The Moho preserves imprints of the regional geodynamic evolution of the lithosphere. As such, its... more The Moho preserves imprints of the regional geodynamic evolution of the lithosphere. As such, its detailed topography in divergence or convergence zones has a strong bearing on any geodynamic model. This is still more critical where 3-D effects are expected, as in the case of the Alpine chain which exhibits in its western part a short radius of curvature while its trend rotates by 180 •. The deep structure of this zone, characterized by a peculiar imbrication of high-density material of lower crust or mantle origin, remains a puzzle. In September 1999, a new controlled-source-seismology experiment was carried out in the southwestern Alps, in the area between the Pelvoux, Dora Maira and Argentera massifs. Five shots were recorded with 130 seismic stations deployed on a total of nine fan-and one in-line profiles. It aimed at getting information on the Moho depth in a hitherto blank area, and discussing the existence of the hypothetical Briançonnais mantle flake mapped in 1986 by the ECORS-CROP experiment. Fan profiles recorded at critical distance for reflections from the European Moho allowed us to map in detail the thickening of the crust from the Mediterranean coastline (27 km) to the root zone (55 km). The zone just south of the Pelvoux massif is characterized by a rather flat, 40-km-deep Moho, which distorts the isobaths in thickening the crust along the Durance valley. Beneath the Argentera massif and just north of it, we evidence a strong dip of the Moho down to 51 km, whereas previous maps predicted depths of 40-46 km only. A new, detailed map of the European Moho can be drawn, which integrates depth data measured at ∼300 reflection midpoints. However, the experiment could not establish the continuity of the Briançonnais mantle flake over a large area in the internal Alps. We observed several reflectors in the 15-31km depth range. One of them is the Ubaye reflector, a 20-km-long, 23-31-km-deep structure. It might correspond to the Briançonnais mantle flake, although it is located much farther south than the reflector mapped in 1986. New investigations will be necessary to state whether its origin is crustal or due to wedging of mantle material.
During the SALIERI cruise conducted by GEOMAR and Géosciences Azur 4000 nautical miles of multibe... more During the SALIERI cruise conducted by GEOMAR and Géosciences Azur 4000 nautical miles of multibeam bathymetry were acquired together with magnetics on the Carnegie Ridge and the convergent margin of Ecuador and Colombia. The Carnegie Ridge is a highly asymmetric dome feature with a steep, faulted northern flank and a gently sloping southern one intruded locally by large, flat-topped and
The February 25th 2001 event (Ml=4.6), occurred offshore, 25-30 km south of Nice, near the locati... more The February 25th 2001 event (Ml=4.6), occurred offshore, 25-30 km south of Nice, near the location of the December 1989 event (Ml=4.6). Both events can be related to the structures observed at the southern Alps and Ligurian Sea margin and their charac- terisation is fundamental in the estimation of the seismic hazard on the French Rivera. The last event was broadly recorded on land by permanent and mobile networks and the focal solution from P-waves first motion polarities shows a reverse faulting with a strike parallel to the coast in agreement with a family of the structural lines observed on seismic profiles. 3 days after, 5 OBS were launched in the epicentral area and the few aftershocks recorded are precisely located : for the first time, we can well con- strain hypocentral depth of such events between 15 and 18 km. The land observations on RAP (Réseau Accélérométrique Permanent) and some calibrated records at mobile stations are used to improve characterisation of the seismic response of the coastal populated region for these offshore events.
ABSTRACT The researches of Arctic region is necessary for beginning from delimitation of Arctic. ... more ABSTRACT The researches of Arctic region is necessary for beginning from delimitation of Arctic. Geographically concept "Arctic" uncertain enough. There is a set of approach to definition of its borders and set the variants of these borders (eternal permafrost, boreal tayga, drifting ice, temperature, etc.). Most correct the point of view of Ecology is realization of Arctic borders on borders of the Arctic geo - depression. Such approach allows to consider in a complex migration of natural substance and polluting substance from orogenes to deep-water hollows of Arctic Ocean. On other hand, it is necessary to take into account natural power flows from zone of Mid-Arctic ridge system at Arctic Ocean to continental land, that is opposition direction process. The certificates of such influence at different levels of Earth's crust already has collected enough (speed of seismic wave on Moho discontinuity; modern vertical movement of Earth's crust; distribution of temperature on depth; structure of basement, etc.). During the last 250 million years the Arctic geo-depression has been developing as an autonomous region with circumpolar zonality, and mass-and-energy transfer in its bowlers as well as shitting of lithospheric plates and expansion of the ocean are caused by rotational forces under conditions of an expanding planet. Four types of geoecological structures have been recorded on the basis of deep structures, position in the over-all structures of regions, place in geological history of its evolution, time of appearance, geodynamic regimes , seismicity, structural-morphological features, specific form of appearance and composition of magmatic and sedimentary formations, compositions of soil, specific metallogenic nature, types of human activity, etc. It is tectonic Segments of Earth, as geoecological global structures; the continental marginal perioceanic zones; the branches of continental marginal perioceanic zones; the mineragenic province. The main criteria of ecological unity of territory under consideration are established to the following: circumpolar position; rigorous climate with short vegetation period and a long winter; extremely wide prevalence of permafrost rocks; multitude of bogs and lake per square unit in basis of many rivers flowing into Arctic Ocean; proximity of the structure and formation of ecosystem; fragility of the Arctic environment; specificity of national economic structure; uniqueness of the raw material base having no equals in the word in its resources and content minerals of certain components; availability of the word's largest Arctic shelf oil-and-gas super basin; common geoecological natural belts; circumpolar belt of earthquakes.
In numerous regions, the focal depths are not precisely determined from standard location methods... more In numerous regions, the focal depths are not precisely determined from standard location methods because of the poor azimuthal coverage by the seismic network or because of the important geological complexity. In this study, we attempt to refine the determination of the focal depth of some events by analyzing the waveforms of the seismograms and the relative amplitude of the crustal phases. The discrete wave number representation method is used to compute synthetics corresponding to sources at various depths. We present three examples of events which occurred in southeast France in different geological units. We conclude that the waveforms are not very sensitive to slight changes of the crustal model or quality factors, but are very dependent on the focal depth. If a convenient crustal model is used in the computations, the numerical simulations constrain the depth in a better way than standard location methods using arrival times. In our examples, when the path from source to station crosses different structures, the best fit of the envelope of the seismograms by synthetics computed in a flat-layered model is reached for a crustal model corresponding to the source region.
Refinement of the seismicity distribution (4574 events) in western and central France, has been d... more Refinement of the seismicity distribution (4574 events) in western and central France, has been done by synthesis of seismological bulletins. Earthquakes have then been relocated by joint hypocentre and velocity structure inversion. The new hypocentre distribution indicates that the seismicity of those regions is much less diffuse than previously thought, mainly with regard to the depth distribution. The hypocentre improvement allows us to compute 44 new focal mechanisms and to revise bibliographic focal mechanism solutions. Then, the regional stress field was determined from 119 available focal mechanisms. It is characterized by a regionally significant strike-slip regime with NW-trending σ 1. However, the refinement in location and increasing available focal mechanism solutions allow us to show that this strike-slip regime is overprinted by local extensional perturbations in three distinct areas. In the Massif Central, the Sillon Houiller, a ancient vertical shear zone, appears to be acting as a passive boundary between a western unit and an eastern unit that is uplifted by the ascension of a hot mantle plume at the base of the lithosphere. Extension is unexpectedly observed in the southern Armorican Massif (SAM), as well as in northwestern Massif Central. One can observe a good relation between the observed perturbed zones and lateral variation of Pn anisotropy in the mantle. This correlation and the scale of these areas are arguments in favour of a lithospheric process as the origin for the stress characteristics. We believe these extensional perturbations could be related to the anticlockwise rotation of the Iberian microplate and/or incipient subduction in the Bay of Biscay.
In order to provide more insight into land propagation of T phases, signal analysis of these phas... more In order to provide more insight into land propagation of T phases, signal analysis of these phases recorded inland has been carried out. We found that the wave train is composed mainly of Pg and Sg waves which reach the seismic station from different azimuths. With the hypothesis that the signal is the sum of discrete arrivals due to different rays crossing the continental slope at different locations, synthetic seismograms of T waves are computed taking into account the variation of the conversion point along the slope. The transmission of energy from the ocean to the continental crust is mainly due to diffraction effects. The duration arid the amplitude of the T phase, which are two criteria used to evaluate tsunami risk, are greatly influenced by the shape of the continental slope.
We present observational evidence of an anomalous propagation of LI7 waves across the southwester... more We present observational evidence of an anomalous propagation of LI7 waves across the southwestern part of the Alpine range, and we use numerical simulations to model these observations. We consider a set of 48 earthquakes which occurred in Switzerland, northern Italy, and southeastern France and were recorded by the French (Laboratoire de Detection et de Geophysique) and northwest Italian (Istituto Geofisico di Genova) seismic networks. While the amplitude of the Pn phase is stable throughout the region studied, LI7 wave amplitude undergoes strong variations. We map this anomaly in LI7 wave propagation by dividing the region into a grid and attributing to each cell a value equal to the mean value of the LII/Pn amplitude ratios computed for all the paths which cross this cell. The image obtained shows that the extinction of LI7 waves occurs in a limited region of the western Alps which corresponds to the zone of highest positive Bouguer anomaly. This zone located to the east of the high peaks of the Massifs Cristallins Externes does not correspond either to the region of the highest topographies or to the one of the deepest Moho. At a frequency of 2 Hz, the crustal waves that cross this anomalous region have amplitudes divided by more than 10 with respect to waves that propagate along other paths. In order to investigate the cause of the anomaly we perform numerical simulations of SH wave propagation through a model of the western Alps which includes the main characteristics of the geological structure. Our results indicate that the geometrical effect of the lateral variations of the medium does not account entirely for the actual vanishing of crustal waves. The simulation predicts a decay of the amplitude by only a factor between 2 and 3. The introduction of corrugation in the interface shapes further adds to the decay. However, other sources of attenuation such as anelasticity or severe heterogeneity must be invoked to explain fully the observations. Asia [Baumgardt, 1991]. We will show in this •tudy that a zone of strong weakening of Lt7 also exists in the western Alps. This region, comprising part of France, Italy, and Switzerland, has been extensively studied by geologists and geophysicists (for a review, see Vialon [1990]). Following numerous refraction and wide angle seismic experiments, a deep reflection profile was recently conducted through the Italian and French parts of the range (ECORS Project). The deep structure along a NW-SE cross section is therefore known in its gross features. In this study we propose first to 1Laboratoire de G•ophysique Interne et Tectonophysique, Uni-versit6 Joseph Fourier,
Bulletin of the Seismological Society of America, Feb 1, 2013
On 1 April 2006, the Taitung earthquake (M w 6.1) occurred in Taiwan at the boundary between the ... more On 1 April 2006, the Taitung earthquake (M w 6.1) occurred in Taiwan at the boundary between the Philippine Sea and Eurasian plates, where high convergence rates contributed to the development of Plio-Pleistocene orogeny in the region. From the joint inversion of seismic and geodetic data, we identified the event's fault geometry and reconstructed the distribution of coseismic fault slip. We modeled fault geometries with increasing complexity and selected the model that best reproduced all datasets, simultaneously. Even though the earthquake magnitude was moderate, rupturing occurred in two steps. The initial rupture was generated on a listric, northsouth-trending fault (for which dip decreases with increasing depth), and was immediately followed by movement along a perpendicular structure that cross-cuts the main fault at 5 km south of the earthquake hypocenter. The average slip along the rupture was 30 cm, with a maximum of 87 cm. Oblique-reverse fault movement was characterized by a predominant left-lateral component. The amount of slip is well constrained for offsets of more than 5 cm, with an associated uncertainty of 32%. For slip amounts greater than 5 cm, uncertainties on rake and rupture time are 11°and 0.54 s, respectively. The rupture propagated from the hypocenter bilaterally, moving slightly faster toward the south (2:5 0:4 km=s) than to the north (1:7 0:1 km=s). To the south, the rupture was rapidly transmitted upward at the junction with the crosscutting east-west segment, whereas in the north, the rupture remained confined to the lower segment of the main fault. From Global Positioning Systems (GPS) and seismic data (time window < 1 min), we infer that the cross-cutting segment was activated following coseismic rupture on the main north-south fault, yet close enough in time to be associated with coseismic movement acquired by GPS (daily solutions).
The region between the inner zones of the Alps and Corsica juxtaposes an overthickened crust to a... more The region between the inner zones of the Alps and Corsica juxtaposes an overthickened crust to an oceanic domain, which makes difficult to ascertain the focal depth of seismic events using routine location codes and average 1D velocity models. The aim of this article is to show that, even with a rather lose monitoring network, accurate routine locations can be achieved by using realistic 3D modelling and advanced location techniques. Previous earthquake tomography studies cover the whole region with spatial resolutions of several tens of kilometres on land, but they fail to resolve the marine domain due to the absence of station coverage and sparse seismicity. To overcome these limitations, we first construct a 3D a-priori P and S velocity model integrating known geophysical and geological information. Significant progress has been achieved in the 3D numerical modelling of complex geological structures by the development of dedicated softwares (e.g. 3D GeoModeller), capable at once of elaborating a 3D structural model from geological and geophysical constraints and, possibly, of refining it by inversion processes (Calcagno et al., 2008). Then, we build an arrival-time catalogue of 1500 events recorded from 2000 to 2011. Hypocentres are then located in this model using a numerical code based on the maximum intersection method (Font et al., 2004), updated by Theunissen et al. (2012), as well as another 3D location technique, the NonLinLoc software (Lomax and Curtis, 2001). The reduction of arrival-time residuals and uncertainties (dh, dz) with respect to classical 1D locations demonstrates the improved accuracy allowed by our approach and confirms the coherence of the 3D geological model built and used in this study. Our results are also compared with previous works that benefitted from the installation of dense temporary networks surrounding the studied epicentre area. The resulting 3D location catalogue allows us to improve the regional seismic hazard assessment, more particularly in the south of the Argentera massif and in the Ligurian basin.
We present a lithospheric section of the northern Tyrrhenian basin based on principal observation... more We present a lithospheric section of the northern Tyrrhenian basin based on principal observations along a transect from Corsica Island to Tuscany margin. Offshore, crustal structure observations come from the LISA deep seismic cruise, composed by Multi-Channel Seismic profiles and wide angle reflection/refraction seismic data. Other parts of the section are constrained by published data, from different geophysical sources. A great detachment between Alpine Corsica and Hercynian Corsica constitutes the base of the Corsica basin. The end of this detachment is probably located below the Elba Island in the ductile lower crust where we observed an east dipping reflector interpreted as ductile shear zone. Similar detachments have been observed beneath the Punta-Ala basin on the Tuscany margin. Our section shows a thin crust below Corsican Basin (15 km thick) and a thicker crust below the Pianosa ridge (25 km). Between Elba Island and the Tuscany coast the Moho reflections shallow to (23 km/depth) as identified on the MCS profiles and from the modelling of the wide-angle reflection/refraction seismic date. An asthenospheric mantle uplift produces a significant heat flow below the Tuscany margin. The base of the lithosphere is located at 75 km depth below the Corsica margin, at 50 km below the Corsica basin and as shallow as 30 km below the Tuscany margin. The shift observed between the Corsica Basin and the Tuscany coast suggests an asymmetric lithospheric thinning which involves a detachment fault located at the base of the Corsican basin.
We investigate how focal solutions and hypocenter locations may depend on the ray tracing algorit... more We investigate how focal solutions and hypocenter locations may depend on the ray tracing algorithm and the strategy of velocity inversion. Using arrival times from a temporary seismological network in the southwestern Alps, a local earthquake tomography has been performed by Paul et al. [
HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Copyright
The Moho preserves imprints of the regional geodynamic evolution of the lithosphere. As such, its... more The Moho preserves imprints of the regional geodynamic evolution of the lithosphere. As such, its detailed topography in divergence or convergence zones has a strong bearing on any geodynamic model. This is still more critical where 3-D effects are expected, as in the case of the Alpine chain which exhibits in its western part a short radius of curvature while its trend rotates by 180 •. The deep structure of this zone, characterized by a peculiar imbrication of high-density material of lower crust or mantle origin, remains a puzzle. In September 1999, a new controlled-source-seismology experiment was carried out in the southwestern Alps, in the area between the Pelvoux, Dora Maira and Argentera massifs. Five shots were recorded with 130 seismic stations deployed on a total of nine fan-and one in-line profiles. It aimed at getting information on the Moho depth in a hitherto blank area, and discussing the existence of the hypothetical Briançonnais mantle flake mapped in 1986 by the ECORS-CROP experiment. Fan profiles recorded at critical distance for reflections from the European Moho allowed us to map in detail the thickening of the crust from the Mediterranean coastline (27 km) to the root zone (55 km). The zone just south of the Pelvoux massif is characterized by a rather flat, 40-km-deep Moho, which distorts the isobaths in thickening the crust along the Durance valley. Beneath the Argentera massif and just north of it, we evidence a strong dip of the Moho down to 51 km, whereas previous maps predicted depths of 40-46 km only. A new, detailed map of the European Moho can be drawn, which integrates depth data measured at ∼300 reflection midpoints. However, the experiment could not establish the continuity of the Briançonnais mantle flake over a large area in the internal Alps. We observed several reflectors in the 15-31km depth range. One of them is the Ubaye reflector, a 20-km-long, 23-31-km-deep structure. It might correspond to the Briançonnais mantle flake, although it is located much farther south than the reflector mapped in 1986. New investigations will be necessary to state whether its origin is crustal or due to wedging of mantle material.
During the SALIERI cruise conducted by GEOMAR and Géosciences Azur 4000 nautical miles of multibe... more During the SALIERI cruise conducted by GEOMAR and Géosciences Azur 4000 nautical miles of multibeam bathymetry were acquired together with magnetics on the Carnegie Ridge and the convergent margin of Ecuador and Colombia. The Carnegie Ridge is a highly asymmetric dome feature with a steep, faulted northern flank and a gently sloping southern one intruded locally by large, flat-topped and
The February 25th 2001 event (Ml=4.6), occurred offshore, 25-30 km south of Nice, near the locati... more The February 25th 2001 event (Ml=4.6), occurred offshore, 25-30 km south of Nice, near the location of the December 1989 event (Ml=4.6). Both events can be related to the structures observed at the southern Alps and Ligurian Sea margin and their charac- terisation is fundamental in the estimation of the seismic hazard on the French Rivera. The last event was broadly recorded on land by permanent and mobile networks and the focal solution from P-waves first motion polarities shows a reverse faulting with a strike parallel to the coast in agreement with a family of the structural lines observed on seismic profiles. 3 days after, 5 OBS were launched in the epicentral area and the few aftershocks recorded are precisely located : for the first time, we can well con- strain hypocentral depth of such events between 15 and 18 km. The land observations on RAP (Réseau Accélérométrique Permanent) and some calibrated records at mobile stations are used to improve characterisation of the seismic response of the coastal populated region for these offshore events.
ABSTRACT The researches of Arctic region is necessary for beginning from delimitation of Arctic. ... more ABSTRACT The researches of Arctic region is necessary for beginning from delimitation of Arctic. Geographically concept &quot;Arctic&quot; uncertain enough. There is a set of approach to definition of its borders and set the variants of these borders (eternal permafrost, boreal tayga, drifting ice, temperature, etc.). Most correct the point of view of Ecology is realization of Arctic borders on borders of the Arctic geo - depression. Such approach allows to consider in a complex migration of natural substance and polluting substance from orogenes to deep-water hollows of Arctic Ocean. On other hand, it is necessary to take into account natural power flows from zone of Mid-Arctic ridge system at Arctic Ocean to continental land, that is opposition direction process. The certificates of such influence at different levels of Earth&#39;s crust already has collected enough (speed of seismic wave on Moho discontinuity; modern vertical movement of Earth&#39;s crust; distribution of temperature on depth; structure of basement, etc.). During the last 250 million years the Arctic geo-depression has been developing as an autonomous region with circumpolar zonality, and mass-and-energy transfer in its bowlers as well as shitting of lithospheric plates and expansion of the ocean are caused by rotational forces under conditions of an expanding planet. Four types of geoecological structures have been recorded on the basis of deep structures, position in the over-all structures of regions, place in geological history of its evolution, time of appearance, geodynamic regimes , seismicity, structural-morphological features, specific form of appearance and composition of magmatic and sedimentary formations, compositions of soil, specific metallogenic nature, types of human activity, etc. It is tectonic Segments of Earth, as geoecological global structures; the continental marginal perioceanic zones; the branches of continental marginal perioceanic zones; the mineragenic province. The main criteria of ecological unity of territory under consideration are established to the following: circumpolar position; rigorous climate with short vegetation period and a long winter; extremely wide prevalence of permafrost rocks; multitude of bogs and lake per square unit in basis of many rivers flowing into Arctic Ocean; proximity of the structure and formation of ecosystem; fragility of the Arctic environment; specificity of national economic structure; uniqueness of the raw material base having no equals in the word in its resources and content minerals of certain components; availability of the word&#39;s largest Arctic shelf oil-and-gas super basin; common geoecological natural belts; circumpolar belt of earthquakes.
In numerous regions, the focal depths are not precisely determined from standard location methods... more In numerous regions, the focal depths are not precisely determined from standard location methods because of the poor azimuthal coverage by the seismic network or because of the important geological complexity. In this study, we attempt to refine the determination of the focal depth of some events by analyzing the waveforms of the seismograms and the relative amplitude of the crustal phases. The discrete wave number representation method is used to compute synthetics corresponding to sources at various depths. We present three examples of events which occurred in southeast France in different geological units. We conclude that the waveforms are not very sensitive to slight changes of the crustal model or quality factors, but are very dependent on the focal depth. If a convenient crustal model is used in the computations, the numerical simulations constrain the depth in a better way than standard location methods using arrival times. In our examples, when the path from source to station crosses different structures, the best fit of the envelope of the seismograms by synthetics computed in a flat-layered model is reached for a crustal model corresponding to the source region.
Refinement of the seismicity distribution (4574 events) in western and central France, has been d... more Refinement of the seismicity distribution (4574 events) in western and central France, has been done by synthesis of seismological bulletins. Earthquakes have then been relocated by joint hypocentre and velocity structure inversion. The new hypocentre distribution indicates that the seismicity of those regions is much less diffuse than previously thought, mainly with regard to the depth distribution. The hypocentre improvement allows us to compute 44 new focal mechanisms and to revise bibliographic focal mechanism solutions. Then, the regional stress field was determined from 119 available focal mechanisms. It is characterized by a regionally significant strike-slip regime with NW-trending σ 1. However, the refinement in location and increasing available focal mechanism solutions allow us to show that this strike-slip regime is overprinted by local extensional perturbations in three distinct areas. In the Massif Central, the Sillon Houiller, a ancient vertical shear zone, appears to be acting as a passive boundary between a western unit and an eastern unit that is uplifted by the ascension of a hot mantle plume at the base of the lithosphere. Extension is unexpectedly observed in the southern Armorican Massif (SAM), as well as in northwestern Massif Central. One can observe a good relation between the observed perturbed zones and lateral variation of Pn anisotropy in the mantle. This correlation and the scale of these areas are arguments in favour of a lithospheric process as the origin for the stress characteristics. We believe these extensional perturbations could be related to the anticlockwise rotation of the Iberian microplate and/or incipient subduction in the Bay of Biscay.
In order to provide more insight into land propagation of T phases, signal analysis of these phas... more In order to provide more insight into land propagation of T phases, signal analysis of these phases recorded inland has been carried out. We found that the wave train is composed mainly of Pg and Sg waves which reach the seismic station from different azimuths. With the hypothesis that the signal is the sum of discrete arrivals due to different rays crossing the continental slope at different locations, synthetic seismograms of T waves are computed taking into account the variation of the conversion point along the slope. The transmission of energy from the ocean to the continental crust is mainly due to diffraction effects. The duration arid the amplitude of the T phase, which are two criteria used to evaluate tsunami risk, are greatly influenced by the shape of the continental slope.
We present observational evidence of an anomalous propagation of LI7 waves across the southwester... more We present observational evidence of an anomalous propagation of LI7 waves across the southwestern part of the Alpine range, and we use numerical simulations to model these observations. We consider a set of 48 earthquakes which occurred in Switzerland, northern Italy, and southeastern France and were recorded by the French (Laboratoire de Detection et de Geophysique) and northwest Italian (Istituto Geofisico di Genova) seismic networks. While the amplitude of the Pn phase is stable throughout the region studied, LI7 wave amplitude undergoes strong variations. We map this anomaly in LI7 wave propagation by dividing the region into a grid and attributing to each cell a value equal to the mean value of the LII/Pn amplitude ratios computed for all the paths which cross this cell. The image obtained shows that the extinction of LI7 waves occurs in a limited region of the western Alps which corresponds to the zone of highest positive Bouguer anomaly. This zone located to the east of the high peaks of the Massifs Cristallins Externes does not correspond either to the region of the highest topographies or to the one of the deepest Moho. At a frequency of 2 Hz, the crustal waves that cross this anomalous region have amplitudes divided by more than 10 with respect to waves that propagate along other paths. In order to investigate the cause of the anomaly we perform numerical simulations of SH wave propagation through a model of the western Alps which includes the main characteristics of the geological structure. Our results indicate that the geometrical effect of the lateral variations of the medium does not account entirely for the actual vanishing of crustal waves. The simulation predicts a decay of the amplitude by only a factor between 2 and 3. The introduction of corrugation in the interface shapes further adds to the decay. However, other sources of attenuation such as anelasticity or severe heterogeneity must be invoked to explain fully the observations. Asia [Baumgardt, 1991]. We will show in this •tudy that a zone of strong weakening of Lt7 also exists in the western Alps. This region, comprising part of France, Italy, and Switzerland, has been extensively studied by geologists and geophysicists (for a review, see Vialon [1990]). Following numerous refraction and wide angle seismic experiments, a deep reflection profile was recently conducted through the Italian and French parts of the range (ECORS Project). The deep structure along a NW-SE cross section is therefore known in its gross features. In this study we propose first to 1Laboratoire de G•ophysique Interne et Tectonophysique, Uni-versit6 Joseph Fourier,
Bulletin of the Seismological Society of America, Feb 1, 2013
On 1 April 2006, the Taitung earthquake (M w 6.1) occurred in Taiwan at the boundary between the ... more On 1 April 2006, the Taitung earthquake (M w 6.1) occurred in Taiwan at the boundary between the Philippine Sea and Eurasian plates, where high convergence rates contributed to the development of Plio-Pleistocene orogeny in the region. From the joint inversion of seismic and geodetic data, we identified the event's fault geometry and reconstructed the distribution of coseismic fault slip. We modeled fault geometries with increasing complexity and selected the model that best reproduced all datasets, simultaneously. Even though the earthquake magnitude was moderate, rupturing occurred in two steps. The initial rupture was generated on a listric, northsouth-trending fault (for which dip decreases with increasing depth), and was immediately followed by movement along a perpendicular structure that cross-cuts the main fault at 5 km south of the earthquake hypocenter. The average slip along the rupture was 30 cm, with a maximum of 87 cm. Oblique-reverse fault movement was characterized by a predominant left-lateral component. The amount of slip is well constrained for offsets of more than 5 cm, with an associated uncertainty of 32%. For slip amounts greater than 5 cm, uncertainties on rake and rupture time are 11°and 0.54 s, respectively. The rupture propagated from the hypocenter bilaterally, moving slightly faster toward the south (2:5 0:4 km=s) than to the north (1:7 0:1 km=s). To the south, the rupture was rapidly transmitted upward at the junction with the crosscutting east-west segment, whereas in the north, the rupture remained confined to the lower segment of the main fault. From Global Positioning Systems (GPS) and seismic data (time window < 1 min), we infer that the cross-cutting segment was activated following coseismic rupture on the main north-south fault, yet close enough in time to be associated with coseismic movement acquired by GPS (daily solutions).
The region between the inner zones of the Alps and Corsica juxtaposes an overthickened crust to a... more The region between the inner zones of the Alps and Corsica juxtaposes an overthickened crust to an oceanic domain, which makes difficult to ascertain the focal depth of seismic events using routine location codes and average 1D velocity models. The aim of this article is to show that, even with a rather lose monitoring network, accurate routine locations can be achieved by using realistic 3D modelling and advanced location techniques. Previous earthquake tomography studies cover the whole region with spatial resolutions of several tens of kilometres on land, but they fail to resolve the marine domain due to the absence of station coverage and sparse seismicity. To overcome these limitations, we first construct a 3D a-priori P and S velocity model integrating known geophysical and geological information. Significant progress has been achieved in the 3D numerical modelling of complex geological structures by the development of dedicated softwares (e.g. 3D GeoModeller), capable at once of elaborating a 3D structural model from geological and geophysical constraints and, possibly, of refining it by inversion processes (Calcagno et al., 2008). Then, we build an arrival-time catalogue of 1500 events recorded from 2000 to 2011. Hypocentres are then located in this model using a numerical code based on the maximum intersection method (Font et al., 2004), updated by Theunissen et al. (2012), as well as another 3D location technique, the NonLinLoc software (Lomax and Curtis, 2001). The reduction of arrival-time residuals and uncertainties (dh, dz) with respect to classical 1D locations demonstrates the improved accuracy allowed by our approach and confirms the coherence of the 3D geological model built and used in this study. Our results are also compared with previous works that benefitted from the installation of dense temporary networks surrounding the studied epicentre area. The resulting 3D location catalogue allows us to improve the regional seismic hazard assessment, more particularly in the south of the Argentera massif and in the Ligurian basin.
We present a lithospheric section of the northern Tyrrhenian basin based on principal observation... more We present a lithospheric section of the northern Tyrrhenian basin based on principal observations along a transect from Corsica Island to Tuscany margin. Offshore, crustal structure observations come from the LISA deep seismic cruise, composed by Multi-Channel Seismic profiles and wide angle reflection/refraction seismic data. Other parts of the section are constrained by published data, from different geophysical sources. A great detachment between Alpine Corsica and Hercynian Corsica constitutes the base of the Corsica basin. The end of this detachment is probably located below the Elba Island in the ductile lower crust where we observed an east dipping reflector interpreted as ductile shear zone. Similar detachments have been observed beneath the Punta-Ala basin on the Tuscany margin. Our section shows a thin crust below Corsican Basin (15 km thick) and a thicker crust below the Pianosa ridge (25 km). Between Elba Island and the Tuscany coast the Moho reflections shallow to (23 km/depth) as identified on the MCS profiles and from the modelling of the wide-angle reflection/refraction seismic date. An asthenospheric mantle uplift produces a significant heat flow below the Tuscany margin. The base of the lithosphere is located at 75 km depth below the Corsica margin, at 50 km below the Corsica basin and as shallow as 30 km below the Tuscany margin. The shift observed between the Corsica Basin and the Tuscany coast suggests an asymmetric lithospheric thinning which involves a detachment fault located at the base of the Corsican basin.
We investigate how focal solutions and hypocenter locations may depend on the ray tracing algorit... more We investigate how focal solutions and hypocenter locations may depend on the ray tracing algorithm and the strategy of velocity inversion. Using arrival times from a temporary seismological network in the southwestern Alps, a local earthquake tomography has been performed by Paul et al. [
HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Copyright
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