Evidence for ongoing extensional deformation in the western Swiss Alps and thrust-faulting in the southwestern Alpine foreland

J Grodynamrts Vol. 26, No. I. pp 7743, 19% c~’1998 Published by Elsevier Science Ltd All rights reserved. Printed in Great Britain

Pergamon PII: SO264-3707(97)00022-7

02663707:98

$19.00+0.00

EVIDENCE FOR ONGOING EXTENSIONAL DEFORMATION IN THE WESTERN SWISS ALPS AND THRUST-FAULTING IN THE SOUTHWESTERN ALPINE FORELAND ELENA EVA,’ STEFANIA PASTORE’ NICHOLAS DEICHMANN2*

and

‘Institute of Geophysics, University of Genova, Genova, Italy ‘Swiss Seismological Service. ETH-Ziirich, Switzerland

Abstract-To verify the discordant orientations of P- and T-axes found by earlier studies in the Penninic domain of the southern Valais, Switzerland, and in the surrounding regions of France and Italy, we have evaluated the focal mechanisms of 11 of the best-recorded earthquakes that occurred in this area between 1985 and 1990. By employing two-dimensional ray-tracing techniques, we have made use of what is known about the lateral variations of the crustal structure to obtain constraints on the possible focal-depth range of the hypocenters and on the take-off angles at the source. In addition, we have been able to identify one of the two nodal planes as the actual fault plane of one of the events, based on high-resolution relative locations of its aftershocks. The resulting normal faulting and oblique-slip focal mechanisms show that, down to depths of about 10 km, the compressional structures of the Penninic nappes, which were formed during the Alpine orogeny, are presently undergoing extensional deformation and that a significant component of this extension is perpendicular to the Alpine arc. Thrust faulting focal mechanisms from events at the northwestern margin of the PO plain, however, indicate that the southern Alpine foreland is still subject to compressional deformation consistent with the large-scale stress field expected from the convergence of the African and European plates. Thus, our results lend support to geodynamic models that predict extensional deformation across the crest of a mountain range, while the flanks and lowlands continue to undergo crustal shortening. CC:,1998 Published by Elsevier Science Ltd. All rights reserved

INTRODUCTION Earlier seismotectonic studies of the western Alps have shown that P-axis directions derived from earthquake focal mechanisms are in overall agreement with directions of maximum *Author to whom all correspondence should be addressed: Schweizerischer Erdbebendienst, sik. CH-8093 Ztirich, Switzerland; Fax: +41 l-633 1065; E-mail: nico(a‘seismo.ifg.ethz.ch. 21

lnstitut

fiir Geophy-

F. Eva

28

(‘I

II/

crustal shortening associated with the Alpine orogeny (e.g. Pavoni, 1986). The fan-like orientation of P-axes perpendicular to the strike of the major Alpine units has been interpreted as a sign of a counterclockwise rotation of the direction of maximum compressive stress from the eastern and central Alps to the Ligurian Sea. Pavoni (1980) and Pavoni and Roth (1990) have interpreted the good agreement between present-day orientations of P-axes and the directions of maximum crustal shortening derived from geological observations as evidence for an essentially unchanged stress field between the time of active mountain building and present. However, a closer look at some of the focal mechanism compilations reveals that in certain areas. such as the region situated between the Dora Maira, Argentera and Pelvoux massifs (Frechet and Pavoni, 1979) and the Penninic domain between the Aar-Gotthard and the Mont Blanc massifs, orientations of the P- and T-axes seem to be more chaotic and discordant with this general picture (Eva et N/.. 1990). In fact, for the southern Valais, Nicolas rt al. (1990) (fig. 1 I) show three focal mechanisms with on the average WNW ESE trending P-axes next to one focal mechanism with a T-axis oriented in the same direction. It is thus possible that in these complex transitional regions, between the internal and external domains of the Alpine mountain belt, orientations of the P- and T-axes are evidence of local stress perturbations. However, since the beginning of systematic instrumental observations, earthquakes in these areas have been small. and station distribution has been far from optimal. Moreover. standard earthquake location procedures rely on one-dimensional crustal velocity models. Considering the strong lateral heterogeneity of the regional crustal structure (e.g. Kissling, 1993), routine focal-depth determinations and, consequently. the calculated take-off angles of the rays at the source are prone to systematic errors. Since, unfortunately, the published focal mechanism compilations rarely include the original data or give details of calculation procedures, it is difficult to assess the reliability of the results. It is thus possible that differences between the regional stress field and many of the focal mechanisms are mostly due to poorly constrained fault-plane solutions. To verify the discordant orientations of P- and T-axes found by earlier studies in the Penninic domain of the southern Valais, Switzerland. and in the surrounding regions of France and Italy, we have evaluated the focal mechanisms, based on the distribution of

Table N

Location

I 2 3 4 5 6 7 8 9 IO II

Mauvoisin Argentiere Argentierc Vissoie Zermatt Dixence Monte Rosa Mont Blanc lvrea Torino I Torino 2

M,

local magnitude;

I. Event hat

Date

Time L’TC

.\I,

Lat

Lon

Z

85.01.04 X5.05.25 86.01.17 86.01. I9 X6.02. I5 86.02.26 87.05.X) 8X.06.1 I 87.07.03 90.02. I I 90.02. I I

lh:S7:37 10:39:5x 07:os.31 06:54:36 01.43:07 l3:07:17 lY:45:1Y 22144146 10:46:57 07:00:3x 07:07:48

-3.2 3.1) 3.4 3.0 3.6 2.‘) 2.7 3.4 3.7 4.1 2.7

46.002 45.994 46.005 46.183 46.05 I 46.034 45.961 45.861 45.399 44.965 44.987

7.269 6.916 6.927 7.640 7.638 7.350 7.909 6.886 7.596 7.547 7.476

7 I3 l--7 o- 6 3-9 2- x 4-10 7 II 5- I I &5 22-26 22-26

Lat. and Lon. in degrees North and East. respectively:

Z, focal depth in km.

Evidence

for ongoing

extensional

deformation

in the western Swiss Alps

29

observed first-motion polarities, of 11 well-recorded earthquakes that occurred in this area between 1985 and 1990 (Fig. 1 and Table 1). Moreover, based on the relative location of the aftershocks of one of the events, it has been possible to identify its actual fault plane. In contrast to earlier studies, we have made use of what is known about the lateral variations of the crustal structure and evaluated secondary arrivals to obtain constraints on the possible focal-depth range of the hypocenters. It is well known that fault-plane solutions of local earthquakes, based on P-wave first-motions, depend strongly on correct identification of the arrivals in the seismogram and on reliable estimates of the take-off angle of the rays at the source. This sensitivity is particularly strong for shallow earthquakes situated in zones of strong upper-crustal velocity gradients and for focal mechanisms that deviate

48-N

47-N

48-N

ITALY 45-N

44-N

MEDITERRANEAN

SEA

43-N 6’E

7’E

6-E

9-E

1

Fig. 1. Map with seismic stations (triangles) and epicenters of the analyzed earthquakes (black diamonds). Straight lines show the general orientation of the three main crustal profiles modelled by two-dimensional ray-tracing.

significantly from a pure strike-slip mechanism. To constrain the take-off angles better, we adopted a two-step location procedure. Using routine location techniques based on onedimensional velocity models. we tirst determined the epicentral coordinates and then used two-dimensional ray-tracing techniques to estimate the possible focal-depth range and derive the corresponding take-off angles. Thus. the more realistic velocity models have also resulted in more reliable take-off angles and. consequently. in more consistent focal mechanisms.

I>ATA

Local magnitudes of the events analyzed in this study range between I ). We have used records from the following networks (Fig. 1): l l l

l l

2.7 and 4.2 (Table

Swiss national network, operated by the Swish Seismological Service in Zurich; regional northwest Italian network, operated by the Lfniversity of Genova; stations in France. operated by the Laboratoire de Dttection et de Geophysique. Parib and by the University of Grenoble; stations in southern Germany. operated by the University of Karlsruhe. local networks in northern Swit/rerland and around the hydroelectric dam of Zeuzier in the Valais. operated by the Swiss Seismological Service.

Most of the records were digital or in form of hard-copy plots of digital data. Only for the stations NEC‘ and BBS as well as for some of the French stations were the available data in form of analog paper records. Thus, all arrival times and first-motions used in this study were reread from the original seismograms. Published data from earthquake bulletins were not used.

EPIC’ENTKAL.

L.OC‘ATIONS

After reevaluation of all available arrival times. epicentral locations were calculated with the widely used program HY PO-7 I (Lee and Lahr. 1972). The velocity-depth model consists of three horizontal layers over a mantle half-space with a lower-crustal layer of variable thickness. to account for the large differences in Moho-depth between foreland and Alpine stations. Variations in near-surface geology and in station elevations are also accounted for by using different velocities in the uppermost layer and by calculating elevation dependent station delays. P-arrival times were in general given full weight (I .(I) in the calculation, whereas S-arrivals, which arc usually subject to a greater uncertainty, were systematically given maximum weights ofO.5 for three-component records and 0.25 for vertical component data alone. In the immediate study area. only stations MMK and SIE (Fig. I) were equipped with three-component seismometers. A first location was calculated using all arrivals. Based on travel-time residuals and on plots of arrival times vs. epicentral distance. it was possible to distinguish arrivals refracted at the Moho (Pn) from direct arrivals (Pg) (Fig. 3). Because the one-dimensional velocity models are poor approximations to the complex three-dimensional crustal structure, the locations were refined using only direct arrivals. The stability of the epicentral location thus obtained was checked by repeated calculations with difl’erent station combinations and c’p/ Vs-ratios. In general. epicenter locations were stable to within f 2 to + 3 km.

Evidence for ongoing extensional deformation 1986.01.19

I

130

1

1

1

140

150

160

Epicentral Fig. 2. Record section velocity Vred = 6 km s

of the seismograms

distance

31

M = 3.0

Z = 06 km

46.183N/07.640E

06:54:36.4

120

in the western Swiss Alps

'-5

170

180

190

200

(km)

of the Vissoie event, recorded

in northern

Switzerland. Reduction arrival from the

‘_Pg, direct arrival; PMP, reflected arrival from the Moho; Pn. refracted upper mantle.

TWO-DIMENSIONAL

RAY-TRACING

With the location algorithm HYPO-71 and simple one-dimensional velocity models, focal depth is generally not well determined, unless it is at least as large as the epicentral distance to the closest station. However, provided one has independent knowledge of the three-dimensional crustal structure, the possible focal-depth range can be constrained by the relative arrival times of direct, refracted and reflected phases (e.g. Deichmann, 1987). As shown in the record section in Fig. 2, both the reflection (PMP) and refraction (Pn) at the Moho can be clearly seen on the seismograms recorded in the northern foreland of the Alps. We have thus constructed two-dimensional crustal cross-sections from the respective epicenters to the north, south and east (Fig. 1). Structural information was obtained from the available seismic refraction and reflection results (Braendli, 1981; Pastore, 1989; Yan and Mechie, 1989; Pfister, 1990; Maurer and Ansorge, 1992). Using the two-dimensional seismic ray-tracing program RAYAMP (McGill University, 1987), we varied the focal depth of each earthquake until we obtained a satisfactory agreement between observed and calculated arrival times of the Pg, Pn and PMP phases (Fig. 3). It is important to note that, because of the trade-off between origin time and focal depth inherent in the location method, we cannot model absolute arrival times by this procedure. However, errors in origin time cause a travel-time offset that is equal for all phases, so that they will not affect arrival-time differences between various phases. Even errors of several km in epicentral coordinates will only have minor repercussions on travel-

E. Eva (21ul

32

DISTANCE (KM)

-

_-______.L_’

Fig. 3. Ray-tracing results for the Mauvo~~n c~ent along a NNE trending protile. Time scale is reduced with a reduction velocity of’6 km s ‘. Observed arrivals are symbolized by squares, calculated arrival times by crosses.

time differences. Thus. the largest source of uncertainty in this procedure is our imperfect knowledge of the Moho depth and of the seismic velocities in the crust. However, the ultimate purpose of this study was not an exact determination of focal depths, but the construction of reliable fault-plane solutions. Therefore. we have used the ray-tracing procedure to estimate the range of likely focal depths for each earthquake (Table 1). Then, we have tested the effect of the resulting focal-depth variability on take-off angles and on fault-plane solutions. In order to allow for uncertainties due to errors in our crustal models, focal depths were considered acceptable if they resulted in discrepancies as large as 0.3 s between observed and calculated travel-time differences. This value is about twice as large as the actual timing uncertainty. Given the large number of stations and the adequate information on the corresponding crustal structure, the best constraints on focal depths were derived from modelling the arrivals observed in northern Switzerland and southern Germany along profiles from south to north. Ray-tracing along profiles striking in other directions was used mainly for calculating take-off angles at the source. The focal-depth ranges obtained for the events in the Penninic domain of the western Swiss Alps and the Mont Blanc Massif agree with earlier focal-depth compilations. which indicate that earthquakes below the Alpine mountain ranges are restricted to the upper 15 km of the crust (Deichmann and Baer, 1990; Eva rt uf., 1990; Roth et al.. 1992; Maurer and Kradolfer, 1996). The two events located at the edge of the PO Plain near Torino, however, occurred at a depth of about 24 km. Such lower crustal earthquakes in the southern Alpine Foreland have also been reported before (Zonno and Kind, 1984; Eva ef al., 1990) and seem to mirror a similar focal-depth distribution observed below the northern Alpine foreland in Switzerland (Garcia-Fernandez and Mayer-Rosa, 1986; Deichmann, 1987, 1992a; Deichmann and Baer. 1990).

Evidence for ongoing extensional deformation in the western Swiss Alps

33

FOCAL MECHANISMS The fault-plane solutions have been calculated with the computer program FPFIT, which determines the best fitting set of nodal planes for a given distribution of first-motions (Reasenberg and Oppenheimer, 1985). The optimum focal mechanism parameters resulting from this algorithm are listed in Table 2 and the corresponding stereographic plots of the first-motion directions are shown in Fig. 4. Determination of the take-off angles at the source, taking into account the lateral heterogeneity of the crust, has resulted in highly consistent fault-plane solutions. The few remaining misfits are either close to the resulting nodal planes or pertain to signals with poor signal-to-noise ratios recorded at large distances. The influence of the uncertainties in focal depth on the focal mechanisms was examined by calculating different fault-plane solutions over the depth range determined from the two-dimensional ray tracing (Eva, 1991; Pastore, 1993). Since these solutions are often constrained by Pn arrivals, which are relatively insensitive to variations of seismic velocities and focal depth, the results are stable even for the normal- and thrust-fault mechanisms. In general, the scatter of strike and plunge of the P- and T-axes calculated by program FPFIT is less than f IO”. Only for the Mauvoisin and Monte Rosa events does the scatter amount to f 15 and f 20’, respectively. The signals of the two Argentiere events (Fig. 1 and Table 1) observed at most stations are almost identical (Pastore, 1993). Consequently, both their hypocentral locations and focal mechanisms must be practically the same, which would justify constructing a composite fault-plane solution. We have tried to construct both a composite and a separate solution for these events. However, at several stations of varying azimuth and distance from the epicenter, the onsets consist of a weak precursor followed about 0.2 s later by a strong arrival of opposite polarity (Fig. 5). This could possibly be a path effect (Pastore, 1993) but the available data are not sufficient to model the ray paths and thus the take-off angles with confidence. Moreover, the signal character at station EMV as well as the fact that the polarity at this station seems to be different for the two events indicate that this station must lie on one of the nodal planes, in contradiction with the fault-plane solutions obtained by fitting the first-motion polarities (Pastore, 1993). For these reasons, we do not consider our results for these two events to be reliable enough to include them here. The Vissoie event, with an almost pure normal faulting mechanism, is located close to two other events (1976.01.29 11:39 and 1982.07.03 04:49) for which Mayer-Rosa and Pavoni

Table 2. Azimuth N

Location

1 4 5 6 I 8 9 10 11

Mauvoisin Vissoie Zermatt Dixence Monte Rosa Mont Blanc Ivrea Torino 1 Torino 2

and plunge of the f- and T-axes P

T

279133 143182 252101 94158 101/33 24913 1 297/10 4614 66112

23121 13/05 345121 188/02 357121 354123 147/78 308/58 317159

ZERh4A-l-r

MAUVOISIN

MONT BLANC

VISSOIE

IVREA

TORINO 1

TORINO 2

(1977) and JimeneL and Pavoni (1984) obtained very similar mechanisms, with a nearly identical orientation of the r-axes. To illustrate the improvement obtained by two-dimensional ray-tracing. Fig. 6 shows the fault-plane solution of the Vissoie event based on the take-off angles calculated using a one-dimensional crustal model without velocity gradients in the upper crust. As can bc seen by comparing Fig. 6 with the corresponding solution in Fig. 4. the two focal mechanisms arc similar despite the obvious misfits and the inaccurate take-off angles. The two misfits in the solution are typical examples of misidentified arrivals. Thus. in this case. two-dimensional ray-tracing has increased the reliability and consistency of the resulting focal mechanism.

35

seconds Fig. 5. Three seismograms (vertical component, ground velocity) of the first Argentiere event. Distance/Azimuth (km/degrees) of the stations: ROM =83/O. MMK = 8 I /86. SIE = 57149. Arrows mark the two P-arrivals discussed in the text.

Fig. 6. Fault-plane solution for the Vissoie event, based on a one-dimensional velocity model. Most take-of angles are inaccurate and many phases are misidentified (see Fig. 4). The compressional data points at the lower edge of the plot, which correspond to observations in northern Switzerland, should actually plot in the upper half of the diagram, either as down-going Pg-arrivals or as Pn-arrivals (see Fig. 2). Similarly, the two discrepant observations at the upper edge of the diagram are actually Pn arrivals, which should plot in the dilatational quadrant.

Eva 1’1

36 1986.02.15

01 :43:07.5

46.051N/07.638E

Z =

06

km

M =

3.6

t

-a

~~7 0

5

i-

15

.'r;

25

30

i .T

40

Azimuth

Fig. 7. Fan-like

record wction 01‘ I’n arrl\al\

between 160 and 201 km. Reduction the NNE

I

.A

10

~SSW striking nodal plane (Fig. 41. The rrlati\c differences in elevation.

(deg

01 the Lermatt

velocity I\ X.0 km \ near-surl’,w

45

55

$0

c~ent. obacrvcd in northern

delay

65

-0

4

75

I

‘. .\rrow\

\elocltir\

50

indicate the polarity

Switxrland

at distance\

change that constrains

between the traces are due to a combination

of

and Moho depth belou each station.

For the Mauvoisin and Zermatt events. focal mechanrsms based on a one-dimensional crustal velocity model have already been published previously (Nicolas rt d., IWO). In both cases, the earlier focal depth determinations (5 km for Mauvoisin and 14 km for Zermatt) differ significantly from our results. Despite the difference in focal depth. the fault-plane solution of the Zermatt event is similar to that shown in Nicolas ct nl. (1990). This is due to the fact that the solution does not deviate much from a pure strike-slip mechanism. which is well constrained by the Pn-arrivals. The seismograms plotted as a function of azimuth in Fig. 7 clearly document the polarity reversal between stations WIL and SAX at an azimuth of about 40 Fig. 7 also illustrates the high signal quality of seismograms from events in the magnitude range between 3 and 4 recorded out to distances of 200 km. In the case of the Mauvoisin event. however. the two focal mechanisms differ significantly: we obtain a strike-slip mechanism with a normal component of slip instead of a thrust mechanism. Since the earlier fault-plane solution of Nicolas C~I(I/. (1990) does not include the actual data points. it is difficult to assess whether the different results are due only to the different velocity model used or also to differences in the available data. RELATIVE

LOCATIONS

OF THE

MAUVOISIN

AFTERSHOCKS

The magnitude 3.2 Mauvoisin event was followed within nine hours by six aftershocks with magnitudes between 1.4 and I .9 (Table 3). The signals of these events are all remarkably similar to each other (Fig. 8). and the first-motion polarities are compatible with the faultplane solution of the main shock. Such signal similarities are due both to the close relative proximity of the hypocenters and to their identical focal mechanisms. Numerous studies

Evidence for ongoing extensional deformation in the western Swiss Alps Table 3. The Mauvoisin Date

GMT

Ml

0

85.01.04 85.0 I .04 85.01.04 85.01.04 85.01.04 85.01.04 85.01.05

16:57 17:40 18:48

3.2 1.9 1.4 1.8 1.8 1.4 1.5

2 3 4 5 6

36

37

38

event and its aftershocks

N

1

39

40

18:50 22:14 22145 01:51

41

42

39

I

/

I

40

41

42

(vertical component,

43

44

seconds

seconds

Fig. 8. Seismograms

31

ground

velocity) of the six aftershocks

at stations

DIX and EMV.

of the Mauvoisin

event, recorded

45

3x

E. Eva

(‘I

trl

based on accurate relative locations of individual events in both aftershock sequences and swarm-like earthquake clusters show that such events often occur on the same fault. To test this hypothesis and thus possibly to identify which of the two nodal planes of the faultplane solution corresponds to the actual fault, we have determined precise arrival-time differences between individual events recorded at each station, using a cross-correlation method in the time domain (e.g. Deichmann and Garcia-Fernandez, 1992: Augliera et d.. 1995). Given the relative proximity of the hypocenters. the location error due to the poorly known seismic velocities is the same for all events. Thus. using a standard master event location technique (e.g. A&era ct rrl., 1995) the relative location errors are only a function of the remaining timing uncertainties. Given timing accuracies of a few milliseconds based on the cross-correlation. it is often possible to determine relative locations of such events with a precision of a few tens of meters. Using records from the stations DIX, EMS, EMV. MMK and SIE, at epicentral distances of I6 55 km and with a good azimuthal distribution. we have relocated all the Mauvoisin events relative to the first aftershock. The results are shown in Fig. 9. In our case. the signal-to-noise ratio at several stations was not optimal, and the mainshock signals were clipped on most records. so that the final resolution of our locations is only in the order of 100 m. Nevertheless, the resulting alignment of the hypocenters matches the steeply dipping NNW SSE striking nodal plane rather than the ENE-~ WSW striking plane (Fig. 9).

Our results give a consistent picture ofthe contemporary deformation within the Penninic nappes of the Valais. The earthquakes in this region (the first tive events in Table 2) are characterized by transcurrent or normal faulting mechanisms. with a nearly Nap-S-trending average orientation of the T-axes. The general trend of the Alps in the Penninic domain of the southern Valais is oriented ENE WSW and thus forms an angle of 60--70 with the average direction of the observed ‘r-axes (Fig. IO). It is therefore evident that the compressional structures of the Penninic nappes. which were formed during the Alpine orogeny, are presently undergoing extensional deformation and that a significant component of this extension is perpendicular to the Alpine arc. The only event analyzed in the external Mont Blanc massif has a predominantly strikeslip mechanisms with a similar N S trend of the T-axis (Fig. 10). Due to the NNE-SSW alignment of the Alpine chain in this area. however. the orientation of the T-axis does not correspond to any significant amount of extension across the mountain range compared to the deformation in the internal Pcnninic domain of the Valais. A more striking contrast can be seen at the southern foot of the Alps, where the observed focal mechanism show thrust faulting with compression perpendicular to the Alpine chain (Fig. 10 and the last three events in Table 2). Fig. I1 gives a synoptic picture of the style and orientation of ongoing deformation across the Swiss Alps and Alpine foreland. compiled from the results presented here together with earlier results of studies by Pavoni ( 1980. t 987). Sambeth and Pavoni (I 988) Pavoni and Roth (1990) Deichmann (1992b) and Roth cjt rl/. (1992). In the northern Alpine f’oreland and Jura Mountains, transcurrent faulting predominates, and the general direction of maximum crustal shortening is oriented more or less perpendicular to the strike of the compressive structural features observed at the surface. This mirrors our observations in the southern Alpine foreland and is in accord with the general stress field associated with

Evidence

46.130h

for ongoing

lr

extensional

A

deformation

in the western

Swiss Alps

39

----.-\ \ \ \ \

B’ ,’

46.120N L 7.250E

7.270E

A’

12.0

Xkm

A

I

1

,

0 4

E ;5

Qqy- 0 0

6

12.5

B

0

m

100

Xktll

1

B’

0 12.5 I _ CfJIoo Fig. 9. Relative locations of the Mauvoisin event and its aftershocks. Top panel is a map view. Lower two panels are vertical cross-sections along A-A’ and B-B’.

40

Fig. IO. Tectomc

sketch-map

with focal mechamsms

compiled

in this study.

the convergence of the African and European plates and with the compressional style of the alpine orogeny (e.g. Miiller c~f~1.. 1992). Against this background, both the predomimantly normal faulting style of the focal mechanisms in the Penninic domain of the Valais and the orientation of the direction of maximum extension at a high angle to the local trend of the Alpine chain seem to be out of place. However, together with the normal faulting focal mechanisms in the eastern parts of the Swiss Alps (Roth c’t ul., 1992) and in the Brianqonnaise Zone between Pelvoux and Argentera (FrCchet and Pavoni. 1979). we must interpret this extensional deformation in terms of a systematic feature found in several regions of high elevation within the Alps. Thus. our results are consistent with geodynamic models of the evolution of mountain ranges. which predict extensional deformation with normal faulting across the crests of the range, while the flanks and lowlands continue to undergo crustal shortening (e.g. Molnar and Lyon-Caen. 1988).

A~,kno~~lec!yonrr,r., -EM. Baer and M. Cattaneo. who are responsible for the software for much of the data acquisition and analysis in Zurich and Genova. contributed significantly to this study. We are grateful to K. Bonjer. Karlsruhe. and to J. FrCchet and F. Thouvenot, Grenoble. for supplying the seismograms recorded by their networks. WC thank C. Eva and H. Maurer for reviewing earlier versions of the manuscript and two anonymous reviewers for their constructive comments. Contribution no. 947. Institute of Geophysics. ETHZtirich.

Evidence for ongoing extensional deformation

in the western Swiss Alps

10SE Crystalline massifs and intrusions

Molasse Basin and POPlain Fig. 1 I. Tectonic sketch-map

with arrows indicating the predominant directions of crustal results derived from this study are shown by the heavier arrows.

deformation.

The

REFERENCES Augliera, P., Cattaneao, M. and Eva, C. (1995) Seismic multiple& analysis and its implications in seismotectonics. Tectonophysics 248,219-234. Braendli, J. (1981) Kompressionswellen-Geschwindigkeiten (Vpg) in den oberflaechennahen Bereichen alpiner Kristallingebiete. Diploma thesis, Institute of Geophysics, ETH-Zurich. Deichmann, N. (1987) Focal depths of earthquakes in northern Switzerland. Ann&es Geophysicae 5B(4), 395402. Deichmann, N. (1992) Structural and rheological implications of lower-crustal earthquakes below northern Switzerland. Phys. Earth Planet. Int. 69, 27&280. Deichmann, N. (1992) Recent seismicity of the northern Alpine foreland of Switzerland. Eclogae geol. Helv. 85(3), 701-705. Deichmann, N. and Baer, M. (1990) Earthquake focal depths below the Alps and northern foreland of Switzerland. In The European Geotraverse: Integrative Studies (Freeman R., Giese P. and Mueller St. eds), pp. 277-288, European Science Foundation, Strasbourg, France. Deichmann, N. and Garcia-Fernandez, M. (1992) Rupture geometry from high-precision

42

E. Eva c’ftrl

relative hypocenter locations of microearthquake clusters. G~oph~~.s. J. Inr. 110, 501 517. Eva. C.. Augliera, P., Cattaneo. M. and Giglia, G. (1990) Some considerations of seismotectonics of northwestern Italy. In The Europcrrtl Gcotrm~~crw: Itzteyrutiw Studiirs, (Freeman, R.. Giese. P. and Mueller. St. eds). pp. 2%2Y6, European Science Foundation. Strasbourg. France. Eva, E. (1991) Sismotettonica delle Alpi Nord-Occidentali. Diploma thesis. University of Genova. Frkchet, J. and Pavoni. N. ( lY79) Etude de sismiciti de la zone Brianqonnaisc entre Pelvoux et Argentera (Alpes Occidentales) LiI’aide d’un rCseau de stations portables. Edqae Grol. Hrk. 72, 763 769. Garcia-Fernandez. M. and Mayer-Rosa. D. (1986) Improved hypocentral parameter determination using secondary phases. Rcr. Grofis. 42, I 75 I X4. Jimenez. M.-J. and Pavoni. N. (19X4) Focal mechanisms of recent earthquakes 1976&iY82 and seismotectonics in Switzerland. In Proc. 1.4SPEl XV/II Gm. A.swmh., 1983 (Stiller H. and Ritsema A. eds), pp. 77 X4. Publ. Zentralinstitut fiir Physik der Erde. Potsdam. Kissling, E. (1993) Deep structure of the Alps What do WC really know? Ph~x. Earth Plmet. Int. 70, 87 112. Let. W. H. K. and Lahr, J. C. (1972) HYPO-71 a computer program for determining hypocenter, magnitude and first motion pattern of local earthquakes. U.S. Geol. Surv.. Open-File Rep. McGill University (1987) RAYAMP-PC. 2-D Raytracing and Synthetic Seismograms, Version 2. I. McGill Ilniversity Geophysics Laboratory. Montreal. Canada. Maurer, H. and Ansorge. J. (1992) Crustal structure beneath the northern margin of the Swiss Alps. T~~~.ro1~opl1~..si~..s 207, I65 I Xl Maurer. H. and Kradofer, lJ. ( 1906) Hypoccntral parameters and velocity estimation in the western Swiss Alps by simultaneous inversion of P- and S-wave data. Bull. Srisnd. Sot. ii/l~. 86, 32 4 I Mayer-Rosa. D. and Pavoni. N. ( I Y77) Fault-plane solutions of earthquakes in Switzerland from 197 I to 1976. Puhl. Inst. Gcwplr~*.s.Pd. Acrrcl. Sci. A-5( I 16). 32 I 326. Molnar. P. and Lyon-Caen. H. ( 198X) Some simple physical aspects of the support. structure and evolution of mountain belts. GcT)/. Sock. .4/17. S/ICT. Prrpw 218, 179-207. Miiller, B.. Zoback. M. L.. Fuchs, K.. Mastin. L.. Grcgersen. S.. Pavoni. N.. Stephansson, 0. and Ljunggren. C. (19Y2) Regional pattern of tectonic stress in Europe. .J. Gcoph~~.s. Rcs. 97(B8). I 1783 I I X03. Nicolas. M.. Santoire. J. P. and Delpech. P. \r (IYYO) Intraplate seismicity: new seismotectonic data in Western Europe. Tl~c,to,loJ)h~..cic~s 179, 27 53. Pastore. S. ( 19X9) Protili sismici crostali nella Liguria Occidentale. Diploma thesis. University of Geneva. Pastore. S. (1993) Analisi del campo di stress attravcrso lo studio di meccanismi focali di terremoti delle Alpi ltalo-Svilzerc. Ph.D. thesis. [Jniversity of Genova. Pavoni. N. (19X0) Crustal stresses inferred from fault-plane solutions of earthquakes and neotectonic deformation in Switzerland. Rod Mcch. 9(Suppl.), 63-68. Pavoni. N. (1986) Regularities in the pattern of major fault zones of the earth and the origin of arcs. In Ori~yin of’Arc.s (Wezel F.C. ed.). pp. 63 7X. Elsevier. Amsterdam. Pavoni. N. (19X7) Zur Seismotektonik der Nordschweiz. I%/O~UP Geol. Hrk. 80(2). 461 472. Pavoni. N. and Roth, P. (lY90) Seismicity and seismotectonics of the Swiss Alps. Results of microearthquake investigations 1983 198X. In IIc~lp Structure of’the Alps. Mew. Sot. ~yd. Frunc~c~.N. S. (Roure F., Heitzmann P. and Polino R. eds), Vol. I%. pp. I29 134. Pfister, M. (I 990) Gemeinsame Auswertung van Erdbeben-. Refraktionsund Reflexionadata in der Nordschweiz. Diploma thesis. Institute of Geophysics. ETH-Zurich.

Evidence

for ongoing

extensional

deformation

in the western Swiss Alps

43

Reasenberg, P. and Oppenheimer, D. (1985) FORTRAN computer programs for calculating and displaying earthquake fault-plane solutions. U.S. Geol. Surv.. Open-File Rep. 85-739. Roth, Ph., Pavoni, N. and Deichmann, N. (1992) Seismotectonics of the eastern Swiss Alps and evidence for precipitation-induced variations of seismic activity. Te~tonoph~wic~s 207, 183-197. Sambeth, U. and Pavoni, N. (1988) A seismotectonic investigation in the Geneva Basin, southern Jura Mountains. Eclogae Geol. Hela. 81,433-440. Yan, Q. Z. and Mechie, J. (1989) A fine structural section through the crust and lower lithosphere along the axial region of the Alps. Geophys. J. ht. 98,465488. Zonno, G. and Kind, R. (1984) Depth determination of north Italian earthquakes using Graefenberg data. Bull. Seismol. Sot. Am. 74(5), 1645-16591.