Pierre Briole

Active deformation of the Corinth rift, Greece : Results from repeated Global Positioning System surveys between 1990 and 1995

Between 1990 and 1995, we carried out seven Global Positioning System (GPS) campaigns in the Corinth rift area in order to constrain the spatial and temporal crustal deformation of this active zone. The network, 193 points over ∼10,000 km2, samples most of the active faults. In order to estimate the deformation over a longer period, 159 of those points are also Greek triangulation pillars previously measured between 1966 and 1972. Two earthquakes of magnitude 6.2 and 5.9 have occurred in the network since it was installed. The extension rate deduced from the analysis of the different GPS data sets is 14±2 mm/yr oriented N9° in the west, 13±3 mm/yr oriented S-N in the center, and 10±4 mm/yr oriented N19°W in the east of the gulf. The comparison between GPS and triangulation gives higher rates and less angular divergence (25±7 mm/yr, N4°E; 22±7 mm/yr, S-N; 20±7 mm/yr, N15°W, respectively). Both sets of data indicate that the deforming zone is very narrow (10–15 km) in the west, might be wider in the center (15–20 km), and is more diffuse in the east. The analysis of the displacements observed after the Ms = 6.2, June 15, 1995, and the Ms = S.9, November 18, 1992, earthquakes, both located in the west of the gulf, together with seismological and tectonic observations shows that these two earthquakes occurred on low-angle (≤35°) north dipping normal faults located between 4.5 and 10 km depth in the inner part of the rift. Assuming that the deformation is concentrated in relatively narrow deforming zones, we use a simple model of a dislocation in an elastic half-space to study the implication of the localization. Using the geometry of the known seismogenic faults, our observations imply continuous aseismic deformation in the uppermost crust of the inner rift. This model predicts geodetic strain rates close to seismic strain rates in opposition to previous estimates. This is because our model takes into account the activity on low-angle normal faults in the inner rift and an effective seismogenic layer of 6–7 km, about half that usually assumed.

The Ms = 6.2, June 15, 1995 Aigion earthquake (Greece): evidence for low angle normal faulting in the Corinth rift

We present the results of a multidisciplinary study of the Ms = 6.2, 1995, June 15, Aigion earthquake (Gulf of Corinth, Greece). In order to constrain the rupture geometry, we used all available data from seismology (local, regional and teleseismic records of the mainshock and of aftershocks), geodesy (GPS and SAR interferometry), and tectonics. Part of these data were obtained during a postseismic field study consisting of the surveying of 24 GPS points, the temporary installation of 20 digital seismometers, and a detailed field investigation for surface fault break. The Aigion fault was the only fault onland which showed detectable breaks (< 4 cm). We relocated the mainshock hypocenter at 10 km in depth, 38 ° 21.7 ′ N, 22 ° 12.0 ′ E, about 15 km NNE to the damaged city of Aigion. The modeling of teleseismic P and SH waves provides a seismic moment Mo = 3.4 1018 N.m, a well constrained focal mechanism (strike 277 °, dip 33 °, rake − 77°), at a centroidal depth of 7.2 km, consistent with the NEIC and the revised Harvard determinations. It thus involved almost pure normal faulting in agreement with the tectonics of the Gulf. The horizontal GPS displacements corrected for the opening of the gulf (1.5 cm/year) show a well-resolved 7 cm northward motion above the hypocenter, which eliminates the possibility of a steep, south-dipping fault plane. Fitting the S-wave polarization at SERG, 10 km from the epicenter, with a 33° northward dipping plane implies a hypocentral depth greater than 10 km. The north dipping fault plane provides a poor fit to the GPS data at the southern points when a homogeneous elastic half-space is considered: the best fit geodetic model is obtained for a fault shallower by 2 km, assuming the same dip. We show with a two-dimensional model that this depth difference is probably due to the distorting effect of the shallow, low-rigidity sediments of the gulf and of its edges. The best-fit fault model, with dimensions 9 km E–W and 15 km along dip, and a 0.87 m uniform slip, fits InSAR data covering the time of the earthquake. The fault is located about 10 km east-northeast to the Aigion fault, whose surface breaks thus appears as secondary features. The rupture lasted 4 to 5 s, propagating southward and upward on a fault probably outcropping offshore, near the southern edge of the gulf. In the shallowest 4 km, the slip – if any – has not exceeded about 30 cm. This geometry implies a large directivity effect in Aigion, in agreement with the accelerogram aig which shows a short duration (2 s) and a large amplitude (0.5 g) of the direct S acceleration. This unusual low-angle normal faulting may have been favoured by a low-friction, high pore pressure fault zone, or by a rotation of the stress directions due to the possible dip towards the south of the brittle-ductile transition zone. This fault cannot be responsible for the long term topography of the rift, which is controlled by larger normal faults with larger dip angles, implying either a seldom, or a more recently started activity of such low angle faults in the central part of the rift.

Active spreading and regional extension at Mount Etna imaged by SAR interferometry

Abstract Large-flank instability has been proposed by most recent authors as a major process in the dynamics of Mount Etna. However, several aspects of the models are still disputed, such as the boundaries of the moving area and the driving force for the instability. In this paper we present synthetic aperture radar interferometric data which allow us to identify two main sectors of active instability where the deformational process differs. An eastern sector is bounded to the north by the Pernicana–Provenzana fault, to the west by the North Rift Zone and the South Rift Zone and to the south by the Mascalucia–Tremestieri–Trecastagni fault system. Interferograms provide new data on the activity and extent of the southern boundary, which was previously underestimated. In this sector the structural features indicate an eastward sliding, driven by E–W regional extension, which can be interpreted as a result of the retreating slab of the Ionian subduction. A second collapsing sector exists along the southern flank of Mount Etna, where the interferograms show the presence of an active anticlinal ridge. This ridge is interpreted as the result of southward gravity spreading over a basal decollement between Etna and its Plio–Quaternary basement and the Hyblean platform. The two sectors first became active in summer 1996, after the beginning of a new cycle of eruptive activity at the summit. The activity was ongoing until January 1998, with the deformation rate ranging from 4–6 mm/yr for the Mascalucia–Tremestieri–Trecastagni faults to 12 mm/yr for the anticlinal ridge.

Tropospheric corrections of SAR interferograms with strong topography. Application to Etna

The accuracy of spaceborne geodetic techniques, including SAR interferometry, is limited by the time and spatial variation and altitude dependance of the propagation delay of electomagnetic waves in the lower troposphere, particularly in mountainous areas. In this paper, we use a 1D model developed for tropospheric corrections of GPS and DORIS measurements to correct SAR data. The differential tropospheric delay is computed at each pixel of the interferogram from ground temperature, humidity and pressure using two empirical parameters calibrated from several radio-soundings acquired in various latitude and climate conditions. It is shown that with such a model, given the 3300 meters topography of Etna, tropospheric variations can generate up to 4π phase rotations between the top and the bottom of the volcano. In 16 out of the 20 interferograms processed with images acquired between August 1992 and October 1993, correction of the tropospheric effect reduces the number of fringes associated with the 1991–93 eruption from previous estimates. The remaining deformation is consistant with a deforming source located at a depth of 14±1 km. During the second half of the eruption, the subsidence rate at the top of the volcano is roughly stable at 13±3 mm/month. These values are in good agreement with tiltmeter data collected on Etna during the same period and with the estimated volume of erupted material. No significant deformation can be observed during the last month of eruption. Inflation of the volcano seems to resume immediately after the end of the eruption at a rate of 3 mm/month.

Volcano‐wide fringes in ERS synthetic aperture radar interferograms of Etna (1992–1998): Deformation or tropospheric effect?

Mount Etna (3300 m) is the volcano that has been first and most studied by differential synthetic aperture radar. Previous papers gave evidence for a large-scale deformation of the entire edifice consistent with unrest episodes but with a poor fit with classical elastic models. Also, atmospheric effects on mountainous areas are known to be very significant. Accordingly, interferograms may reflect both deformation and tropospheric effects. We investigate here the possibility of evaluating and correcting tropospheric effects directly from the interferograms. From 38 ERS synthetic aperture radar ascending scenes taken from 1992 to 1999, we have computed 238 interferograms. The amount of data allows us to analyze how the coherence is maintained over long periods of time and thus how it reveals the location of permanent scatters. From a simple analysis of phase-elevation relation, we give evidence for the clear difference in pattern between stratified tropospheric effects and large-scale deformation produced by a source in a Three-dimensional elastic body with topography. Using combination of inversion processes, we analyze both deformation source and tropospheric effects for the complete set of 238 interferograms. Finally time evolution of the parameters and modeling probability estimation are retrieved for each SAR image. Estimated relative change in tropospheric delay varies in time from -2.7 to +3.0 (± 1.2) fringes. They are consistent with other tropospheric estimations at Etna based on ground met and Global Positioning System observations on the same period of time. Magmatic volume variations remain below error bars and thus cannot be estimated properly. Finally, some of the interferograms may be entirely explained by the difference in tropospheric conditions between the two acquisition periods, without the need to invoking a deformation of the volcano.

The MW=8.1 Antofagasta (North Chile) Earthquake of July 30, 1995: First results from teleseismic and geodetic data

A strong (Mw = 8.1) subduction earthquake occurred on July 30, 1995 in Antofagasta (northern Chile). This is one of the largest events during this century in the region. It ruptured the southernmost portion of a seismic gap between 18°S and 25°S. In 1992 we had used GPS to survey a network with about 50 benchmarks covering a region nearly 500 km long (N-S) and 200 km wide (E-W). Part of these marks were re-surveyed with GPS after the 1995 earthquake. Comparison with 1992 positions indicate relative horizontal displacement towards the trench reaching 0.7 m. The inland subsided several decimeters. The Mejillones Peninsula was uplifted by more than 15 cm. Teleseismic body-wave modelling of VBB records gives a subduction focal mechanism and source time function with three distinct episodes of moment release and southward directivity. Modelling the displacement field using a dislocation with uniform slip in elastic half-space suggests a rupture zone with 19°–24° eastward dip extending to a depth no greater than 50 km with N-S length of 180 km and an average slip of about 5 m. The component of right-lateral slip inferred both from the teleseismic and geodetic data does not require slip partitioning at the plate boundary. That the well-constrained northern end of the 1995 rupture zone lies under the southern part of the Mejillones Peninsula increases the probability for a next rupture in the gap north of it.

Post‐eruptive deformation associated with the 1986–87 and 1989 lava flows of Etna detected by radar interferometry

We analysed 92 interferograms produced using ERS1 SAR images taken on Etna between May 1992 and October 1993. Nineteen show a local subsidence in the eastern flank of the volcano, correlated with the location of the 30 October 1986 – 1 March 1987 and 27 September – 9 October 1989 lava flows. Using fringe unwrapping and data gridding techniques, and assuming that over the sampled time-window, deformation was a linear function of time, we derive a map of along range rate of motion. The correlation between the deformation field and the area covered by recent lavas suggests that compaction of the lava flows continues several years after the eruptions. The area of maximum subsidence (47 mm/yr.) is localised at the narrowing of the 1989 flow, between 1500 and 1700 m a.s.l‥ We observe that subsidence extends outside the lava flows, accounting for at least 12 mm/yr. Assuming a relaxation process of the substrate in response to loading produced by recent lavas, a simple 1D Maxwell visco-elastic model predicts a maximum subsidence rate of 25 mm/yr and a relaxation time of about 3.5 years. The relaxation time agrees with those derived from post-eruptive displacements observed by levelling on Etna and Piton de la Fournaise after several eruptions. We conclude that at the time of our measurements, 25 to 50% of the deformation was related to relaxation of the substrate and the other part due to compaction of the lava flows.

Contemporary, Holocene, and Quaternary Deformation of the Asal Rift, Djibouti: Implications for the Mechanics of Slow Spreading Ridges

Because the frequency and character of rifting events along mid-ocean ridges are largely unknown, how the repetition of such events gives rise to rift structures is unexplored. The Asal rift in the Afar depression of Djibouti, Africa, provides the worlds best subaerial analogue for young slow spreading mid-ocean ridges. Seismic, geodetic, and field observations of a seismovolcanic event in 1978 at Asal yield estimates of the fault and dike locations, geometry, displacement, and volume of basalt extruded in a rifting event. A 6–9 kyr-old lake shore highstand at Asal has been warped downward by 70 m, providing a Holocene measure of the vertical deformation across the rift. The rift topography furnishes an older datum, which we infer to be 34±6 kyr old using the Holocene deformation rate. We find that faults throughout the rift valley are active; Holocene slip rates diminish beyond 4 km from the rift axis; late Quaternary rates decrease beyond 6–7 km. The Holocene slip rates are used to estimate repeat times by taking the displacement on the faults which slipped in 1978 as characteristic; we find tectonic events on individual faults recur every 200– 300 years. Half the rift faults slipped together in the 1978 event. If this is typical, then groups of faults are activated every 100–150 years. We suggest that half the events take place in the rift axis accompanied by volcanic extrusion; the remainder occur peripheral to the neovolcanic zone and involve fault slip only, both events having a repeat time of 200–300 years. Given the 10 km width of the rift and its 16 mm yr−1 spreading rate, the mean age of the material in the rift should be ∼350 kyr, an order of magnitude older than the inferred age of the formation of the rift topography. The subsidence rate of the rift axis during the past 35 kyr is 8–9 mm yr−1, with the rate of infilling by volcanic extrusion <1 mm yr−1. The resulting net subsidence rate, about equal to the half-spreading rate of the rift, could not be sustained for 300 kyr without significant infilling by lavas. Thus both observations suggest that the long-term vertical deformation in the rift has not been steady state. Instead, we suggest that there is a rifting/filling cycle at Asal, with the most recent filling episode ending ∼35 kyr.

The September 26, 1997 Colfiorito, Italy, earthquakes: Modeled coseismic surface displacement from SAR interferometry and GPS

The largest events of the 1997 Umbria-Marche seismic sequence were the two September 26 earthquakes of Mw = 5.7 (00:33 GMT) and Mw = 6.0 (09:40 GMT), which caused severe damage and ground cracks in a wide area around the epicenters. We created an ERS-SAR differential interferogram, where nine fringes are visible in and around the Colfiorito basin, corresponding to 25 cm of coseismic surface displacement. GPS data show a maximum horizontal displacement of (14±1.8) cm and a maximum subsidence of (24±3) cm. We used these geodetic data and the seismological parameters to estimate geometry and slip distribution on the fault planes. Modeled fault depths and maximum slip amplitudes are 6.5 km and 47 cm for the first event and 7 km and 72 cm for the second one, in good agreement with those derived from the seismological data.

The 1995 Grevena (northern Greece) Earthquake: Fault model constrained with tectonic observations and SAR interferometry

After the 1995 Grevena Ms=6.6 event in northern Greece, we mapped the earthquake fault break in detail. The surface break is small (8–12 km long, 4 cm slip) compared to the moment release of the event. However, the morphologic and tectonic study of the active faults, in the field and using the SPOT satellite imagery, suggests that the earthquake ruptured part of a much larger fault system including interconnecting segments. We used SAR interferometry of the satellite ERS-1 imagery to characterize the coseismic displacement field. This shows a kidney-shaped zone of subsidence reaching 30 cm flanked by an uplift zone reaching 5 cm. We reproduce this field using dislocations in an elastic half-space and our observations of the fault system. This requires 1 m slip from 4 to 15 km depth on a main normal fault segment dipping NNW. Our preliminary model includes significant NE-dipping scissors faulting at the eastern end of the rupture, clearly seen in the interferograms.