No abstract

Teleseismic shear-wave splitting and magnetotelluric experiments across the Grenville front, between the Archean craton and the Proterozoic Grenville province in the regions of the Pontiac subprovince and northwestern Grenville province (Canada), show a consistent obliquity between the polarization direction of the fast split shear wave () and the most electrically conductive direction (MT) in the upper mantle transcurrent shear zones. At all wellrecorded stations, is nearly N103؇E, and MT is approximately N80؇E. The obliquity may be considered a potential kinematic indicator, because the seismic and electrical anisotropies are thought to be controlled by lattice-preferred and shape-preferred orientations of mantle minerals (mainly olivine), respectively. The dextral movement sense of the transcurrent shear zones in the mantle, inferred from the observed obliquity, is consistent with that inferred from surface geology of the crustal shear zones. This consistency implies that deformation of the crust and the subcrustal upper mantle in the lithosphere was largely coherent in the study region.

S U M M A R YIn the present paper, we present a generalization of the wavelet transform, known as chirplet transform, specially designed to quantify the morphological attributes of individual seismic sections (packets) constituting the seismic waveforms. The proposed transform relies on an atomic decomposition of individual seismograms based on local multiscale chirps (swept frequency wave packets) of various shape and duration. We developed an algorithm that provides an optimal representation of the waveform packets in terms of (i) arrival time, (ii) central frequency, (iii) modulus, (iv) phase, (v) duration, (vi) envelope shape and (vii) frequency modulation compacting the information contained in each seismogram into a reduced set of parameters particularly well suited to describe seismic waveforms. In the present work, we focus on the ability of atomic decomposition to classify seismic events. We illustrate the developed methodology and resulting hierarchical classification scheme (agglomerative clustering displayed as a dendrogram) to seismograms of the induced seismicity recorded in the Lacq gas field between 1989 and 1997 by a local seismic network. For the present case-study, the resulting classification reveals different levels of similarity between seismic events of a same swarm. Accurate analysis of the subsequences of seismic events associated to an injection well shows temporal changes in the morphological attributes of the recorded seismic waveforms. These changes are highly correlated with water over-pressure records of the reservoir demonstrating the capability of the method to guide investigation of the underlying processes (properties of propagation media, source, rupture processes), and in a general manner the physical properties of the reservoir. Two major difficulties in earthquake studies are the lack of (1) controlled direct and near-field observations (essential for the validation of models and concepts) and (2) signal processing tools and analysis methods closely connected to the physics of wave propagation in a heterogeneous and dispersive medium (scattering). In this way, seismologists attempt to bridge the gap between laboratory experiments and tectonic earthquakes in the crust by investigating intermediate-scale systems where the cause of seismicity is more or less understood, as it is controlled by anthropogenic activities or by visible volcano activities (Ruiz et al. 1998). Providing that seismologists are able to relate the cause of seismicity to the observed seismic activity, systems associated to subsidence caused by fossil fuel extraction, local pressure changes over large water reservoirs, alteration of the local stress field of significant volume of rock around mining area, extraction of geothermal energy, or volcano activity can be seen as natural laboratories useful for investigating seismogenic processes.The physics of earthquakes (i.e. including nucleation and rupture initiation, rupture propagation) is complex and its understanding requires to efficiently monitor small pe...

Abstract:In the past decade, the Abitibi-Grenville Lithoprobe transect has been the site of numerous geological and geophysical surveys oriented towards understanding the lithospheric evolution of the southeastern Superior and adjoining Grenville provinces. Among the different geophysical methods that have been employed, earthquake seismology provides the widest range of information on the deep structures of the upper mantle. This paper presents a review of studies, both complete and ongoing, involving teleseismic datasets that were collected in 1994 and 1996 along the transect. A complete shear-wave splitting analysis has been performed on the 1994 dataset as part of a comparative study on electrical and seismic anisotropies. Results suggest a correlation between the two anisotropies (supported by xenolith data) and favour a lithospheric origin for the seismic anisotropy. The two anisotropies are believed to represent the fossilized remnants of Archean strain fields in the lithospheric roots of the Canadian Shield. Preliminary splitting results for the 1996 experiment suggest that the S-wave azimuthal anisotropy may be depth dependent and laterally varying. Ongoing receiver function analysis and traveltime inversion studies provide velocity models of the crust and upper mantle beneath the study area. Preliminary receiver function results reveal the presence of an S-velocity increase at -90-100 km depth which appears to be laterally continuous over 200 km. Traveltime inversion models indicate the presence of an elongate, low-velocity anomaly beneath the southern portion of the 1996 array which strikes obliquely to major geological structures at the surface (e.g., Grenville Front). Preliminary interpretation relates this anomaly to the same process (e.g., fixed mantle plume, continental rifting) responsible for the emplacement of the Monteregian Hills igneous province.Résumé : Au cours de la dernière décennie, de nombreuses campagnes géologiques et géophysiques ont été effectuées le long de la traverse Abitibi-Grenville du projet Lithoprobe, dans le but de comprendre l'évolution lithosphérique de la partie sud-est de la Province du Supérieur et de la Province de Grenville. Parmi les différentes méthodes géophysiques employées, la sismologie passive est sans doute celle qui procure le plus d'informations sur les structures profondes du manteau supérieur. Cet article passe en revue les études, complétées et en cours, s'appuyant sur les données télésismi-ques enregistrées en 1994 et en 1996, le long de la traverse. Une analyse complète de biréfringence des ondes S a été effectuée sur les données de 1994, dans le cadre d'une étude comparative entre l'anisotropie électrique et sismique. Les résultats obtenus suggèrent une corrélation entre les deux anisotropies (observation supportée par l'analyse de xénoli-thes), et favorisent une anisotropie sismique d'origine lithosphérique. Les deux anisotropies semblent associées à des champs de déformation archéens fossilisés dans les racines lithosphériques du Bouclier canadien...

S U M M A R YThe goal of this paper is to propose a model of deformation pattern for the Lacq gas field (southwest of France), considering the temporal and spatial evolution of the observed induced seismicity. This model of deformation has been determined from an updating of the earthquake locations and considering theoretical and analogue models usually accepted for hydrocarbon field deformation. The Lacq seismicity is clearly not linked to the natural seismicity of the Pyrenean range recorded 30 km farther to the south since the first event was felt in 1969, after the beginning of the hydrocarbon recovery. From 1974 to 1997, more than 2000 local events (M L < 4.2) have been recorded by two permanent local seismic networks. Unlike previously published results focusing on limited time lapse studies, our analysis relies on the data from 1974 to 1997. Greater accuracy of the absolute locations have been obtained using a well adapted algorithm of 3-D location, after improvement of the 3-D P-wave velocity model and determination of specific station corrections for different clusters of events. This updated catalogue of seismicity has been interpreted taking into account the structural context of the gas field. The Lacq gas field is an anticlinal reservoir where 3-D seismic and borehole data reveal a pattern of high density of fracturing, mainly oriented WNW-ESE. Seismicity map and vertical cross-sections show that majority of the seismic events (70 per cent) occurred above the gas reservoir. Correlation is also observed between the orientation of the pre-existent faults and the location of the seismic activity. Strong and organized seismicity occurred where fault orientation is consistent with the poroelastic stress perturbation due to the gas recovery. On the contrary, the seismicity is quiescient where isobaths of the reservoir roof are closed to be perpendicular to the faults. These quiescient areas as well as the central seismic part are characterized by a surface subsidence determined by repeated levelling profiles. Moreover, the temporal evolution of the distribution of the seismicity clearly exhibits a spatial migration from the centre to the boundaries of the reservoir. We conclude that the entire field is strained but this deformation is seismically expressed only where faults are parallel to the isobaths of the reservoir roof and where these faults plunge towards outside the field according to one of the two theoretical deformation models considered in our study. Then we propose a temporal scenario of deformation along the principal axis of seismic deformation.

Abstract:In the past decade, the Abitibi-Grenville Lithoprobe transect has been the site of numerous geological and geophysical surveys oriented towards understanding the lithospheric evolution of the southeastern Superior and adjoining Grenville provinces. Among the different geophysical methods that have been employed, earthquake seismology provides the widest range of information on the deep structures of the upper mantle. This paper presents a review of studies, both complete and ongoing, involving teleseismic datasets that were collected in 1994 and 1996 along the transect. A complete shear-wave splitting analysis has been performed on the 1994 dataset as part of a comparative study on electrical and seismic anisotropies. Results suggest a correlation between the two anisotropies (supported by xenolith data) and favour a lithospheric origin for the seismic anisotropy. The two anisotropies are believed to represent the fossilized remnants of Archean strain fields in the lithospheric roots of the Canadian Shield. Preliminary splitting results for the 1996 experiment suggest that the S-wave azimuthal anisotropy may be depth dependent and laterally varying. Ongoing receiver function analysis and traveltime inversion studies provide velocity models of the crust and upper mantle beneath the study area. Preliminary receiver function results reveal the presence of an S-velocity increase at -90-100 km depth which appears to be laterally continuous over 200 km. Traveltime inversion models indicate the presence of an elongate, low-velocity anomaly beneath the southern portion of the 1996 array which strikes obliquely to major geological structures at the surface (e.g., Grenville Front). Preliminary interpretation relates this anomaly to the same process (e.g., fixed mantle plume, continental rifting) responsible for the emplacement of the Monteregian Hills igneous province.Résumé : Au cours de la dernière décennie, de nombreuses campagnes géologiques et géophysiques ont été effectuées le long de la traverse Abitibi-Grenville du projet Lithoprobe, dans le but de comprendre l'évolution lithosphérique de la partie sud-est de la Province du Supérieur et de la Province de Grenville. Parmi les différentes méthodes géophysiques employées, la sismologie passive est sans doute celle qui procure le plus d'informations sur les structures profondes du manteau supérieur. Cet article passe en revue les études, complétées et en cours, s'appuyant sur les données télésismi-ques enregistrées en 1994 et en 1996, le long de la traverse. Une analyse complète de biréfringence des ondes S a été effectuée sur les données de 1994, dans le cadre d'une étude comparative entre l'anisotropie électrique et sismique. Les résultats obtenus suggèrent une corrélation entre les deux anisotropies (observation supportée par l'analyse de xénoli-thes), et favorisent une anisotropie sismique d'origine lithosphérique. Les deux anisotropies semblent associées à des champs de déformation archéens fossilisés dans les racines lithosphériques du Bouclier canadien...

The Grenville Front is a prominent tectonic feature of the Canadian Shield which has been the focus of a LITHOPROBE transect. A strong electrical anisotropy has been detected in a frequency range of 0.1–0.01 Hz (≈ 50 to 150 km), the most conductive direction being oriented N80E. In order to compare electrical and seismic anisotropies, ten seismometers have been deployed across the Grenville Front to record teleseismic events. S‐wave splitting measurements show an average delay of 1.54±0.21 s between the fast and the slow components. The fast axis direction is approximately E‐W at all stations, showing a strong coherence between the stations along the complete transect. This azimuth correlates well with that of the electrical anisotropy. The electrical anisotropy originates in the subcrustal lithosphere between 50 and 150 km. The similarity of the magnetotelluric and seismic anisotropy directions across the Grenville Front is an argument in favor of an origin of the seismic anisotropy in the subcrustal lithosphere as advocated by Silver and Kaneshima (1993).

International audienceIn places where sedimentation and erosion compete at fast rates, part of the record of past earthquakes on faults may be buried, hence hidden, in the first few metres below the surface. We developed a novel form of palaeoseismology, of geophysical type, based on the use of a dense pseudo-3-D Ground Penetrating Radar (GPR) survey to investigate such possible buried earthquake traces, on a long, fast-slipping strike-slip fault (Hope fault, New Zealand), at a site (Terako) where marked alluvial conditions prevail. We first used LiDAR data to analyse the ground surface morphology of the 2 km2 site at the greatest resolution. Nineteen morphological markers were observed, mainly alluvial terrace risers and small stream channels that are all dextrally offset by the fault by amounts ranging between 3 and 200 m. The measurements document about 10 past earthquake slip events with a mean coseismic slip of 3.3 +/- 1 m, with the most recent earthquake event having a slip of 3 +/- 0.5 m. We then investigated a detailed area of the site (400 x 600 m2) with pseudo-3-D GPR. We measured 56, similar to 400 m long, 510 m spaced GPR profiles (250 MHz), parallel to the fault and evenly distributed on either side. The analysis revealed the existence of a palaeosurface buried at about 3 m depth, corresponding to the top of alluvial terraces of different ages. That buried surface is incised by a dense network of stream channels that are all dextrally offset by the fault. We measured 48 lateral offsets in the buried channel network, more than twice than at the surface. These offsets range between 6 and 108 m, as observed at the surface, yet provide a more continuous record of the fault slip. The similarity of the successive slip increments suggests a slip per event averaging 4.4 +/- 1 m, fairly similar to that estimated from surface data. From the total surface and buried 67 offset collection, we infer that a minimum of 30 large earthquakes have broken the Hope fault at the Terako site in the last about 67 kyr, with an average coseismic slip of 3.2 +/- 1 m, a minimum average recurrence time of about 200 yr, and a magnitude of at least Mw 7.07.4. Our study therefore confirms that part of the record of past earthquakes may indeed reside in the first few metres below the surface, where it may be explored with geophysical, GPR-based palaeoseismology. Developing such a new palaeoseismological tool should provide rich information that may complement surface observations and help to document the past earthquakes on faults