Response of a multi-domain continental margin to compression: study from seismic reflection-refraction and numerical modelling in the Tagus Abyssal Plain

The effects of the Miocene through Present compression in the Tagus Abyssal Plain are mapped using the most up to date available to scientific community multi-channel seismic reflection and refraction data. Correlation of the rift basin fault pattern with the deep crustal structure is presented along seismic line IAM-5. Four structural domains were recognized. In the oceanic realm mild deformation concentrates in Domain I adjacent to the Tore-Madeira Rise. Domain 2 is characterized by the absence of shortening structures, except near the ocean-continent transition (OCT), implying that Miocene deformation did not propagate into the Abyssal Plain, In Domain 3 we distinguish three sub-domains: Sub-domain 3A which coincides with the OCT, Sub-domain 3B which is a highly deformed adjacent continental segment, and Sub-domain 3C. The Miocene tectonic inversion is mainly accommodated in Domain 3 by oceanwards directed thrusting at the ocean-continent transition and continentwards on the continental slope. Domain 4 corresponds to the non-rifted continental margin where only minor extensional and shortening deformation structures are observed. Finite element numerical models address the response of the various domains to the Miocene compression, emphasizing the long-wavelength differential vertical movements and the role of possible rheologic contrasts. The concentration of the Miocene deformation in the transitional zone (TC), which is the addition of Sub-domain 3A and part of 3B, is a result of two main factors: (1) focusing of compression in an already stressed region due to plate curvature and sediment loading; and (2) theological weakening. We estimate that the frictional strength in the TC is reduced in 30% relative to the surrounding regions. A model of compressive deformation propagation by means of horizontal impingement of the middle continental crust rift wedge and horizontal shearing on serpentinized mantle in the oceanic realm is presented. This model is consistent with both the geological interpretation of seismic data and the results of numerical modelling. (C) 2008 Elsevier B.V. All rights reserved.Instituto Nacional de Engenharia, Tecnologia e Inovacao(INETI); Landmark Graphics Corporation; Landmark University Grant Program; LATTEX/IDL [ISLF-5-32]; FEDERinfo:eu-repo/semantics/publishedVersio

Neotectonics of the SW Iberia margin, Gulf of Cadiz and Alboran Sea: a reassessment including recent structural, seismic and geodetic data

We use a thin-shell approximation for the lithosphere to model the neotectonics of the Gulf of Cadiz, SW Iberia margin and the westernmost Mediterranean, in the eastern segment of the Azores-Gibraltar plate boundary. In relation to previous neotectonic models in the region, we utilize a better constrained structural map offshore, and the recent GPS measurements over NW Africa and Iberia have been taken into account, together with the seismic strain rate and stress data, to evaluate alternative geodynamic settings proposed for the region. We show that by assuming a relatively simple, two-plate tectonic framework, where Nubia and Eurasia converge NW-SE to WNW-ESE at a rate of 4.5-6 mm yr-1, the models correctly predict the amount of shortening and wrenching between northern Algeria-Morocco and southern Spain and between NW Morocco and SW Iberia, as estimated from both GPS data and geological constraints. The consistency between modelled and observed velocities in the vicinity of Gibraltar and NW Morocco indicates that forcing by slab sinking beneath Gibraltar is not required to reproduce current horizontal deformation in these areas. In the Gulf of Cadiz and SW Iberia, the modelling results support a diffuse Nubia-Eurasia Plate boundary, where the convergence is accommodated along NNE-SSW to NE-SW and ENE-WSW thrust faults and WNW-ESE right-lateral strike-slip faults, over an area >200 km wide, in good general agreement with the distribution of the seismic strain rate and associated faulting mechanisms. The modelling results are robust to regional uncertainties in the structure of the lithosphere and have important implications for the earthquake and tsunami hazard of Portugal, SW Spain and Morocco. We predict maximum, long-term average fault slip rates between 1-2 mm yr-1, that is, less than 50 per cent the average plate relative movement, suggesting very long return periods for high-magnitude (Mw > 8) earthquakes on individual structures.publishe

Geophysical detection of underground cavities

International audienceIn this paper, we present a synthesis of the geophysical investigations conducted on experimental sites selected in the Lorraine salt basin and Haute-Normandie marlpits. These investigations were carried out in the frame of a partnership (scientific and industrial) research program on cavities detection and characterization using techniques of high resolution seismic, microgravity and electric resistivity. The objective of the research is three folds: (1) to develop and optimize P and S seismic vibratory sources with an enhanced procedure of generation and optimization of sweeps (2) for each technique, to define the privileged application field and limitations regarding the general context of the considered site (3) to progress in the joint interpretations of the geophysical data. Despite the good quality of the data, the results evidence the complexity in the interpretation of the geophysical response in cavities environment due mainly to the compromise between the resolution and the ratio depth/dimension of the cavities, and the nature of the filling (brine, water, air). In the case of the marlpit mines, the corresponding geophysical anomalies could be correlated to the exact position of the marlpit known according to the archive records. Drilling campaign has confirmed a local collapse of the marlpit identified on the only High Resolution Seismic dataCe papier présente une synthèse des investigations géophysiques menées sur des sites expérimentaux du bassin salifère lorrain et les marnières de Haute-Normandie. Ces travaux ont été réalisés dans le cadre d'un programme de recherche partenarial axé sur la détection et la caractérisation des cavités à partir des techniques de Sismique Haute résolution (SHR), de la microgravimétrie et de la résistivité électrique. L'objectif du programme est triple : (1) développer et optimiser des sources vibratoires en ondes P et S avec une Procédure de Génération et d'Optimisation de Sweep adapté ; (2) définir pour chaque technique le champ d'application privilégié et les limitations en fonction du contexte général des sites considérés ; (3) progresser dans les interprétations conjointes des données géophysiques. Malgré la bonne qualité des mesures, des difficultés ont été rencontrées lors des interprétations de la réponse géophysique de l'environnement des cavités avec, pour raison essentielle le compromis entre la résolution de l'imagerie et le rapport profondeur/dimension de la cavité ainsi que la nature du remplissage (saumure, air, eau). Dans le cas des marnières, les anomalies géophysiques ont été carrelées à la position exacte de marnière pilote bien documentée dans les archives. Une campagne de forage a permis de confirmer un éboulement local de la marnière observée sur les données de sismique haute résolutio

Relevance of Seismic Risk Assessment in Abandoned Mining Districts : the Case of the Gardanne Coal Mine, Provence, France

International audienceMining shutdowns have increased significantly in last century, but seismic risk in post mining districts and consequent damage from ground shaking is still poorly understood. Large induced seismic events with M > 5 are known from active mining districts. Their origin is widely directly linked to stress perturbations related to mining activity. In post mining districts, especially when they are flooded, the bandwidth of potential seismic source origins is comparatively large and has been observed in the context of partial underground collapses, fluid induced redistribution of the environmental stresses, and reactivation of pre existing fault structures next to the mining district. The estimation of the associated seismic hazard is quite challenging, depending on many complexly interacting factors, such as the mine geometry and geological constitution, its long term alteration behaviour (modified by the presence of fluids), meteorological impacts and climate changes, triggering from regional or global natural earthquakes, and the presence of pre existing fault structures and tectonic stresses. Such challenges are today encountered in the case of the underground flooded, abandoned coal mine at Gardanne in the Provence region (in SE France). Local microseismic monitoring highlights the presence of significant periodic seismic swarming activity, including events of magnitudes close to 2 which have been several times felt by the nearby living population. Seismic analysis demonstrates that most of the events appear to be located below the excavated, flooded mine workings and seems to be spatially and temporally correlated with the flooding evolution, controlled by meteorological conditions and active pumping operations. Results from source mechanism analysis showed that swarming activity is probably related to rupture along a network of pre existing fault structures, which are favourably oriented with respect to the local tectonic stress field. Based on these observations, we suggest that some mine workings (especially room and pillar) act as a very efficient "anthropogenic" aquifer, whose water level fluctuations trigger reactivation of these faults, e.g. via a poroelastic effect or pore pressure increase. The nature of the detailed triggering mechanism remains however speculative and is part of currently ongoing investigations. This example shows the necessity and relevance of the understanding of this mechanism in order to reason if variations of the mine water level might potentially trigger larger local tectonic events, as recorded in the past (Mw 3.6, 1984), or simply episodes of swarm activity associated to small transient creep. In the light of these developments, seismic hazard analysis in post mining risk assessment, which is non standard today, could be assessed

Location of microseismic swarms induced by salt solution mining

International audienceGround failures, caving processes and collapses of large natural or man-made underground cavities can produce significant socio-economic damages and represent a serious risk envisaged by the mine managements and municipalities. In order to improve our understanding of the mechanisms governing such a geohazard and to test the potential of geophysical methods to prevent them, the development and collapse of a salt solution mining cavity was monitored in the Lorraine basin in northeastern France. During the experiment, a huge microseismic data set (similar to 50 000 event files) was recorded by a local microseismic network. 80 per cent of the data comprised unusual swarming sequences with complex clusters of superimposed microseismic events which could not be processed through standard automatic detection and location routines. Here, we present two probabilistic methods which provide a powerful tool to assess the spatio-temporal characteristics of these swarming sequences in an automatic manner. Both methods take advantage of strong attenuation effects and significantly polarized P-wave energies at higher frequencies (>100 Hz). The first location approach uses simple signal amplitude estimates for different frequency bands, and an attenuation model to constrain the hypocentre locations. The second approach was designed to identify significantly polarized P-wave energies and the associated polarization angles which provide very valuable information on the hypocentre location. Both methods are applied to a microseismic data set recorded during an important step of the development of the cavity, that is, before its collapse. From our results, systematic spatio-temporal epicentre migration trends are observed in the order of seconds to minutes and several tens of meters which are partially associated with cyclic behaviours. In addition, from spatio-temporal distribution of epicentre clusters we observed similar epicentre migration in the order of hours and days. All together, we suggest that the recorded microseismicity mainly represents detachment and block breakage processes acting at the cavity's roof, indicating a zone of critical state of stress and where partial fractures cause chain reaction failures as a result of stress redistribution processes

Automatic Detection and Location of Microseismic Events from Sparse Network and Its Application to Post-mining Monitoring

International audienceIn post-mining regions with seismic hazard, timely decision making for risk management faces the challenge of quick and reliable detection and location of seismic events. As a response to the increasing density of monitoring stations, generating large volumes of seismic data, automatic, full waveform-based methods have been developed in recent years in global seismology. Such methods often cannot be directly applied to post-mining monitoring with a limited station coverage, as it is the case when temporarily networks are installed as an emergency response. In this paper we propose a new methodology that bridges this gap and enables the application of a full waveform, backprojection based method (BackTrackBB) to data of sparse network. The methodology was successfully tested on an abandoned and flooded underground coalmine in South-eastern France. Steps preceding BackTrackBB application were implemented in order to remove the coherent noise that otherwise results in numerous false detections. First results indicate that seismic activity in the study area is controlled by water level variation within former room-and-pillar mine works and fault segments (re)activation below them

Resolving source mechanisms of microseismic swarms induced by solution mining

International audienceIn order to improve our understanding of hazardous underground cavities, the development and collapse of a ∼200 m wide salt solution mining cavity was seismically monitored in the Lorraine basin in northeastern France. The microseismic events show a swarm-like behaviour, with clustering sequences lasting from seconds to days, and distinct spatiotemporal migration. Observed microseismic signals are interpreted as the result of detachment and block breakage processes occurring at the cavity roof. Body wave amplitude patterns indicated the presence of relatively stable source mechanisms, either associated with dip-slip and/or tensile faulting. Signal overlaps during swarm activity due to short interevent times, the high-frequency geophone recordings and the limited network station coverage often limit the application of classical source analysis techniques. To overcome these shortcomings, we investigated the source mechanisms through different procedures including modelling of observed and synthetic waveforms and amplitude spectra of some well-located events, as well as modelling of peak-to-peak amplitude ratios for the majority of the detected events. We extended the latter approach to infer the average source mechanism of many swarming events at once, using multiple events recorded at a single three component station. This methodology is applied here for the first time and represents a useful tool for source studies of seismic swarms and seismicity clusters. The results obtained with different methods are consistent and indicate that the source mechanisms for at least 50 per cent of the microseismic events are remarkably stable, with a predominant thrust faulting regime with faults similarly oriented, striking NW–SE and dipping around 35°–55°. This dominance of consistent source mechanisms might be related to the presence of a preferential direction of pre-existing crack or fault structures. As an interesting byproduct, we demonstrate, for the first time directly on seismic data, that the source radiation pattern significantly controls the detection capability of a seismic station and network

The crustal structure of the NW-Moroccan Continental Margin for Wide-angle and Reflection Seismic Data

11 pages, 11 figures, 1 tableThe Atlantic margin off Morocco with its neighbouring Jurassic oceanic crust is one of the oldest on earth. It is conjugate to the Nova Scotia margin of North America. The SISMAR marine seismic survey acquired deep reflection seismic data as well as wide-angle seismic profiles in order to image the deep structure of the margin, characterize the nature of the crust in the transitional domain and define the geometry of the synrift basins. We present results from the combined interpretation of the reflection seismic, wide-angle seismic and gravity data along a 440-km-long profile perpendicular to the margin at 33-34°N, extending from nearly normal oceanic crust in the vicinity of Coral Patch seamount to the coast at El Jadida and approximately 130 km inland. The shallow structure is well imaged by the reflection seismic data and shows a thick sedimentary cover that is locally perturbed by salt tectonics and reverse faulting. The sedimentary basin thickens from 1.5 km on the normal oceanic crust to a maximum thickness of 6 km at the base of the continental slope. Multichannel seismic (MCS) data image basement structures including a few tilted fault blocks and a transition zone to a thin crust. A strong discontinuous reflection at 12 s two-way travel-time (TWT) is interpreted as the Moho discontinuity. As a result of the good data quality, the deep crustal structure (depth and velocity field) is well constrained through the wide-angle seismic modelling. The crust thins from 35 km underneath the continent to approximately 7 km at the western end of the profile. The transitional region has a width of 150 km. Crustal velocities are lowest at the continental slope, probably as a result of faulting and fracturing of the upper crust. Upper-mantle velocities could be well defined from the ocean bottom seismometer (OBS) and land station data throughout the modelWe gratefully acknowledge financial support support provided by Institut National des Sciences de l'Univers, Total, as well as the Portugese Grant MATESPRO PDCTM/P/MAR/15264/9