner, 1975; Leydecker and of HYP0-71 (Lee and Lahr, 1972). The program HYP­ Harjes, 1978), can be extended by short-term but high-sensi­ GRAD utilizes a travel-time subroutine which allows the tive observations, labelled as instrumental data. For the use of arbitrary velocity-depth distributions (e.g. gradients, whole graben area, the historical, mainly macroseismic velocity inversions). In addition to the first P- and S-arriv­ data, and the new instrumental data yield the same b-value als, later P- and S-phases, including overcritically reflected of - O. 73. By extrapolating both data sets towards one an­ waves, can be used to augment the number of observations other, they show compatible annual occurrence rates. How­ and to considerably increase the reliability of the focal pa­ ever, as shown by Lippert (1979), the b = -0. 73 value 1s rameter determinations (Gelbke, 1978). For most of the only an average value which can vary significantly (i.e. events their accuracy is better than ± 3.5 km. This has been -0.5 > b > -1.0) if the graben area is divided into smaller verified by relocating calibration shots at various locations focal regions. However, in performing such a subdivision, within the Rhinegraben area (Gelbke, 1978). care has to be taken to define the appropriate location and size of every focal region so that it covers an area with The epicentre distribution a uniform seismotectonic regime. The strong variation of b-values and the annual occurrence rate of earthquakes Figure 6 summarizes the epicentres of the period (with respect to a fixed magnitude) found by Lippert (1979) 1971-1980. During this time the sourthern and northern might either reflect true seismotectonic differen~es or an end of the Upper Rhinegraben were considerably more ac­ artifact of an inappropriate choice of the focal reg10ns and/ tive than the middle part between Strasbourg and Karls­ or insufficient data. The latter possibility might be true, ruhe. On the other hand, the historical epicentre distribu­ given the rather short period of 9 years of instrumental tion (Hiller et al., 1967) shows a seismic activity for the 4

8~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~-r 6 4 2 u 0 :z -2 w - 4 r -6 >-- 10 ~ -8 ~ - 10 20 -- -1 2 "' "'~ 30 Cl w -1 4 ~ u fri 40 6-16 0 w "'- 10 50 - 20 3 4 5 6 7 6 9 10 -22 VE LOCITY IN KM/$ - 24 5 1ERENTZ MS L ANGZEITKURVE

0 40 80 120 160 200 240 280 320 360 400 440 480 520 560 600 DISTANCE IN Kr! Fig. 5. Average velocity-depth model and travel times used for hypocentre determinations (+=observed travel times of the Sierentz shock July 15, 1980 - after Gruber, 1983). Reduced travel times (v,= 6.0 km/s) as a function of epicentre distance

50 70 gO 5• 70 go 10° 80 10° 50°30' a• 50°30' 51oirr~t:;:i;;~;=:t;=:t;-t--+-+-l-+-+--+-+-+-t-+--+-+-+:--r 5 1° • ~'t • • ' 0 X·lO x · 2 0 X·'O 50° ... 50° x

50° 50°

.. 49° ......

48° 48°

. ~ • 0 0 ~ · o a 000 47°20' ~ 47°20' 5• 1° a• g• 10° Fig. 7. Comparison between historical and present day seismic ac­ tivity. squares: historical (mainly macroseismic) epicentres (1000-1970); circles: instrumentally determined epicentres (1971- 1980) ~~~::::;'.".'.:;:::;:::;:.:+-+--+-+-+-+-+-+--+-l---+--+-1--+--+-+L7° ~ ~ ~ ~ 1if Fig. 6. Epicentres in the Upper Rhinegraben area (l 971- 1980) partly due to these two areas that the number of earth­ quakes east of the Rhine river is greater than west of it. Such an asymmetry can be recognized in the historical data, middle part which is of about the same order as at the too (Fig. 7). Neither the geological setting nor the topogra­ graben ends (Fig. 7). This may be seen more convincingly phy (Illies, 1974) give any hint in furnishing an explanation when the seismic energy flux is considered (Fig. 1 of Hagele for this asymmetric distribution of foci. Although no hypo­ and Wohlenberg, 1970). Thus the short observation time centre has been detected in the lower crust near the Moho may account for the low activity reported here. (see next section), it is worthwhile noticing that the border The areas with the highest seismicity are found in the between the more and the less active areas closely follows Swabian Jura and near Saulgau, south of Stuttgart between the crest line of the updomed crust-mantle boundary the Danube and rivers. The data are taken from (Fig. 8) with a strike direction of about N30°E (Gelbke, Earthquakes in the Federal Republic ofGerman y (Seismolog­ 1978; Bonjer, 1979a). ical Central Observatory Graefenberg, 1971- 1980). It is One of the remarkable features of the seismicity pattern 5

8. 9° 10° 11° the eastern border faults do not show earthquakes, at least down to a depth of about 10 km. Furthermore, the most 30 prominent faults at the graben flanks do not show enhanced so· seismic activity. It remains an open question whether or not the foci apparently following the strike of the Bonndorf graben zone between Freiburg (48°00'N/7°50'E) and (Fig. 1) may be attributed to this fault system, as favoured 49° by Gelbke (1978) and Bonjer and Fuchs (1979), or to a number of parallel arranged conjugate faults. Fault-plane solutions (Fig. 11) show that both directions are possible. At the southernmost end of the Rhinegraben (Basel-Dinkel­ berg area, 47° 35' N/7° 40' E) a short lineation of epicentres with an abrupt change of strike from N to NE is found 48° 48° (Fig. 6). As a sufficient number of reliable fault-plane solu­ tions is not yet available, further discussion is not justified at this time. The distribution of epicentres seems to be even more complicated in the graben proper (Fig. 6), especially in the northern and southern parts. Here the foci are spread out, approximately over the entire width of the graben, and no Fig. 8. Epicentres (1971 - 1980) and depth contour-lines of the crust­ clear alignment of epicentres exceeding a length of 10 km mantle boundary (Edel et al., 1975) is noticeable. If there were fault segments active over a length of 10 km or more, they should have been detected is connected with the western and eastern main border fault due to the sufficient accuracy of the localizations. systems of the graben. Both fault systems create only negli­ The most pronounced seismic activity is located in the gible seismic activity, if at all. The low activity of the bor­ western Swabian Jura (48°17'N/9°01'E) and the Epinal dering faults has already been stated by Hiller et al. (1967), (48°18'N/6° 30' E) - Remiremont (47° 59'N/6° 31' E) re­ on the basis of epicentres from 1021 to 1965 (Fig. 7, open gions, roughly situated at the same distance from the Rhine­ squares). As regards the instrumental data presented in this graben "hinge-lines" (Mueller, 1970) of the uplifted graben paper, already this lack of activity seems to be obvious shoulders. Comparing historical and instrumental data at the western border faults on rough inspection of the (Fig. 7), a shift of activity from south (Remiremont) to epicentre map (Fig. 6). Apparently, such a deficit of seismi­ north (Epinal) can be seen. Quite a similar migration of city does not exist at the eastern border faults, if only the the strongest events towards the north was found in the epicentre distribution is considered. However, it emerges, western Swabian Jura, starting around 1900 (Brenner, 1966; when discussing the focal depths (Figs, 9, 10), that even Schneider, 1971 ). Another point of comparison between

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60 70 80 9• 10° 110 Epinal is concerned, no statement can be made yet because 51°+------1---~--""---~---j 51° of the scarcity and uncertainty of the seismological observa­ tions. The comparison of historical and instrumental data (Fig. 7) reveals three facts. Firstly, local source areas exhibit a significant geographical consistency for the past millen­ nium. Secondly, the short-time, high-sensitivity observa­ 50° tions show that most of these focal areas are marked by persistent microseismicity which reflects the general pattern of the Rhinegraben seismicity. This pattern has already been discussed by Bonjer and Fuchs (1974). Statistical anal­ yses of N(ML) corroborate this result when historical data 49° are matched with instrumental observations (Fig. 4). Thirdly, the Kraichgau area, in the middle part of the Rhinegraben (Fig. 1) north of Karlsruhe, which was already considered to be aseismic by Hiller et al. (1967) and Schneider (1968), is still seen to be a zone of seismic quies­ cence. No earthquake with a local magnitude ML > 0.5 was 48° 48° detected there.

Focal depths / )~ " The accuracy of focal-depth determinations has been in­ ,---~o~ .. creased due to the enlargement of the seismic network, to 47° 47° much more detailed knowledge of the crust and uppermost 60 70 80 90 10° 11° mantle structure, and to the additional use of later body­ wave phases, which give strong constraints when no near­ Fig. 11. Fault-plane solutions (Wulff projection) in the Upper Rhinegraben area. (References: [1 , 5, 6, 8, 9, 21, 23, 24, 27] = by observations are available (Gelbke, 1978). Focal depths A horner et al., 1983; [3] =Neugebauer and Tobias, 1977 ; [4] = based on instrumental data for certain time spans and gra­ Baier and Wernig, 1983; [22] = Turnovsky, 1981; [31 , 33] = Mayer­ ben regions have already been reported by Gelbke (1978), Rosa and Pavoni, 1977 ; [2, 7, 10-20, 25, 26, 28- 30, 32] = this Bonjer (1979a), Gilg (1980) and Gruber (1983). In Fig. 9 paper). Black quadrants: compression. White quadrants: dilatation. a presentation of the focal depths for the years 1971 - 1980 Black dots: P-axes. White dots: T-axes in five different cross-sections is given. The most conspicu­ ous details are presented in Fig. 9c, which shows a cross­ section perpendicular to the strike of the southern graben, these two areas is the dominance of strike-slip mechanisms roughly half-way between Strasbourg and Basel. All hypo­ (Fig. 11). Even though the seismotectonics of the western centres of that region are projected onto this cross-section. Swabian Jura have been studied in detail during the past The greatest focal depths of about 20 km are found in the decades (e.g. Schneider, 1968; Schick, 1968, 1970 ; Haessler Black Forest, whereas in the graben proper and in the et al. 1980; Turnovsky, 1981 ; Turnovsky and Schneider, Vosges mountains focal depths of 16 and 13 km, respective­ 1982; Illies, 1982; Illies and Baumann, 1982), no convincing ly, are not exceeded. This maximum depth of about 20 km correlation of the strike directions of seismic shear zones is not only found in the southern, but also in the northern­ at depth with those of surface geology was found. As far most part of the Black Forest at the latitude of Karlsruhe as the corresponding shear zone between Remiremont and (Fig. 9a). Shallower depths seem to prevail in the middle |00000013|| -.I 2SW 3SW 6NW 6SW 4SE ONE 18NW 30SE 30NW 54SE 50SE 57NE 5SSW 5SSW Plunge 20SE 28NW 24NW 25NW 90 71NW 75NE 6SSE 68NE 6SSE 33SE 39NE 80SW sosw 46NW 40NE 45NW 41NW 45E 51 30 55 45 25 90 48 1SO 192 120 170 150 130 AZ 2SO 16S B-Axis 165 277 262 225 150 200 308 325 333 345 355 310 338 224 279 220 206 3NW 3SW 5NE 5NE 5NE SNE SSW SE SSW 2NE ONE 7NE 6NE 13SW Plunge 10SW 10SW 21SE 10SE 21SW 14SW 12SW 34SW 30SW 35NW 34NE 50SE 51SW 49NE 44W 22SW 26NE ONE 90 0 12 57 59 73 3S 77 77 92 76 66 69 62 S6 41 T-Axis 185 195 108 115 31S AZ 215 22S 234 246 21S 249 233 242 275 227 255 237 240 7SE 3SE 8NW SSE SN 4SW OSE ONW 19SE 15SE 1SSE 1SNW 15SE 55NW 57NW 55SE 28SE 20NW 23NW 23NW 72SE 70NW 78NW 79NW 7SE 65SE 39SE 30SE 63SE 83SW 44NE 45NW 41SE Plunge 80 90 150 110 16S P-Axis 106 1S6 147 137 135 168 156 135 157 133 120 162 130 130 33S 32S 300 340 326 336 300 325 354 330 326 354 346 AZ 2S5 0 10SE 35NW 35SE 50NE 56SW 58SW 54NW 54NE 50SW 50NE sosw sosw Dip 25NW 75NE 70NE 70NE 77NE 70SW 70SW 60SW 60SW 60NE 60SW 80SW 42NE 45SE 45NE 40NE 45NE 41NE 49SW 45SW 2 60 62 45 45 94 19S 102 177 118 162 153 145 153 142 195 117 119 113 106 120 170 115 103 170 120 163 105 133 130 290 305 330 Strike Plane 296 0 Dip 29NW 70NW 72SE 75NW 70S 60SW 64SW 35NE 65NE 65NW 60SW 65NE 60SE 50SW 50NE 52NE 54SE SOSE SOSE SOSE 88NW 80NE 85NE 80SE S2SE SONW 42NW 45NW 42NE 40SW 90 90 1 2 4 13 12 12 30 30 20 28 25 22 20 90 62 63 37 60 44 45 42 40 181 12S 100 105 190 100 110 113 145 160 140 Plane 201 Strike 1.S 1.7 1.6 1.9 3.1 3.1 3.9 3.7 3.3 3.0 3.7 3.9 5.7 2.3 2.6 2.S 2.1 2.9 2.1 2.1 2.S 2.1 2.4 2.3 2.9 2.3 3.1 3.6 3.3 ML 3.6 3.2 3.2 4.7 7 7 7.4 6.7 5 S.1 S.9 s 4.6 2.2 9.6 6 6 11 11 17 10 16.S 17.5 11.5 19.9 12 11 11 15 13.3 13.7 20 20 20 11.7 20 10 h (km) 7.06 7.94 7.97 7.65 9.4 7.70 7.63 7.4S S.66 S.56 S.51 S.22 8.40 S.87 S.30 9.03 Long.E 07.44 07.90 07.77 07.79 07.02 07.59 07.64 OS.14 OS.2S OS.67 OS.34 OS.21 OS.50 07.01 07.52 08.48 06.5S Lat.N 50.41 50.1S 50.25 50.07 50.04 47.55 49.91 49.65 47.65 49.85 49.55 4S.81 4S.13 4S.OO 4S.17 4S.12 47.9S 4S.29 4S.29 47.S7 47.79 47.6 49.06 4S.10 47.20 49.36 4S.65 48.59 47.60 49.87 47.6S 48.46 4S.2S 1971~1980 1976 19SO 19S1 1971 1977 1979 1977 1977 1977 1976 1979 1976 1979 1971 197S 1975 1975 1975 1980 1976 1978 1974 1976 1972 1980 1977 1974 1979 1980 1979 1974 1979 2, 7, 3, 5, S, 7, 7, 9, 6, 8, 4, 4, 15, 11, 197S 15, 12, 29, 18, 26, 15, 11, 1 12, 16, 12, 21, 23, 31, 30, 31, 26, 27, 27, 28, earthquakes of Feb. Febr. Mar. Feb. May Mar. Feb. Nov. Nov. Nov. Nov. Aug. Apr. Apr. Apr. Mar. Date Dec. May Dec. Dec. Dec. Oct. Oct. Setp. Nov. Aug. Jul. Jan. Jun. Jul. Jul. Jan. Jul. solutions berg Goar olterdingen Fault-plane Langenthal Dinkel Bodensee Rastatt Rhinau Kaiserstuhl Bantzenheim Delle Region Lorsch Echzell Epinal Schopfheim Sierentz Undenheim Albstadt Wyhl Waldkirch W Waldau Wiesbaden Darmstadt Hambach Dornstetten Heitersheim Molsheim Enzklosterle ldstein Oppenheim Geroldstein Grof3sachsen St. Speyer Schlucht 1. 1 3 5 2 7 9 6 8 4 19 31 33 30 32 11 1S 13 15 21 2S 26 24 10 14 16 22 23 25 27 29 12 17 Table No. 20 |00000014||

8

NORTHERN R.G. N N

•. up 0 • down SOUTHERN R.G.

"-120' Pattern of fault strike Slip vectors (from f.p.s.) (from f.p.s.J Fig. 12a + b. a Rose diagram of fault and auxiliary planes; b distribution of slip vectors a) bl deduced from fault-plane solutions (Wulff net)

part around Freudenstadt (Fig. 9a). In contrast to the of approximately 10 km. The lateral extent of the earth­ southern graben with greatest depths of 16 km, the northern quake-free zone is about 8 km. Due to an insufficient part shows focal depths down to 20 km (Fig. 9 b, d). Only number of events a similar experiment cannot be done on few data are so far available from the central part of the the western border fault of the graben. graben between Karlsruhe and Strasbourg but they indicate a restriction of the foci to the uppermost 10-15 km of the Focal mechanisms crust (Fig. 9 b ). Since the focal depths of about 20 km practically coin­ It has often been questioned whether fault-plane solutions cide with the sharp velocity increase at the boundary be­ derived from microearthquakes can serve as a clue in under­ tween upper and lower crust (Fig. 5), we checked the error standing the seismotectonic regime of a broader region, e.g. in focal depth due to inaccuracies in the depth of this dis­ the Upper Rhinegraben, or whether they reflect solely the continuity. It was varied in both directions with appropriate seismic dislocation at a particular site. Whatever the case, velocity modifications in order to correct for travel times. we are forced to use microearthquakes when investigating In this way we could confirm these great focal depths, i.e. the seismic dislocations and the regional stress field acting no earthquake occurred below 20 km. Hence, the lower in the Rhinegraben area. Otherwise, we would not have crust in the Rhinegraben area is free of seismic activity. sufficient data in an acceptable short period of time (see (On the basis of prevailing macroseismic investigations, this Fig. 4) for a reliable interpretation. fact has already been mentioned by Ahorner et al. (1972).) Several studies of focal mechanisms in the Rhinegraben This important result will be discussed in detail later. As area were performed in the past (e.g. Schneider et al., 1966; documented in Fig. 9, the thickness of the seismogenic part Schneider, 1968; Ahorner and Schneider, 1974; Mayer­ of the crust varies laterally, but no fine structure of the Rosa and Pavoni, 1977; Bonjer, 1979b; Bonjer and Fuchs, depth distribution of the hypocentres is recognizable with 1979; Haessler et al., 1980; Rouland et al., 1980; Tur­ certainty. Whether or not the relatively small number of novsky, 1981). In this paper, older data were omitted, events in the uppermost 5 km is only an artifact (earth­ mainly because of their poor quality, and as far as possible quakes with small magnitudes are likely to be recorded by replaced by more recent solutions. However, the former an insufficient number of stations resulting in unreliable solutions did display the general trend of the regional stress depth determinations), or is caused by the seismotectonic field (Schneider et al., 1966; Ahorner, 1970; Ahorner and regime, can only be resolved by a greater concentration Schneider, 1974). of seismic stations in areas with supposedly shallow foci. Figure 11 summarizes the fault-plane solutions deduced In discussing the epicentre map (Fig. 6), it was men­ from earthquakes which occurred during the period tioned that the main western and eastern border faults are 1971-1980. Details are given in Table 1. The type of seismic lacking seismic activity. This is hard to accept with regard dislocation changes from north to south at a latitude of to the eastern fault, but it emerges from Fig. 9c that this about 49°N. In the north, normal faulting is predominant, might indeed be the fact. Gelbke (1978) was able to prove whereas in the south, strike-slip mechanisms partly prevail, convincingly the lack of seismicity by correcting the associated with small dip components (Bonjer, 1979b). In crooked course of the eastern masterfault and projecting the middle section of the graben, the fault-plane solutions the hypocentres onto cross-sections perpendicular to the are indicative of a greater amount of dip. From south to local strike of the fault and assembling these sections into north, i.e. with increasing distance from the Alps, the strike a master-section. From Fig. 9 it is quite obvious that the of the P-axes rotates counter-clockwise by about 40°. In eastern master fault is free of earthquakes down to a depth the southern graben area the strike of the P-axes closely |00000015||

9

approaches the north-south direction. A further detail pared with other continental rift systems (e.g. Ahorner, should be stressed. At a latitude of about 48°N, the strike 1975; Fairhead and Stuart, 1982). of the P-axes remains constant from east to west, i.e. we As was discussed in the preceding sections, no convinc­ find the same seismotectonic regime in the western Swabian ing evidence of seismic activity in the southern area was Jura, the Black Forest, the graben proper and the Vosges found to be associated with the main border faults and mountains. Despite the ambiguity of associating the actual the master faults of the grahen shoulders. Furthermore, rupture plane (the same holds for the slip vector) with one the earthquakes do not follow the contact zone between of the two nodal planes, it is worthwhile constructing a granite and gneiss in the southern Black Forest (Gelbke, rose diagram with the strike of the nodal planes in order 1978). Delays of Pg-waves crossing the eastern border faults to look for prevailing directions. In Fig. 12a such rose dia­ led Gelbke (1978) to the conclusion that the uppermost grams are shown for the northern and the southern graben. 5-10 km of this zone are strongly fractured and, therefore, Note that there is a striking difference between the north unable to accumulate sufficient strain to produce earth­ and the south. In the north, only one direction, of N150° E, quakes. If this zone can be loaded at all by the regional is dominant. This direction is nearly perpendicular to the stress field, strain should be released by creep processes strike of the SE border fault of the Rhenish Massif and or "nano-earthquakes" which are below the detection level can be followed by fault-plane solutions, alignment of epi­ of the existing seismic network. A different explanation is centres (Ahorner et al., 1983) and tectonic features through given by Illies et al. (1981) for the absence of seismic activi­ the Massif up to the Embayment. In the ty. They argue that the frictional resistance of faults dipping southern graben area, two prominent conjugate directions at about 63° is high enough to withstand the shear stress ofN20°E (Rhenish) and N120°E (Hercynian) emerge. produced by the actual regional stress field. However, this All these directions well exceed the scatter of the data. model does not explain the occurrence of earthquakes The slip vector, another seismotectonic parameter, can also which are below a depth of approximately 10 km. At this be extracted from the fault-plane solutions. Again, this was depth, due to an increase of confining pressure, the fric­ performed separately for the northern and the southern tional resistance might be even greater if temperature and graben areas (Fig. 12 b ). In the north the majority of slip pore pressure effects are neglected. On the other hand, we vectors dip more or less steeply with an average trend to­ do not want to deny either the inaccuracy of the position wards SW. In the southern graben area, the situation seems of the foci by projecting them onto a vertical master section to be chaotic. Even a trend of the slip is not recognizable, as shown in Fig. 10, or a possible mislocation due to lateral and the number of directions downwards and upwards is velocity changes, e.g. by a crushed zone (Gelbke, 1978; about the same. It is impossible to calculate an average Gilg, 1980). Anyway, it is a challenge for future research annual slip rate of the seismic dislocation unless one can to map the spatial extent and the rheology of this zone define source areas with uniform slip. Seismic slip rates, of seismic quiescence more precisely, in order to answer calculated simply by adding up the amount of slip (without the implicit question of whether we have to regard this taking into account the azimuth, dip angle and sense) are zone as being highly strained or just the opposite. grossly in error in a region like the southern Rhinegraben. In the northern part of the graben, adjacent to the Oden­ wald south of Frankfurt (Fig. 1), the eastern border fault Discussion does not show a zone of seismic quiescence, at least not as pronounced as in the southern graben (Gilg, 1980). A cor­ In the magnitude-frequency distribution of earthquakes responding zone at the western border fault system, as sug­ from 1971 to 1978 for the area of the graben proper (after gested by the epicentre distribution, cannot be traced due Lippert, 1979), to the small number of events at adjacent sides of the fault. log(Ncla)=2.24-0.73 ML 1.00.5 is less than 150. This is not very impressive care has to be taken, if epicentre maps are interpreted with­ for an area of about 16 x 104 km2 . Furthermore, since a out taking the focal depths into account. In the southern local magnitude greater than ML= 5 is seldom reached, and graben, at least, the depths are known for a considerable omitting the exceptional Basel event of 1356 A.D., we can number of earthquakes. But what is known about the dip regard the Rhinegraben as an area of low seismicity com- and the extension with depth of the particular Riedel 10

61 Strike Direction of ( ' { Ft,B-axes' • · · · · · · · 50°

No 6 49 49° 9 = 140° 5 S(G1l= 141° 4 0!P, Bl= 139°

3

2

0 47° '--~---~----'-----' 47° 70 ao 90 -e Fig. 13. Comparison of in situ stress measurements (a 1-direction) and strike direction of P- and B-axes from fault-plane solutions shears? At least, the strike directions are known. We will Combining Byerlee's law (Byerlee, 1968) with flow laws come back to this point, when we discuss the focal mecha­ for quartz or olivine to account for temperature effects and msms. correction of pore pressure, Brace and Kohlstedt (1980) According to Fig. 9 the thickness of the seismogenic derived upper limits for the lithospheric stress as a function part of the crust varies laterally in the Upper Rhinegraben of depth. The introduction of the quartz flow law results area. The lower boundary was found by mapping the grea­ in a very abrupt drop in frictional resistance, close to zero, test focal depths. If they represent maximum depths down at shallow depths between 10 and 20 km. This shallower to which earthquakes are occurring in the Rhinegraben depth offers an explanation for the rather sharp transition area, we can assume that such a boundary marks the transi­ from seismic to aseismic behaviour, beneath the Black For­ tion from brittle, or frictional, to ductile behaviour of rocks. est, Upper Rhinegraben and Vosges mountains. The model In the Black Forest, this transition zone coincides with the of Brace and Kohlstedt (1980) has been successfully applied rapid increase of P-velocity from slightly more than 6 km/s to several seismically active regions outside subduction to about 7 km/s, starting at a depth of approximately 20 km zones (e.g. Meil3ner, 1980; Sibson, 1982 ; Mei13ner and (Edel et al., 1975). Strehlau, 1982). Although Kirby (1980) has questioned the A P-wave velocity of about 7 km/s in the depth range use of the quartz flow law for the middle and lower crust, of 20-25 km can be assigned to gabbroic rocks (Ringwood it can at least give a lower bound to the crustal shear stresses and Green, 1966). Above this transition zone, the average (Brace and Kohlstedt, 1980). velocity of 6.0- 6.1 km/s of the upper crust can be attributed As mentioned earlier, most events which have been in­ to granitic rocks, and in the lowermost few kilometres velo­ vestigated for the deduction of fault-plane mechanisms are cities of up to 6.5 km/s in the original model of Edel et al. microearthquakes. The mechanism of a single event is not (1975) correspond to granodioritic rocks (Edel, 1975). Thus necessarily representative for a broader region. On the other the die-off of seismic activity at a depth of about 20 km hand, Fig. 11 shows that the mechanisms of events in par­ in the Black Forest may be linked to the change from gran­ ticular areas, e.g. the northern or southern graben, the east­ itic to gabbroic rocks (Bonjer, 1979b). In the graben proper, ern graben shoulders etc. reveal a remarkable consistency. the same velocity-depth distribution is found, as in the This fact is supported by Fig. 12. The mean strike directions Black Forest. However, here the drop-off in the number of the nodal planes (Fig. 12) follow, with surprisingly small of earthquakes occurs in the granitic part of the crust at scatter, the prevailing direction of the border-fault system a depth of about 16 km. Upwarping of the isotherms from as well as the conjugate direction in the southern graben the Black Forest towards the Rhinegraben (Mueller and area. A strike direction of N170°- 180° E, claimed by Illies Rybach, 1974) results in a temperature of about 460° C et al. (1981) for the present seismotectonic active direction at 16 km depth in the graben. The heat flow in the Vosges of the Riedel shears, is not found in the rose diagram. We, mountains is supposed to be even higher than in the graben therefore, favour the idea that the two expressions of active (e.g. Gelbke, 1978), and accordingly the maximum focal tectonics occur at different levels of the crust, i.e. second­ depths are even shallower. It seems obvious that the maxi­ order shear seems to be restricted to the uppermost part mum focal depths are intrinsically connected with the tem­ of the crust. In the northern graben area, an average strike perature at those particular depths (Bonjer, 1977; Gelbke, direction ofN150°E for the fault planes prevails (Fig. 12a). 1978). The same strike also dominates in the Rhenish Massif and |00000017||

11

the Lower Rhine Embayment (Ahorner et al., 1983). These mantle from long-range seismic observations in southern Ger­ areas, including the northern Upper Rhinegraben, are pre­ many and the Rhinegraben area. Tectonophysics 56, 31-48 sently experiencing active rifting (Ahorner et al., 1983). 1979 Baier, B. Wernig, J.: Micro-earthquake activity near the southern Figure 13 shows a comparison of the (J 1-directions from in situ stress measurements in the Upper Rhinegraben area, border of the Rhenish Massif. In: Plateau Uplift - The Rhenish Massif, a case history. Fuchs, K., v. Gehlen, K., Miilzer, H., as summarized by Baumann (1981), and strike directions Murawski, H., Semmel, A. eds.: pp 222-227; Springer-Verlag, of P-axes derived from earthquakes, which occurred close 1983 (less than about 20 km) to the site of stress measurements. Baumann, H.: Regional stress field and rifting in Western Europe. The maxima of the most frequently obtained strike direc­ In: Mechanism of Graben Formation, Illies, H. ed.: pp 105- tions from in situ measurements and P-axes differ only by 111, Tectonophysics 73, 1981 a few degrees. Both parameters may reflect the same cause: Bonjer, K.P.: Seismicity of the Upper Rhinegraben - relations to (the strike of) the regional stress field in the ­ the vertical and lateral temperature variations? In: Geother­ graben. If this is the case, the regional stress field at the mische Forschung im Oberrheingraben. , K., ed.: pp 29- surface and at depth would have the same strike. On the 31, 1977 Bonjer, K.P.: The seismicity of the Upper Rhinegraben as an ex­ other hand, in the southernmost graben, more than an pression of active tectonics and their deeper origin. Allgemeine order of magnitude difference exists for the amplitude of Vermessungs-Nachrichten 10, 383-386, 1979a the maximum horizontal shear stress ( <4 bars) deduced Bonjer, K.P.: The seismicity of the Upper Rhinegraben. A conti­ from in situ data (Illies and Baumann, 1982) and the aver­ nental rift system. In: Proc. Int. Research Conf. on Intra-Conti­ age static stress drops (30