Jean-Claude Ruegg

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.

Propagation of rifting along the Arabia-Somalia Plate Boundary: Into Afar

It is generally accepted that the Aden ridge has propagated westward from ∼58°E to the western tip of the Gulf of Aden/Tadjoura, at the edge of Afar. Here, we use new tectonic and geochronological data to examine the geometry and kinematics of deformation related to the penetration of that ridge on dry land in the Republic of Djibouti. We show that it veers northward, forming a narrow zone of dense faulting along the northeastern edge of the Afar depression. The zone includes two volcanic rifts (Asal-Ghoubbet and Manda Inakir), connected to one another and to the submarine part of the ridge by transfer zones. Both rifts are composite, divided into two or three disconnected, parallel, NW-SE striking subrifts, all of which appear to have propagated northwestward. In Asal-Ghoubbet as in Manda Inakir, the subrifts appear to have formed in succession, through north directed jumps from subrifts more farther south. At present, the northernmost subrifts (Manda and Dirko Koma) of the Manda Inakir rift, form the current tip of the northward propagating Arabia-Somalia plate boundary in Afar. We account for most observations by a mechanical model similar to that previously inferred for the Gulf of Aden, in which propagation is governed by the intensity and direction of the minimum horizontal principal stress, σ3. We interpret the northward propagation on land, almost orthogonal to that in the gulf, to be related to necking of the Central Afar lithosphere where it is thinnest. Such necking may be a consequence of differential magmatic thickening, greater in the center of the Afar depression where the Ethiopian hot spot enhanced profuse basaltic effusion and underplating than along the edges of the depression. The model explains why the Aden ridge foregoes its WSW propagation direction, constant from ∼58°E to Asal-Ghoubbet. At a smaller scale, individual rifts and subrifts keep opening perpendicular to the Arabia-Somalia (or Danakil-Somalia) motion vector and propagate northwestward. Concurrently, such lithospheric cracks are forced to jump northward, such that the plate boundary remains inside the regional N-S necking zone. Changes of obliquity between the directions of overall and local propagation may account for different segmentation patterns, a small angle promoting long, en echelon subrifts, and a high-angle, smaller, nested, “subrifts within subrifts.” The propagation mechanism is thus similar, whether in oceanic or continental lithosphere, the principal change being the overall propagation path, here governed by thickness changes rather than by the geometry in map view as previously inferred for the rest of the Aden ridge. Finally, because the same mechanism has led rifting along the Red Sea to propagate southward and jump to the western edge of Afar, the Arabia-Somalia and Arabia-Nubia plate boundaries tips have missed each other and keep overlapping further, leading to strain transfer by large-scale bookshelf faulting.

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.

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.

Geodetic displacements and aftershocks following the 2001 Mw = 8.4 Peru earthquake: Implications for the mechanics of the earthquake cycle along subduction zones

We analyzed aftershocks and postseismic deformation recorded by the continuous GPS station AREQ following the M_w = 8.4, 23 June 2001 Peru earthquake. This station moved by 50 cm trenchward, in a N235°E direction during the coseismic phase, and continued to move in the same direction for an additional 15 cm over the next 2 years. We compare observations with the prediction of a simple one-dimensional (1-D) system of springs, sliders, and dashpot loaded by a constant force, meant to simulate stress transfer during the seismic cycle. The model incorporates a seismogenic fault zone, obeying rate-weakening friction, a zone of deep afterslip, the brittle creep fault zone (BCFZ) obeying rate-strengthening friction, and a zone of viscous flow at depth, the ductile fault zone (DFZ). This simple model captures the main features of the temporal evolution of seismicity and deformation. Our results imply that crustal strain associated with stress accumulation during the interseismic period is probably not stationary over most of the interseismic period. The BCFZ appears to control the early postseismic response (afterslip and aftershocks), although an immediate increase, by a factor of about 1.77, of ductile shear rate is required, placing constraints on the effective viscosity of the DFZ. Following a large subduction earthquake, displacement of inland sites is trenchward in the early phase of the seismic cycle and reverse to landward after a time t i for which an analytical expression is given. This study adds support to the view that the decay rate of aftershocks may be controlled by reloading due to deep afterslip. Given the ratio of preseismic to postseismic viscous creep, we deduce that frictional stresses along the subduction interface account for probably 70% of the force transmitted along the plate interface.

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.

Geodetic investigation of the 13 May 1995 Kozani‐Grevena (Greece) Earthquake

The Ms=6.6 13 May 1995 Kozani-Grevena earthquake struck a region of low historical seismic activity which includes a 10-year-old triangulation network in northern Greece. After the earthquake, monuments from this network were occupied with GPS to measure co-seismic displacements. Inversion of the co-seismic displacement field to yield a source mechanism is achieved by use of a hybrid simplex-Monte-Carlo method which requires no a priori constraints. The model focal mechanism agrees well with the global CMT solution and locally observed aftershocks, but implies a significantly higher scalar moment than do seismological or SAR interferometry studies, and has a longer fault length than the model based on SAR interferometry.

Geodetic evidence for rifting in Afar a brittle-elastic model of the behaviour of the lithosphere

Abstract An important episode of rifting occurred in November 1978 in southwest Afar, in the first subaerial section of the accreting plate boundary between the African and Arabian plates. Horizontal rifting of more than 2 m took place, with vertical displacements of about 1 m, earthquakes of magnitude up to 5.3, and a fissural volcanic eruption of basaltic lavas. Very precise geodetic measurements were carried out in order to study this crisis and strains of the order of 3 × 10−4 were measured, both tensile and compressive. This paper presents an analysis of the mechanical behaviour of the lithosphere. It is shown that an elastic-brittle model with a rebound mechanism fits very well the data, and it is suggested that such a model, with magma injection in the resulting open fissures, should be used to describe accretion at plate boundaries.

Geodetic evidence for rifting in afar, 2. Vertical displacements

Abstract In a previous paper we have proposed a two-dimensional elastic model which accounts for the horizontal displacements measured for the rifting episode which occurred in November 1978 in Afar. In this paper we demonstrate that the same model accounts for the measured vertical displacements as well as for the horizontal ones when the thickness of the lithosphere is introduced in the model as a new parameter. After comparison with similar data obtained in recent years in Iceland, we propose a global interpretation for the rifting phenomenon.

Seismic activity triggered by stress changes after the 1978 events in the Asal Rift, Djibouti

Following a sequence of earthquakes in the Asal Rift in November 1978, the deformation resulting from dyke emplacement and associated faulting was measured geodetically and using teleseismic data. Using this information we have computed the resulting Coulomb stress changes, which can explain the features of the distribution of seismicity for the following six weeks. A further major shock then occurred. When this event is included in the modelling, the distribution of all the seismicity within a radius of 40 km in the following 4 years is predicted. The study confirms that increases of Coulomb stress as small as 0.3 bars are sufficient to trigger earthquakes and shows that dykes as well as fault slip can result in stress changes that influence the seismicity distribution over a broad region.