Geodetic measurement of tectonic deformation in the southern Alps and Provence, France, 1947–1994
Introduction
The theory of global plate tectonics makes two assumptions which are only approximately correct. The first is that the lithosphere behaves as a set of rigid caps (plates) rotating on the surface of a sphere. The second is that their rate of relative rotation is constant on time scales of the order of a million years. Both these approximations break down in plate boundary regions, where episodic earthquakes on faults produce deformation which is neither rigid nor constant in rate. In these regions, the complexity of the faulting, topography, and seismicity suggests that a better mechanical understanding requires observations at length scales shorter than the characteristic size of a continent and at time scales shorter than the recurrence interval for large earthquakes. This poses the problem of how the crustal deformation is distributed in time and space. Precise geodetic measurements can help improve our understanding of the shortcomings of these approximations mapping the rate of deformation on the continents.
In approaching this problem, we note a serious trade-off between spatial and temporal resolution. We choose to favour the former at the expense of the latter in this paper. Increasing spatial resolution requires a geodetic network with closely spaced stations. To save time and expense, we choose to remeasure an existing triangulation network by GPS rather than install a new set of points and wait for them to move. These two choices lead to a map of deformation at the spatial scale of several tens of kilometres and a temporal scale of four decades. Our approach contrasts markedly with a network of continuously recording GPS receivers, which samples the deformation field, much more coarsely in space (several hundred kilometres between stations) but much more frequently (position estimates once per day). As a consequence, however, our approach inherits two (minor) drawbacks from the triangulation surveys. The existing networks are regional, not global, because triangulation requires visibility along the line of sight between stations, thereby degrading precision over long distances. Furthermore, triangulation measurements are not as precise as GPS, primarily because of optical refraction in the lowermost atmosphere. The major advantage, of course, is that our approach can estimate the rate of deformation without waiting any longer. In this paper, we use the fine-scale approach, favouring spatial resolution, to focus on a small, but active, part of a complex plate boundary.
In the western Mediterranean, Africa and Eurasia are colliding with a convergence rate of 6.2±0.5 mm/yr in the direction N17°W ± 9°, assuming two rigid plates and constant velocity over the last three million years 1, 2. A relative lack of seismicity, combined with space geodetic measurements, suggest that northern Europe is currently deforming quite slowly 3, 4. The central and eastern parts of the Mediterranean region, however, exhibit higher rates of deformation, in the presence of sustained seismicity, particularly in Greece 5, 6, 7, Turkey [8], and Italy [9]. The western part of the Mediterranean coast of Europe, on the other hand, appears to be less active. Here, the most pertinent geodetic observation involves the Satellite Laser Ranging station at Grasse (near Nice) in southeast France (Fig. 1Fig. 2). Its velocity is 3±2 mm/yr of southward displacement with respect to northern Europe [4] (Fig. 3). In the northern Alps, geodetic data have been interpreted in terms of some 4 mm/yr of horizontal shortening in the Jura range 10, 11 and 3–5 mm/yr of shortening between the Belledonne range and the subalpine chains [12]. Such displacement rates, when distributed over small geodetic networks, imply strain rates in excess of 2×10−7/yr. These rates are startling because they are as high as those observed around the active trace of the San Andreas fault system in California [13].
In this paper, we map crustal deformation in southeast France during this century. The study area lies at the crossroads between three different tectonic domains: (1) the high, active Alpine mountain range; (2) the relatively undeformable Massif Central; and (3) the relatively malleable offshore oceanic basin of the Ligurian Sea (Fig. 1). In the study area, the level of seismic activity is moderate (Fig. 2). The largest known and most recent earthquake is the Lambesc event of 1909 with intensity IX (on the MSK scale [14]), attributed to the Trévaresse thrust fault 15, 16. Several large (intensity VIII–IX) earthquakes also occurred near Vésubie, north of Nice, in 1564, 1618, and 1644 [16]. The largest earthquakes recorded instrumentally are the 1959 magnitude 5.3 Haute-Ubaye event and the 1963 magnitude 5.6 event in the Ligurian Sea 16, 17. The geodetic network spans the Provence domain (Fig. 3), which is characterized by strike-slip faults striking NE–SW (Durance, Nı̂mes, and Cévennes) and thrusts trending E–W (Ventoux, Lure, Lubéron, Trévaresse, and Costes). In the northeastern part of the study area, the Embrunais–Ubaye region includes folds and faults striking NNW–SSE (Durance–Serenne–Roburent). The eastern edge of the study area includes the outer folded parts of the Digne and Castellane nappes. Field observations and palaeostress determinations from fault slip data analysis suggest that the tectonic regime in the study area is characterized by north–south compression 18, 19, 20. In this paper, we examine the historical geodetic data set, the longest available instrumental geophysical time series, to measure the rate of tectonic deformation.
The objectives of this study are four in number. First, to validate a geodetic technique for measuring low rates of crustal deformation from a heterogeneous triangulation survey from the late 1940s. Second, to estimate the fraction of the inter-plate convergence accommodated in our study area. Third, to estimate the amount of deformation accommodated in the absence of earthquakes. Fourth, to test the kinematic consequences on short time scales of geophysical models intended to explain long-term tectonic deformation in the area.
Section snippets
Data selection
We analyze two types of geodetic data at 17 geodetic sites measured at least twice by triangulation or GPS (Table 1). The early surveys are first- and second-order triangulation observations performed by the French national survey agency, Institut Géographique National (IGN), primarily during campaigns in 1947–1952 and 1981–1983 with a few measurements between 1887 and 1931. The early first-order campaigns before 1947 used azimuth circles and reduced the eccentric observations to the primary
Estimation procedure
To estimate the strain rates, we use the forward modelling network deformation analysis software package to adjust the network 28, 29. For each station, we can estimate as many as four parameters: the two horizontal components of both the position and velocity vectors in a so-called `free network' solution. The velocity does not vary with time.
We pay particular attention to the occupation history of each benchmark. For example, we do not estimate a velocity for a station with only a single
Geodetic estimates of strain rate
To evaluate the robustness of our solutions to the estimation strategy, we have performed a suite of sensitivity tests on the following issues: (1) a priori values of the vertical component of station position; (2) correcting for the deflection of the vertical where possible; and (3) choice of fixed stations or a `free network' adjustment. None of these perturbations alters the estimated values of and by more than about a third of their stated uncertainty [22].
We have rejected one
Interpretation
The maximum shear strain rate is significantly different from zero (with 68% confidence) in only six of nineteen subnetworks (Table 2). The uncertainty is typically about 0.1 μrad/yr, which corresponds to 1 mm/yr of shearing motion across a zone 10 km wide.
To visualize the shear strain rates, we plot them on a map in Fig. 3. We group the subnetworks into three categories according to the values of the maximum shear strain rate and the azimuth θ of the compressive principal axis of the
Conclusions
We have shown that a rigorous analysis of historical data in a tightly spaced geodetic network can provide useful geophysical information, even in areas of relatively low rates of deformation. Although most of the geodetic triangles yield strain rate estimates which are not significantly different from zero, they tend to support the kinematic model proposed previously on the basis of other geological and geophysical observations 18, 19. Eight of nineteen geodetic subnetworks indicate
Acknowledgements
We wish to thank Danan Dong for generously sharing and supporting his Forward Modelling Network Deformation Analysis software. Georges Ferhat and Vincent Deschaux tirelessly carried batteries to mountain tops. In addition, we thank all the members of the `GPS-Alpes' working group [23] for collecting the 1993 GPS data under difficult conditions as well as the surveyors of the Institut Géographique National (IGN). We appreciate the conscientious efforts of IGN in recovering the historical data.
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