Alexis Rigo

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.

Crustal deformation near Hengill volcano, Iceland 1993–1998: Coupling between magmatic activity and faulting inferred from elastic modeling of satellite radar interferograms

Tectonic activity in the Hengill volcanic area in southwestern Iceland accelerated in July 1994, when an unusually persistent swarm of moderate-sized earthquakes began. Although the largest events were magnitude 5, the pattern of upward crustal deformation at 2 cm/yr indicates that most of the activity is related to inflation of a magma chamber at depth. To monitor this activity, we analyze synthetic aperture radar (SAR) images acquired by the ERS-I and ERS-2 satellites between July 1993 and September 1998 using interferometry. Interferograms composed of images acquired during the snow-free summer months remain coherent on Holocene lava flows, even after 4 years. Some of the interferograms show a discontinuity in the fringe pattern, which we interpret as 8 mm of (aseismic) dip slip on a 3-km-long segment of a N5° striking normal fault, part of which had been mapped previously This slip must have occurred between July 31 and September 3, 1995 (inclusive), and has been confirmed by observations in the field. The predominant signature in all the interferograms spanning at least 1 year, however, is a concentric fringe pattern centered just south of the Hromundartindur volcanic center. This we interpret as mostly vertical uplift caused by increasing pressure in an underlying magma source. The volume source that best fits the observed interferograms lies at 7±1 km depth and remains in the same horizontal position to within 2 km. It produces 19±2 mm/yr of uplift. This deformation accumulates as elastic strain energy at a rate 2.8 times the rate of seismic moment release. Accumulated over 5 years, it increases the Coulomb failure stress by >0.6 bar in an area that includes some 84% of the earthquakes recorded between 1993 and 1998. Under our interpretation, magma is injected at 7 km depth, just below the seismogenic zone formed by colder, brittle rock. There the inflation induces stresses that exceed the Coulomb failure criterion, triggering earthquakes, possibly in a cyclical fashion.

Soil gas profiles as a tool to characterise active tectonic areas: the Jaut Pass example (Pyrenees, France)

A new method to investigate active tectonic structures, using soil gas composition at faults, suggests relevant information about regional stress conditions which can be obtained rapidly and at relatively low cost. In 1995, we carried out geochemical profiles around the epicentre of the M=5.1 1980 earthquake near Arudy in the French Pyrenees, where the presence of minor fractures is evident in the field, and confirmed using satellite SPOT imagery. Fractures are conduits facilitating fluid migrations in the crust, and are also pathways for the release of deep-seated gases to the atmosphere. In order to investigate the implication of these fractures in the present deformation, i.e. if they are connected to the Hercynian substratum at a depth of about 1800 m, soil gases were measured along four traverses crossing the observed structures. Gases determined were 222Rn, CO2 and 4He, each of them for their characteristic source: 222Rn has essentially a shallow origin due to its short half-life, whereas CO2 is the major soil gas component with a mainly surficial biogenic source. However, there could additionally be CO2 from crustal or mantle degassing, which would also be the principal sources of He. Data analysis clearly reveals anomalous values for each gas at specific positions along the traverses. Two sets of fractures corresponding to different observed trends are distinguished: the one characterised by He anomalies accompanied by other gases, and the second with few identified He anomalies. The agreement between the geochemical data and the field observations leads us to propose a deformation model for the area studied, analogous to a pull-apart system located in a right lateral shear zone.

Erosion-induced isostatic rebound triggers extension in low convergent mountain ranges

Mechanisms that control seismic activity in low strain rate areas such as western Europe remain poorly understood. For example, in spite of low shortening rates of <0.5 mm/ yr, the Western Alps and the Pyrenees are underlain by moderate but frequent seismicity detectable by instruments. Beneath the elevated part of these mountain ranges, analysis of earthquake focal mechanisms indicates extension, which is commonly interpreted as the result of gravitational collapse. Here we show that erosional processes are the predominant control on present-day deformation and seismicity. We demonstrate, using fi nite element modeling, that erosion induces extension and rock uplift of the elevated region of mountain ranges accommodating relatively low overall convergence. Our results suggest that an erosion rate of ~1 mm/yr can lead to extension in mountain ranges accommodating signifi cant shortening of <3 mm/yr. Based on this study, the seismotectonic framework and seismic hazard assessment for low strain rate areas need to be revisited, because erosion-related earthquakes could increase seismic hazard.

The February 1996 earthquake sequence in the eastern Pyrenees: first results

An earthquake with local magnitude (ML) 5.2 occurred February 18, 1996 in the eastern Pyrenees (France) near the town of Saint-Paul de Fenouillet. This event is the first of this magnitude in France to be well recorded instrumentally. Less than 24 hours after the main shock, we installed a temporary network of 30 seismological stations in the epicentral area to record the aftershock sequence. In this paper, we analyse the main shock and present the 37 largest aftershocks (1.8 ≤ Ml ≤ 3.4) in the two months following the main shock. These events are located using data from the permanent Pyrenean seismological network and the temporary network when available. We also determined eight fault plane solutions using the P-wave first motions. The main shock and the aftershocks are located inside the small Agly massif. This Hercynian structure sits some 8 km north of the North Pyrenean Fault, which is usually considered to be the suture between the Iberian and Eurasian plates. The mechanism of the main shock is a left-lateral strike-slip on an E–W trending fault. The fault plane solutions of the aftershocks are mostly E–W striking reverse faults, in agreement with the general north-south shortening of the Pyrenees. The aftershocks located down to 11 km depth, indicating that the Agly massif is deeply fractured. The main interpretations of these results are: (i) The main shock involved an E–W trending fault inside the highly fractured Agly massif, relaying the North Pyrenean Fault which had, at least in the last 35 years, a poor seismic activity along this segment; (ii) The Saint-Paul de Fenouillet syncline to the north and the North Pyrenean Fault to the south delimit a ∼15 km wide senestral shear zone. Such a structure is also suggested by the highly fractured pattern of the Agly massif and by small en echelon faults and secondary folds in the Saint-Paul de Fenouillet syncline; (iii) we suggest that the North Pyrenean Frontal Thrust, located less than 10 km north of the Agly massif, has a ramp geometry at depth below the Agly massif.

Earthquake-related elemental and isotopic lead anomaly in a springwater

Abstract Elemental and isotopic analyses of lead present at ultra-trace levels were obtained with a double focusing inductively coupled plasma mass spectrometer on a Pyrenean springwater. The sampling was carried out across a time series of 598 days encompassing an earthquake of magnitude 5.2. Lead concentrations show a tenfold increase and isotopic compositions shift toward anthropogenic values four days prior to the quake. The time lag between these geochemical anomalies and the earthquake, combined with hydrogeological constraints, are then used to infer where and when strain changes during earthquake preparation processes went beyond the threshold required to temporarily mix waters from normally independent aquifers. The occurrence of pre-seismic centrifugal strain waves propagating from the epicenter area is proposed on the basis of the observed geochemical anomalies.

Investigating the 1996 Pyrenean Earthquake (France) with SAR interferograms heavily distorted by atmosphere

We form interferograms by pairwise combination of four synthetic aperture radar (SAR) images acquired by the ERS-1/2 satellites before and after the ML 5.2, earthquake of February 18, 1996 near Saint-Paul de Fenouillet in the eastern Pyrenees, France. Of the four interferograms we interpret, two span the seismic event. The interferometric fringes, which correspond to contours of equal satellite-to-ground line length change (28.4 mm), are distorted by atmosphere in such a way that the amplitude of the coseismic signature is much smaller than that of the atmospheric disturbances. These noisy conditions contrast with previous earthquake studies with radar interferometry. Nevertheless, we are able to obtain a realistic elastic dislocation model fitting the observations. By comparisons with forward modeling, our results can discriminate between competing models that are not resolved by previous seismological studies. The data derived from radar are especially useful for a more accurate characterization of the vertical motion.

Effets topographiques sur les comparaisons de profils de nivellement: Cas français de Saint-Paul-de-Fenouillet (Pyrénées-Orientales) et d'Arudy (Pyrénées-Atlantiques)

Differences between repeated levelling surveys measure vertical movements. We show that these comparisons are correlated with the topography of the measured lines. This correlation constitutes an important additional source of error in levelling. We analyse levelling lines in two regions in the French Pyrenees where earthquakes occurred recently (Saint-Paul-de-Fenouillet and Arudy) and correct the levelling comparisons using the correlation between the topographic slope and the measured tilt. This correction reduces the vertical signature by a factor of 2. This demonstrates that it is absolutely necessary to take into account the topographic effect in analysing and interpreting levelling comparisons.