The displacement field has been monitored in the vicinity of the crater rim at Mount Merapi (Indonesia) from 1993 to 1997. During this period the volcanic activity has been quasicontinuous with dome growth, explosions, and pyroclastic flows. We measured a nine‐point network every year with the Global Positioning System static method. Interpretation of results is conducted with a three‐dimensional elastostatic boundary elements code that takes into account topography, fractures, and complex magma source geometry. The inversion technique yields an estimate of the variation with time of the boundary conditions at the magma duct interface together with the probability associated with the best model. The Young's modulus of the equivalent continuum is found to be very low (of the order of 1 GPa), a feature which suggests that a viscoelastic behavior may be more appropriate for this rock mass, given the observed seismic velocities for the domain of interest. A striking compatibility is outlined between observed deformations and the rate of occurrence of multiphase seismic events, once the main fractures of the structure have been taken into consideration. This suggests that the summit elastic (or viscoelastic) deformation field is controlled by the magma flux within the duct rather than by magma pressure variations. In addition, a nonelastic displacement has been identified at the westernmost point of the network. This was considered critical for the stability of the summit structure, a concern whose validity has been verified a posteriori by the July 1998 explosion.
Abstract The kinematics of the present-day deformation in the western Alps is still poorly known, mostly because of a lack of direct measurements of block motion and internal deformation. Geodetic measurements have the potential to provide quantitative estimates of crustal strain and block motion in the Alps, but the low expected rates, close to the accuracy of the geodetic techniques, make such measurements challenging. Indeed, an analysis of 2.5 years of continuous GPS data at Torino (Italy), Grasse (France), and Zimmerwald (Switzerland), showed that the present-day differential motion across the western Alps does not exceed 3 mm/yr [Calais, 1999]. Continuous measurements performed at permanent GPS stations provide unique data sets for rigorously assessing crustal deformation in regions of low strain rates by reducing the amount of time necessary to detect a significant strain signal, minimizing systematic errors, providing continuous position time series, and possibly capturing co- and post-seismic motion. In 1997, we started the implementation of a network of permanent GPS stations in the western Alps and their surroundings (REGAL network). The REGAL network mostly operates dual frequency Ashtech Z12 CGRS GPS stations with choke-ring antennae. In most cases, the GPS antenna is installed on top of a 1.5 to 2.5 m high concrete pilar directly anchored into the bedrock. The data are currently downloaded once daily and sent to a data center located at Geosciences Azur, Sophia Antipolis where they are converted into RINEX format, quality checked, archived, and made available to users. Data are freely available in raw and RINEX format at http://kreiz.unice.fr/regal/. The GPS data from the REGAL network are routinely processed with the GAMIT software, together with 10 global IGS stations (KOSG, WZTR, NOTO, MATE, GRAZ, EBRE, VILL, CAGL, MEDI, UPAD) that serve as ties with the ITRF97. We also include the stations ZIMM, TORI, GRAS, TOUL, GENO, HFLK, OBER because of their tectonic interest. We obtain long term repeatabilities on the order of 2-3 mm for the horizontal components, 8-10 mm for the vertical component. Using a noise model that combines white and coloured noise (flicker noise, spectral index 1), we find uncertainties on the velocities ranging from 1 mm/yr for the oldest stations (ZIMM, GRAS, TOUL, TORI, SJDV) to 4-5 mm/yr for the most recently installed (CHAT, MTPL). Station velocities obtained in ITRF97 are rotated into a Eurasian reference by substracting the rigid rotation computed from ITRF97 velocities at 11 central European sites located away from major active tectonic structures (GOPE, JOZE, BOR1, LAMA, ZWEN, POTS, WETT, GRAZ, PENC, Effelsberg, ONSA). The resulting velocity field shows residual motions with respect to Eurasia lower than 3 mm/yr. We obtain at TORI, in the Po plain, a residual velocity of 2.3+ or -0.8 mm/yr to the SSW and a velocity of 1.9+ or -1.1 mm/yr at SJDV, on the Alpine foreland. These results indicate that the current kinematic boundary conditions across the western Alps are extensional, as also shown by the SJDV-TORI baseline time series. We obtain at MODA (internal zones) a residual velocity of 1.2+ or -1.2 mm/yr to the SSE. The MODA-FCLZ baseline show lengthening at a rate of 1.6+ or -0.8 mm/yr. These results are still marginally significant but suggest that the current deformation regime along the Lyon-Torino transect is extension, as also indicated by from recent seismotectonic data. It is in qualitative agreement with local geodetic measurements in the internal zones (Briancon area) but excludes more than 2.4 mm/yr of extension (FCLZ-MODA baseline, upper uncertainty limit at 95% confidence). Our results indicate a different tectonic regime in the southern part of the western Alps and Provence, with NW-SE to N-S compression. The GRAS-TORI baseline, for instance, shows shortening at a rate of 1.4+ or -1.0 mm/an. This result is consistent with seismotectonic data and local geodetic measurements in these areas. The Middle Durance fault zone, one of the main active faults in this area, is crossed by the GINA-MICH baseline, which shows shortening at a rate of 1.0+ or -0.8 mm/an. This result is only marginally significant, but confirms the upper bound of 2 mm/yr obtained from triangulation-GPS comparisons. The REGAL permanent GPS network has been operating since the end of 1997 for the oldest stations and will continue to be densified. Although they are still close to or within their associated uncertainties, preliminary results provide, for the first time, a direct estimate of crustal deformation across and within the western Alps.
A trilateration network was set up in early 1982 by a team of French and Mexican institutions across the central part of the Gulf of California in order to study the plate boundary related movements in this transition area between the San Andreas fault system and the East Pacific Rise. The reobservation of this network in March 1986 provides a first set of data on the present day deformations in this area. Both surveys used AGA8 Laser geodimeter measurements between 11 stations located on elevated points of Baja California and Sonora and on the islands between the peninsula and mainland coasts. Deformation patterns during the 1982‐1986 interval, obtained through three different methods indicate mainly a right lateral shear movement in the Gulf axis direction N46°W. Between Baja California Peninsula and Angel de la Guarda Island 17 ± 4 cm of dextral slip occurred. Between the coast of Sonora and the central islands of the Gulf the mean displacement amount to about 23 ± 12 cm. In the southwestern part of the network, weaker movements seem to have occurred, and are smaller than estimated errors. This may indicate either that the boundary is locked in this part or is deflected towards the ESE. These results, which give an estimation of the relative plate velocity of 8 ± 3 cm/a, are consistent with the generally accepted relative movement between North American and Pacific plates (about 6 cm/a).
A wide-angle laser ranging system has been developed for high-precision multilateration. A laser transmits short pulses in a widely diverging beam toward a network of cube-corner retroreflectors (CCRs). Returned echoes are detected, amplified, digitized, and timed. Impact of instrumental error sources, atmospheric refraction, and scintillation are discussed. The precision of the correlation method for the estimation of times of arrival and a least-squares adjustment method for the estimation of relative CCR coordinates is studied. Data from several ground-based experiments are analyzed. The positioning precision achieved is of 1-2.3 mm in relative radial coordinates and 1.8-3.8 cm in relative transverse coordinates. Results are consistent with a covariance analysis, demonstrating that, as follows: 1) systematic instrumental errors are well corrected; 2) outliers arising from coincident echoes are properly detected; 3) models for forward and inverse problems and estimation techniques are correct. A numerical simulation program based on these models and techniques can be used for optimizing instrumental aspects, providing extensive simulations in various conditions and processing data from future airborne experiments to which the system is devoted.
Geodetic trilateration/GPS measurements were acquired from experiments conducted in 1991 and 1993 in the central part of the Philippine Fault on Leyte island. Displacement vectors deduced from two least squares adjustments indicate a mean slip rate of 26±10 mm/yr at an azimuth of about N130°E. These values are consistent with the tectonic and kinematic data. Our results indicate that the displacements are: (1) almost parallel to the Philippine Fault, and (2) related to creep along the fault, because no moderate seismic events occurred between the two observation epochs.
The Asal Ghoubbet rift in the Republic of Djibouti is a subaerial continuation of the Gulf of Aden ridge system. Many geological and tectonic arguments indicate that it represents one of the most typical spreading segments of the world rift system. Geodetic measurements regularly conducted from 1979 to 1983 across this area show an accumulation of extension in the inner floor of the rift valley with a rate of about 60 mm/yr perpendicular to the normal faults that bound the rift. Periodic leveling surveys across major faults in the rift show vertical creep movements of about 5 to 10 mm/yr. Although the observed deformations could be contamined by postseismic relaxation after a rifting episode in November 1978, the regularity and the magnitude of these movements during the last two years argue for a steady state mechanism of deformation of the rift valley. The deformation is uniformly distributed in time and in space inside the inner floor of the rift. The necking of the lithosphere in the central part of the rift and the high temperature gradient imply a ductile rheology in the inner floor which is stretched in a steady state process.
abstract The resurvey of both a geodetic network and a leveling net was carried out in June 1981 8 months after the MS = 7.3 El Asnam earthquake of 10 October 1980. Previous seismological and neotectonic studies (Ouyed et al., 1981; Yielding et al., 1981) indicate that this event results from a northeast-trending overthrust complex fault of about 40-km length, which shows at least three principal segments with slightly different directions. Vertical movements, evaluated by means of a trigonometric leveling method, show an uplift of the northwestern side of the thrust fault of about 5 m and a depression of the southeastern edge of about 1 m. These movements are progressively attenuated away from the fault trace. Horizontal movements have been evaluated by a classical first-order triangulation method. The resulting mean strain tensors, calculated for different triangles of the geodetic network, indicate a shortening of about 2.50 m which is consistent with the SE-NW direction of compression determined from neotectonic evaluations and focal mechanisms. Dislocation models are used to explain and discuss the observed deformations in light of the seismological data and observed ground breakages. Five segments are required to explain both horizontal and vertical deformations. The magnitude of vertical displacements (about 6 m) at the junction between the southwest and the central segments of the fault argues for the breaking of this area during the main shock and for a slip vector of about 8 m, at least in the central segment.
