Alfredo Taboada - Academia.edu (original) (raw)

Papers by Alfredo Taboada

Research paper thumbnail of Landsliding generated by thermomechanical interactions between rock columns and wedging blocks: Study case from the Larzac Plateau (Southern France)

EPJ Web of Conferences, 2017

Research paper thumbnail of Shear strength and fabric properties in granular media with interlocked particles

AIP Conference Proceedings, 2009

We study granular materials characterized by the possibility of interlocking between the particle... more We study granular materials characterized by the possibility of interlocking between the particles. The Interlocking is modeled as the combined effect of sliding and rolling resistance at the contacts, and it involves two contact parameters: a coefficient of sliding friction and a coefficient of rolling friction. This model is introduced within the framework of the Contact Dynamics method, and it

Research paper thumbnail of Mouvement de versant : un processus géomorphologique long terme

Research paper thumbnail of Strain Localisation in Porous Sandstone as a Function of Tectonic Setting, Burial and Material Properties

Proceedings, 2012

The analysis of three cataclastic band sets from Provence (France) reveals that the band density,... more The analysis of three cataclastic band sets from Provence (France) reveals that the band density, their conjugate angles, their ratio of shear displacement to compaction, and the amount of cataclasis are different and can be expressed as a function of tectonic setting and petrophysical properties. We identify (1) a dense and closely spaced network of shear enhanced (reverse) compaction bands, (2) a regularly spaced less dense network of reverse compactional shear bands, and (3) a localized network of normal shear bands. The field data show that the localization of strain is favored in an extensional setting and is characterized by shear bands with a large shear to compaction ratio and a low conjugate band angle. In contrast, distributed strain is favored in a contractional setting and is characterized by compactional bands with a low ratio of shear to compaction and a large conjugate band angle. To explain the mechanical origin of this strain localization, we quantify the yield strength and the stress evolution in extensional and contractional regimes. We propose a model of strain localisation in porous sandstone as a function of tectonic stresses, burial depth, material properties, strain hardening and fluid pressure.

Research paper thumbnail of The 1999 Tsao-Ling Rockslide: Source Area, Debris, and Life Cycle of Associated Rockslide-Dammed Lake (Central Taiwan)

Lecture Notes in Earth Sciences, 2010

The Tsao-Ling rock avalanche was the largest landslide triggered by the 1999 Chi–Chi earthquake i... more The Tsao-Ling rock avalanche was the largest landslide triggered by the 1999 Chi–Chi earthquake in central Taiwan. In addition to detailed and extensive field observations, several Digital Elevation Models (DEMs) generated from sets of aerial photos have been utilized to document and measure the coseismic and post-seismic morphological changes at Tsao-Ling. The estimated volume of the initial rock avalanche is about 125.5 and 138 Mm3 for the depleted and accumulated zones, respectively, indicating an increase in volume due to fragmentation. The average thickness was about 150–170 m, up to 195 m of the slid mass–over 140 m on the Chingshui River channel while between 30 and 90 m debris covered on the preexisting debris deposit hill. The landslide debris blocked the river channel and formed a dammed lake, with a maximum volume of 45 Mm3. In the deposit area, strong river erosion has removed 72 Mm3 of the debris since the earthquake. By August 2004, the rockslide-dammed lake had filled up completely with sediment. The filling materials originated in numerous landslides in the upstream area.

Research paper thumbnail of Rôle des paramètres locaux de cohésion dans la résistance mécanique d'un milieu granulaire cohésif

Nous présentons une étude paramétrique de la résistance au cisaillement d'un milieu granulaire co... more Nous présentons une étude paramétrique de la résistance au cisaillement d'un milieu granulaire cohésif simulé par la méthode de dynamique des contacts. Cette étude est basée sur un modèle de cohésion qui tient compte de la résistance à la traction, au glissement, et au roulement dans les contacts entre les particules. La résistance macroscopique est évaluée par la compression biaxiale des échantillons numériques comportant plusieurs milliers de particules en deux dimensions. Nos résultats suggèrent que : 1) l'angle de frottement interne peut être vu comme la somme de trois contributions : une géométrique, une frictionnelle, et une associée à la dilatance ; 2) les coefficients de frottement sont les seuls paramètres de résistance microscopiques à l'origine de l'angle de frottement interne et de l'angle de dilatance ; et 3) l'adhésion entre particules est le seul paramètre de résistance microscopique à l'origine de la résistance à la traction.

Research paper thumbnail of Recent Morphological Changes of the Nice Continental Slope

Engineering Geology for Society and Territory - Volume 4, 2014

Time-series bathymetric data acquired between 1991 and 2011 have been used to evaluate the recent... more Time-series bathymetric data acquired between 1991 and 2011 have been used to evaluate the recent morphological evolution of the Nice upper continental slope (SE France, Ligurian Sea). Small-scale landslides lead to a retrogressive evolution of the continental shelf/upper slope transition but their frequency, size and impact are not well known. Mapping was undertaken to identify the morphology of landslide scarps and the location of the shelfbreak. Map comparisons were performed using ArcGIS “raster calculator”. Sediment remobilization on the upper slope (up to depths of 200 m) is fast and significant; landslide scars with volumes greater than 25,000 m3 can appear with a frequency less than 8 years. Shelfbreak migration toward the coastline can reach rates of 60 m over 7–8 years where the continental shelf is over 200 m wide. Furthermore, this quantitative analysis highlights alternations between periods of strongly erosive events and sedimentation periods. On the upper slope, eroded volumes can be multiplied by 10 during periods of enhanced landslide patterns (1999–2006). Such cycle-like landslide activity raises the issue of the triggering processes. On the Nice continental slope thick poorly consolidated beds rapidly deposited on a steep slope, earthquakes and rainfall leading to fresh water circulation below the shelf were identified as potential triggers. Our 4D bathymetric study suggests that over the last 20 years the greatest impacting factor may be freshwater outflows.

Research paper thumbnail of The 19 January 1995 Tauramena (Colombia) earthquake: geometry and stress regime

Tectonophysics, 2003

The Tauramena (Colombia) earthquake, M w = 6.5, occurred on January 19, 1995, in the Andean Easte... more The Tauramena (Colombia) earthquake, M w = 6.5, occurred on January 19, 1995, in the Andean Eastern Cordillera foothill region, the so-called Piedemonte Llanero. The Harvard CMT focal mechanism indicates an almost pure reverse fault rupture. There was no surface faulting associated with this earthquake. This event was located at the northern tip of a zone, about 90 km in length, with relatively low microseismic activity along the central segment of the Piedemonte Llanero in Colombia. A field expedition to the epicentral area was organized and a temporary portable network was installed for 1 month to register aftershock activity. More than 800 events were recorded during this period. A subset of the best located aftershocks (319 events) shows epicenters extended over an area of 800 km 2 , and suggests two antithetic planes on which most of the activity was concentrated. The main event and a subset of 41 aftershocks occurred before the installation of the portable network but were recorded by the Colombian National Seismic Network and were relocated. They show an epicentral distribution similar to that of the subsequent events. Based on geologic information, aftershock locations, and focal mechanisms, we built a model for the Tauramena earthquake. Our results indicate that the Tauramena earthquake was produced by reverse faulting along a steep-dipping plane (dip f 50jNW) associated with the Guaicaramo System. The fault plane cuts through basement rocks and folded Mesozoic and Cenozoic sedimentary rocks. Distribution of aftershocks in depth, suggesting the presence of splay reverse faults and a backthrust, is consistent with the hypothesis that the Guaicaramo Fault System was an old Mesozoic normal fault, reactivated during the Andean compression as a reverse fault. Stress tensor inversion of P-wave first motion polarities was performed, and focal mechanisms for the best recorded aftershocks were calculated. We found a well-defined sub-horizontal principal compression axis oriented in the NW-SE direction. The predominant reverse faulting for the Tauramena earthquake and the stability of r 1 suggest that the tectonics in the

Research paper thumbnail of Geodynamics of the northern Andes: Subductions and intracontinental deformation (Colombia)

Tectonics, 2000

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.

Research paper thumbnail of Additive Decomposition of Shear Strength in Cohesive Granular Media from Grain-Scale Interactions

Physical Review Letters, 2006

We study cemented granular media by introducing cohesive bonding (sliding or rolling friction and... more We study cemented granular media by introducing cohesive bonding (sliding or rolling friction and tensile strength) between grains in the framework of the contact dynamics method. We find that, for a wide range of bond parameters, the macroscopic angle of friction at the peak state can be split into three distinct terms of collisional, frictional and dilational origins. Remarkably, the macroscopic tensile strength depends only on the bond tensile strength, and the friction angle at the peak state is proportional to the dilatancy angle which varies linearly with sliding friction.

Research paper thumbnail of Simulation of cemented granular materials. I. Macroscopic stress-strain response and strain localization

Physical Review E, 2010

This is the first of two papers investigating the mechanical response of cemented granular materi... more This is the first of two papers investigating the mechanical response of cemented granular materials by means of contact dynamics simulations. In this paper, a two-dimensional polydisperse sample with high-void ratio is constructed and then sheared in a simple shear numerical device at different confinement levels. We study the macroscopic response of the material in terms of mean and deviatoric stresses and strains. We show that the introduction of a local force scale, i.e., the tensile strength of the cemented bonds, causes the material to behave in a rigid-plastic fashion, so that a yield surface can be easily determined. This yield surface has a concave-down shape in the mean:deviatoric stress plane and it approaches a straight line, i.e., a Coulomb strength envelope, in the limit of a very dense granular material. Beyond yielding, the cemented structure gradually degrades until the material eventually behaves as a cohesionless granular material. Strain localization is also investigated, showing that the strains concentrate in a shear band whose thickness increases with the confining stress. The void ratio inside the shear band at the steady state is shown to be a material property that depends only on contact parameters.

Research paper thumbnail of Simulation of cemented granular materials. II. Micromechanical description and strength mobilization at the onset of macroscopic yielding

Physical Review E, 2010

This is the second of two papers investigating the mechanical response of cemented granular mater... more This is the second of two papers investigating the mechanical response of cemented granular materials by means of contact dynamics simulations. In this paper, a two-dimensional polydisperse sample with high void ratio is sheared in a load-controlled simple shear numerical device until the stress state of the sample reaches the yield stress. We first study the stress transmission properties of the granular material in terms of the fabric of different subsets of contacts characterized by the magnitude of their normal forces. This analysis highlights the existence of a peculiar force carrying structure in the cemented material, which is reminiscent of the bimodal stress transmission reported for cohesionless granular media. Then, the evolution of contact forces and torques is investigated trying to identify the micromechanical conditions that trigger macroscopic yielding. It is shown that global failure can be associated to the apparition of a group of particles whose contacts fulfill at least one of the local rupture conditions. In particular, these particles form a large region that percolates through the sample at the moment of failure, evidencing the relationship between macroscopic yielding and the emergence of large-scale correlations in the system.

Research paper thumbnail of Impact of erosion and décollements on large-scale faulting and folding in orogenic wedges: analogue models and case studies

Journal of the Geological Society, 2013

Deformation mechanisms, long-term kinematics and evolution of fold and thrust belts subjected to ... more Deformation mechanisms, long-term kinematics and evolution of fold and thrust belts subjected to erosion are studied through 2D analogue experiments involving large convergence. First-order parameters tested include (1) décollements and/or plastic layers interbedded at different locations within analogue materials and (2) synconvergence surface erosion. Weak layers, depending on their location in the model, favour deformation partitioning characterized by the simultaneous development of underplating domains in the inner part of the wedge (basal accretion) and frontal accretion where the wedge grows forward. Interaction between tectonics and surface processes influences this behaviour. Development of antiformal thrust stacks controlled by underplating shows small-and large-scale cyclicity. Thin plastic layers induce folding processes, which are studied at wedge scale. recumbent and overturned folds, with large inverted limbs, develop in a shear-induced asymmetric deformation regime via progressive unrolling of synclinal hinges. Surface erosion and underplating at depth induce further rotation (passive tilting) and horizontalization of fold limbs. Model results give insights to discuss the mechanisms responsible for the large-scale structures (i.e. antiformal nappe stacks, klippen and kilometre-scale recumbent fold-nappes) encountered in several mountain belts such as the Montagne Noire (French Massif Central), the Galicia Variscan belt (Spain) and the northern Apennines (Italy).

Research paper thumbnail of Stress and strain from striated pebbles. Theoretical analysis of striations on a rigid spherical body linked to a symmetrical tensor

Journal of Structural Geology, 1993

Striations on a pebble are interpreted as resulting from slip due to either a homogeneous stress ... more Striations on a pebble are interpreted as resulting from slip due to either a homogeneous stress state or a small homogeneous coaxial deformation in the matrix. In terms of stress, striations are assumed to be parallel to the applied shear stress. In terms of strain, striations are considered to be parallel to the relative tangential displacement between the pebble and adjacent matrix particles. Slip on the surface of a spherical rigid body enclosed in a deformable matrix (brittle or ductile) is theoretically analysed for different stress and strain regimes. The analysis predicts the topology of the resulting striations and singularity distribution on the sphere. Both in terms of stress and strain, the tangential vector field on the sphere's surface derives from a potential function proportional to the magnitude of the normal vector field. Tangential and normal vectors represent either shear and normal stresses, or displacement components (in terms of strain). The plot of continuous curves parallel to striations (integral curves) and of equipotential curves on the sphere, allows simultaneously the magnitude and orientation of the tangential and normal vector fields to be visualized. Close to singular points, the integral curves correspond to power laws and the equipotentials correspond to conic sections. This theoretical analysis allows graphical method for estimating the stress ratio (02-aa)/(ot-03) from striated faults to be proposed, once the orientations of the principal stress directions are known (i.e. by means of other graphical methods).

Research paper thumbnail of A model of strain localization in porous sandstone as a function of tectonic setting, burial and material properties; new insight from Provence (southern France)

Journal of Structural Geology, 2013

The analysis of three cataclastic band sets from Provence (France) reveals that the band density,... more The analysis of three cataclastic band sets from Provence (France) reveals that the band density, their conjugate angles, their ratio of shear displacement to compaction, and the amount of cataclasis within the bands differ and can be expressed as functions of tectonic setting and petrophysical properties. We identify (1) a dense and closely spaced network of shear-enhanced (reverse) compaction bands; (2) a regularly spaced less dense network of reverse compactional shear bands; and (3) a localized network of normal shear bands. The field data show that strain localization is favored in an extensional regime and is characterized by shear bands with a large shear to compaction ratio and a small conjugate band angle. In contrast, distributed strain is favored in a contractional regime and is characterized by compactional bands with a low ratio of shear to compaction and a large conjugate band angle. To explain the mechanical origin of this strain localization, we quantify the yield strength and the stress evolution in extensional and contractional regimes in a frictional porous granular material. We propose a model of strain localization in porous sands as a function of tectonic stresses, burial depth, material properties, strain hardening and fluid pressure. Our model suggests that stress reduction, inherent to extensional regime, favors strain localization as shear bands, whereas stress increase during contraction favors development of compactional bands.

Research paper thumbnail of Correction to “Discrete element simulation of the Jiufengershan rock-and-soil avalanche triggered by the 1999 Chi-Chi earthquake, Taiwan”

Journal of Geophysical Research, 2009

Research paper thumbnail of Correction to “Rock-and-soil avalanches: Theory and simulation”

Journal of Geophysical Research, 2009

Research paper thumbnail of Rheology, force transmission, and shear instabilities in frictional granular media from biaxial numerical tests using the contact dynamics method

Journal of Geophysical Research: Solid Earth, 2005

Research paper thumbnail of Discrete element simulation of the Jiufengershan rock-and-soil avalanche triggered by the 1999 Chi-Chi earthquake, Taiwan

Journal of Geophysical Research, 2009

We present Contact Dynamics discrete element simulations of the earthquaketriggered Jiufengershan... more We present Contact Dynamics discrete element simulations of the earthquaketriggered Jiufengershan avalanche, which mobilized a 60 m thick, 1.5 km long sedimentary layer, dipping $22°SE toward a valley. The dynamic behavior of the avalanche is simulated under different assumptions about rock behavior, water table height, and boundary shear strength. Additionally, seismic shaking is introduced using strong motion records from nearby stations. We assume that seismic shaking generates shearing and frictional heating along the surface of rupture, which, in turn, may induce dynamic weakening and avalanche triggering; a simple ''slip-weakening'' criterion was adopted to simulate shear strength drop along the rupture surface. We investigate the mechanical processes occurring during triggering and propagation of an avalanche mobilizing shallowly dipping layers. Incipient deformation forms a pop-up structure at the toe of the dip slope. As the avalanche propagates, the pop-up deforms into an overturned fold, which overrides the surface of separation along a décollement. Simultaneously, uphill layers slide at high velocity (125 km/h) and are folded and disrupted as they reach the toe of the dip slope. The avalanche foot forms a wedge that is pushed forward as deformed rocks accrete at its rear. We simulated five cross sections across the Jiufengershan avalanche, which differ in the geometry of the surface of separation. Topographic and simulated surface profiles are similar. The friction coefficient at the surface of separation determined from back analysis is abnormally low (m SS = 0.2), possibly due to lubrication by liquefied soils. The granular deposits of simulated earthquake-and rain-triggered avalanches are similar.

Research paper thumbnail of Modern tectonic stress field in the Mediterranean region: evidence for variation in stress directions at different scales

Geophysical Journal International, 1992

A compilation of more than one thousand stress indicators (which include in situ stress measureme... more A compilation of more than one thousand stress indicators (which include in situ stress measurements, focal mechanisms, microtectonic and other geological data) allowed us to reconstruct the modern stress field in the Mediterranean region and the surrounding area. Average stress directions at different scales have been reconstructed by means of a linear interpolation method. This method takes into account the distribution, scale and quality of stress data. The results of the interpolation at plate scale, allow us to recognize slightly deformed regions such as the northwestern European platform, where average maximum horizontal stress direction is oriented roughly NNW-SSE, subparallel to absolute and relative plate velocity directions. Other regions such as the Caucasus, Alps and Pyrenees, where recent tectonic deformation and seismicity are present, display important variations of stress directions. The reconstruction of the average stress directions at different scales within the French Alps pointed out that the average stress field pattern may vary from one scale to another. Nevertheless, variations of stress directions at a given scale are consistent with the kinematics of faults of the same scale.

Research paper thumbnail of Landsliding generated by thermomechanical interactions between rock columns and wedging blocks: Study case from the Larzac Plateau (Southern France)

EPJ Web of Conferences, 2017

Research paper thumbnail of Shear strength and fabric properties in granular media with interlocked particles

AIP Conference Proceedings, 2009

We study granular materials characterized by the possibility of interlocking between the particle... more We study granular materials characterized by the possibility of interlocking between the particles. The Interlocking is modeled as the combined effect of sliding and rolling resistance at the contacts, and it involves two contact parameters: a coefficient of sliding friction and a coefficient of rolling friction. This model is introduced within the framework of the Contact Dynamics method, and it

Research paper thumbnail of Mouvement de versant : un processus géomorphologique long terme

Research paper thumbnail of Strain Localisation in Porous Sandstone as a Function of Tectonic Setting, Burial and Material Properties

Proceedings, 2012

The analysis of three cataclastic band sets from Provence (France) reveals that the band density,... more The analysis of three cataclastic band sets from Provence (France) reveals that the band density, their conjugate angles, their ratio of shear displacement to compaction, and the amount of cataclasis are different and can be expressed as a function of tectonic setting and petrophysical properties. We identify (1) a dense and closely spaced network of shear enhanced (reverse) compaction bands, (2) a regularly spaced less dense network of reverse compactional shear bands, and (3) a localized network of normal shear bands. The field data show that the localization of strain is favored in an extensional setting and is characterized by shear bands with a large shear to compaction ratio and a low conjugate band angle. In contrast, distributed strain is favored in a contractional setting and is characterized by compactional bands with a low ratio of shear to compaction and a large conjugate band angle. To explain the mechanical origin of this strain localization, we quantify the yield strength and the stress evolution in extensional and contractional regimes. We propose a model of strain localisation in porous sandstone as a function of tectonic stresses, burial depth, material properties, strain hardening and fluid pressure.

Research paper thumbnail of The 1999 Tsao-Ling Rockslide: Source Area, Debris, and Life Cycle of Associated Rockslide-Dammed Lake (Central Taiwan)

Lecture Notes in Earth Sciences, 2010

The Tsao-Ling rock avalanche was the largest landslide triggered by the 1999 Chi–Chi earthquake i... more The Tsao-Ling rock avalanche was the largest landslide triggered by the 1999 Chi–Chi earthquake in central Taiwan. In addition to detailed and extensive field observations, several Digital Elevation Models (DEMs) generated from sets of aerial photos have been utilized to document and measure the coseismic and post-seismic morphological changes at Tsao-Ling. The estimated volume of the initial rock avalanche is about 125.5 and 138 Mm3 for the depleted and accumulated zones, respectively, indicating an increase in volume due to fragmentation. The average thickness was about 150–170 m, up to 195 m of the slid mass–over 140 m on the Chingshui River channel while between 30 and 90 m debris covered on the preexisting debris deposit hill. The landslide debris blocked the river channel and formed a dammed lake, with a maximum volume of 45 Mm3. In the deposit area, strong river erosion has removed 72 Mm3 of the debris since the earthquake. By August 2004, the rockslide-dammed lake had filled up completely with sediment. The filling materials originated in numerous landslides in the upstream area.

Research paper thumbnail of Rôle des paramètres locaux de cohésion dans la résistance mécanique d'un milieu granulaire cohésif

Nous présentons une étude paramétrique de la résistance au cisaillement d'un milieu granulaire co... more Nous présentons une étude paramétrique de la résistance au cisaillement d'un milieu granulaire cohésif simulé par la méthode de dynamique des contacts. Cette étude est basée sur un modèle de cohésion qui tient compte de la résistance à la traction, au glissement, et au roulement dans les contacts entre les particules. La résistance macroscopique est évaluée par la compression biaxiale des échantillons numériques comportant plusieurs milliers de particules en deux dimensions. Nos résultats suggèrent que : 1) l'angle de frottement interne peut être vu comme la somme de trois contributions : une géométrique, une frictionnelle, et une associée à la dilatance ; 2) les coefficients de frottement sont les seuls paramètres de résistance microscopiques à l'origine de l'angle de frottement interne et de l'angle de dilatance ; et 3) l'adhésion entre particules est le seul paramètre de résistance microscopique à l'origine de la résistance à la traction.

Research paper thumbnail of Recent Morphological Changes of the Nice Continental Slope

Engineering Geology for Society and Territory - Volume 4, 2014

Time-series bathymetric data acquired between 1991 and 2011 have been used to evaluate the recent... more Time-series bathymetric data acquired between 1991 and 2011 have been used to evaluate the recent morphological evolution of the Nice upper continental slope (SE France, Ligurian Sea). Small-scale landslides lead to a retrogressive evolution of the continental shelf/upper slope transition but their frequency, size and impact are not well known. Mapping was undertaken to identify the morphology of landslide scarps and the location of the shelfbreak. Map comparisons were performed using ArcGIS “raster calculator”. Sediment remobilization on the upper slope (up to depths of 200 m) is fast and significant; landslide scars with volumes greater than 25,000 m3 can appear with a frequency less than 8 years. Shelfbreak migration toward the coastline can reach rates of 60 m over 7–8 years where the continental shelf is over 200 m wide. Furthermore, this quantitative analysis highlights alternations between periods of strongly erosive events and sedimentation periods. On the upper slope, eroded volumes can be multiplied by 10 during periods of enhanced landslide patterns (1999–2006). Such cycle-like landslide activity raises the issue of the triggering processes. On the Nice continental slope thick poorly consolidated beds rapidly deposited on a steep slope, earthquakes and rainfall leading to fresh water circulation below the shelf were identified as potential triggers. Our 4D bathymetric study suggests that over the last 20 years the greatest impacting factor may be freshwater outflows.

Research paper thumbnail of The 19 January 1995 Tauramena (Colombia) earthquake: geometry and stress regime

Tectonophysics, 2003

The Tauramena (Colombia) earthquake, M w = 6.5, occurred on January 19, 1995, in the Andean Easte... more The Tauramena (Colombia) earthquake, M w = 6.5, occurred on January 19, 1995, in the Andean Eastern Cordillera foothill region, the so-called Piedemonte Llanero. The Harvard CMT focal mechanism indicates an almost pure reverse fault rupture. There was no surface faulting associated with this earthquake. This event was located at the northern tip of a zone, about 90 km in length, with relatively low microseismic activity along the central segment of the Piedemonte Llanero in Colombia. A field expedition to the epicentral area was organized and a temporary portable network was installed for 1 month to register aftershock activity. More than 800 events were recorded during this period. A subset of the best located aftershocks (319 events) shows epicenters extended over an area of 800 km 2 , and suggests two antithetic planes on which most of the activity was concentrated. The main event and a subset of 41 aftershocks occurred before the installation of the portable network but were recorded by the Colombian National Seismic Network and were relocated. They show an epicentral distribution similar to that of the subsequent events. Based on geologic information, aftershock locations, and focal mechanisms, we built a model for the Tauramena earthquake. Our results indicate that the Tauramena earthquake was produced by reverse faulting along a steep-dipping plane (dip f 50jNW) associated with the Guaicaramo System. The fault plane cuts through basement rocks and folded Mesozoic and Cenozoic sedimentary rocks. Distribution of aftershocks in depth, suggesting the presence of splay reverse faults and a backthrust, is consistent with the hypothesis that the Guaicaramo Fault System was an old Mesozoic normal fault, reactivated during the Andean compression as a reverse fault. Stress tensor inversion of P-wave first motion polarities was performed, and focal mechanisms for the best recorded aftershocks were calculated. We found a well-defined sub-horizontal principal compression axis oriented in the NW-SE direction. The predominant reverse faulting for the Tauramena earthquake and the stability of r 1 suggest that the tectonics in the

Research paper thumbnail of Geodynamics of the northern Andes: Subductions and intracontinental deformation (Colombia)

Tectonics, 2000

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.

Research paper thumbnail of Additive Decomposition of Shear Strength in Cohesive Granular Media from Grain-Scale Interactions

Physical Review Letters, 2006

We study cemented granular media by introducing cohesive bonding (sliding or rolling friction and... more We study cemented granular media by introducing cohesive bonding (sliding or rolling friction and tensile strength) between grains in the framework of the contact dynamics method. We find that, for a wide range of bond parameters, the macroscopic angle of friction at the peak state can be split into three distinct terms of collisional, frictional and dilational origins. Remarkably, the macroscopic tensile strength depends only on the bond tensile strength, and the friction angle at the peak state is proportional to the dilatancy angle which varies linearly with sliding friction.

Research paper thumbnail of Simulation of cemented granular materials. I. Macroscopic stress-strain response and strain localization

Physical Review E, 2010

This is the first of two papers investigating the mechanical response of cemented granular materi... more This is the first of two papers investigating the mechanical response of cemented granular materials by means of contact dynamics simulations. In this paper, a two-dimensional polydisperse sample with high-void ratio is constructed and then sheared in a simple shear numerical device at different confinement levels. We study the macroscopic response of the material in terms of mean and deviatoric stresses and strains. We show that the introduction of a local force scale, i.e., the tensile strength of the cemented bonds, causes the material to behave in a rigid-plastic fashion, so that a yield surface can be easily determined. This yield surface has a concave-down shape in the mean:deviatoric stress plane and it approaches a straight line, i.e., a Coulomb strength envelope, in the limit of a very dense granular material. Beyond yielding, the cemented structure gradually degrades until the material eventually behaves as a cohesionless granular material. Strain localization is also investigated, showing that the strains concentrate in a shear band whose thickness increases with the confining stress. The void ratio inside the shear band at the steady state is shown to be a material property that depends only on contact parameters.

Research paper thumbnail of Simulation of cemented granular materials. II. Micromechanical description and strength mobilization at the onset of macroscopic yielding

Physical Review E, 2010

This is the second of two papers investigating the mechanical response of cemented granular mater... more This is the second of two papers investigating the mechanical response of cemented granular materials by means of contact dynamics simulations. In this paper, a two-dimensional polydisperse sample with high void ratio is sheared in a load-controlled simple shear numerical device until the stress state of the sample reaches the yield stress. We first study the stress transmission properties of the granular material in terms of the fabric of different subsets of contacts characterized by the magnitude of their normal forces. This analysis highlights the existence of a peculiar force carrying structure in the cemented material, which is reminiscent of the bimodal stress transmission reported for cohesionless granular media. Then, the evolution of contact forces and torques is investigated trying to identify the micromechanical conditions that trigger macroscopic yielding. It is shown that global failure can be associated to the apparition of a group of particles whose contacts fulfill at least one of the local rupture conditions. In particular, these particles form a large region that percolates through the sample at the moment of failure, evidencing the relationship between macroscopic yielding and the emergence of large-scale correlations in the system.

Research paper thumbnail of Impact of erosion and décollements on large-scale faulting and folding in orogenic wedges: analogue models and case studies

Journal of the Geological Society, 2013

Deformation mechanisms, long-term kinematics and evolution of fold and thrust belts subjected to ... more Deformation mechanisms, long-term kinematics and evolution of fold and thrust belts subjected to erosion are studied through 2D analogue experiments involving large convergence. First-order parameters tested include (1) décollements and/or plastic layers interbedded at different locations within analogue materials and (2) synconvergence surface erosion. Weak layers, depending on their location in the model, favour deformation partitioning characterized by the simultaneous development of underplating domains in the inner part of the wedge (basal accretion) and frontal accretion where the wedge grows forward. Interaction between tectonics and surface processes influences this behaviour. Development of antiformal thrust stacks controlled by underplating shows small-and large-scale cyclicity. Thin plastic layers induce folding processes, which are studied at wedge scale. recumbent and overturned folds, with large inverted limbs, develop in a shear-induced asymmetric deformation regime via progressive unrolling of synclinal hinges. Surface erosion and underplating at depth induce further rotation (passive tilting) and horizontalization of fold limbs. Model results give insights to discuss the mechanisms responsible for the large-scale structures (i.e. antiformal nappe stacks, klippen and kilometre-scale recumbent fold-nappes) encountered in several mountain belts such as the Montagne Noire (French Massif Central), the Galicia Variscan belt (Spain) and the northern Apennines (Italy).

Research paper thumbnail of Stress and strain from striated pebbles. Theoretical analysis of striations on a rigid spherical body linked to a symmetrical tensor

Journal of Structural Geology, 1993

Striations on a pebble are interpreted as resulting from slip due to either a homogeneous stress ... more Striations on a pebble are interpreted as resulting from slip due to either a homogeneous stress state or a small homogeneous coaxial deformation in the matrix. In terms of stress, striations are assumed to be parallel to the applied shear stress. In terms of strain, striations are considered to be parallel to the relative tangential displacement between the pebble and adjacent matrix particles. Slip on the surface of a spherical rigid body enclosed in a deformable matrix (brittle or ductile) is theoretically analysed for different stress and strain regimes. The analysis predicts the topology of the resulting striations and singularity distribution on the sphere. Both in terms of stress and strain, the tangential vector field on the sphere's surface derives from a potential function proportional to the magnitude of the normal vector field. Tangential and normal vectors represent either shear and normal stresses, or displacement components (in terms of strain). The plot of continuous curves parallel to striations (integral curves) and of equipotential curves on the sphere, allows simultaneously the magnitude and orientation of the tangential and normal vector fields to be visualized. Close to singular points, the integral curves correspond to power laws and the equipotentials correspond to conic sections. This theoretical analysis allows graphical method for estimating the stress ratio (02-aa)/(ot-03) from striated faults to be proposed, once the orientations of the principal stress directions are known (i.e. by means of other graphical methods).

Research paper thumbnail of A model of strain localization in porous sandstone as a function of tectonic setting, burial and material properties; new insight from Provence (southern France)

Journal of Structural Geology, 2013

The analysis of three cataclastic band sets from Provence (France) reveals that the band density,... more The analysis of three cataclastic band sets from Provence (France) reveals that the band density, their conjugate angles, their ratio of shear displacement to compaction, and the amount of cataclasis within the bands differ and can be expressed as functions of tectonic setting and petrophysical properties. We identify (1) a dense and closely spaced network of shear-enhanced (reverse) compaction bands; (2) a regularly spaced less dense network of reverse compactional shear bands; and (3) a localized network of normal shear bands. The field data show that strain localization is favored in an extensional regime and is characterized by shear bands with a large shear to compaction ratio and a small conjugate band angle. In contrast, distributed strain is favored in a contractional regime and is characterized by compactional bands with a low ratio of shear to compaction and a large conjugate band angle. To explain the mechanical origin of this strain localization, we quantify the yield strength and the stress evolution in extensional and contractional regimes in a frictional porous granular material. We propose a model of strain localization in porous sands as a function of tectonic stresses, burial depth, material properties, strain hardening and fluid pressure. Our model suggests that stress reduction, inherent to extensional regime, favors strain localization as shear bands, whereas stress increase during contraction favors development of compactional bands.

Research paper thumbnail of Correction to “Discrete element simulation of the Jiufengershan rock-and-soil avalanche triggered by the 1999 Chi-Chi earthquake, Taiwan”

Journal of Geophysical Research, 2009

Research paper thumbnail of Correction to “Rock-and-soil avalanches: Theory and simulation”

Journal of Geophysical Research, 2009

Research paper thumbnail of Rheology, force transmission, and shear instabilities in frictional granular media from biaxial numerical tests using the contact dynamics method

Journal of Geophysical Research: Solid Earth, 2005

Research paper thumbnail of Discrete element simulation of the Jiufengershan rock-and-soil avalanche triggered by the 1999 Chi-Chi earthquake, Taiwan

Journal of Geophysical Research, 2009

We present Contact Dynamics discrete element simulations of the earthquaketriggered Jiufengershan... more We present Contact Dynamics discrete element simulations of the earthquaketriggered Jiufengershan avalanche, which mobilized a 60 m thick, 1.5 km long sedimentary layer, dipping $22°SE toward a valley. The dynamic behavior of the avalanche is simulated under different assumptions about rock behavior, water table height, and boundary shear strength. Additionally, seismic shaking is introduced using strong motion records from nearby stations. We assume that seismic shaking generates shearing and frictional heating along the surface of rupture, which, in turn, may induce dynamic weakening and avalanche triggering; a simple ''slip-weakening'' criterion was adopted to simulate shear strength drop along the rupture surface. We investigate the mechanical processes occurring during triggering and propagation of an avalanche mobilizing shallowly dipping layers. Incipient deformation forms a pop-up structure at the toe of the dip slope. As the avalanche propagates, the pop-up deforms into an overturned fold, which overrides the surface of separation along a décollement. Simultaneously, uphill layers slide at high velocity (125 km/h) and are folded and disrupted as they reach the toe of the dip slope. The avalanche foot forms a wedge that is pushed forward as deformed rocks accrete at its rear. We simulated five cross sections across the Jiufengershan avalanche, which differ in the geometry of the surface of separation. Topographic and simulated surface profiles are similar. The friction coefficient at the surface of separation determined from back analysis is abnormally low (m SS = 0.2), possibly due to lubrication by liquefied soils. The granular deposits of simulated earthquake-and rain-triggered avalanches are similar.

Research paper thumbnail of Modern tectonic stress field in the Mediterranean region: evidence for variation in stress directions at different scales

Geophysical Journal International, 1992

A compilation of more than one thousand stress indicators (which include in situ stress measureme... more A compilation of more than one thousand stress indicators (which include in situ stress measurements, focal mechanisms, microtectonic and other geological data) allowed us to reconstruct the modern stress field in the Mediterranean region and the surrounding area. Average stress directions at different scales have been reconstructed by means of a linear interpolation method. This method takes into account the distribution, scale and quality of stress data. The results of the interpolation at plate scale, allow us to recognize slightly deformed regions such as the northwestern European platform, where average maximum horizontal stress direction is oriented roughly NNW-SSE, subparallel to absolute and relative plate velocity directions. Other regions such as the Caucasus, Alps and Pyrenees, where recent tectonic deformation and seismicity are present, display important variations of stress directions. The reconstruction of the average stress directions at different scales within the French Alps pointed out that the average stress field pattern may vary from one scale to another. Nevertheless, variations of stress directions at a given scale are consistent with the kinematics of faults of the same scale.