Papers by Franck Audemard
PLOS ONE, 2020
Intriguing latest Eocene land-faunal dispersals between South America and the Greater Antilles (n... more Intriguing latest Eocene land-faunal dispersals between South America and the Greater Antilles (northern Caribbean) has inspired the hypothesis of the GAARlandia (Greater Antilles Aves Ridge) land bridge. This landbridge, however, should have crossed the Caribbean oceanic plate, and the geological evolution of its rise and demise, or its geodynamic forcing, remain unknown. Here we present the results of a land-sea survey from the northeast Caribbean plate, combined with chronostratigraphic data, revealing a regional episode of mid to late Eocene, trench-normal, E-W shortening and crustal thickening by *25%. This shortening led to a regional late Eocene-early Oligocene hiatus in the sedimentary record revealing the location of an emerged land (the Greater Antilles-Northern Lesser Antilles, or GrANoLA, landmass), consistent with the GAARlandia hypothesis. Subsequent submergence is explained by combined trench-parallel extension and thermal relaxation following a shift of arc magmatism, expressed by a regional early Miocene transgression. We tentatively link the NE Caribbean intra-plate shortening to a well-known absolute and relative North American and Caribbean plate motion change, which may provide focus for the search of the remaining connection between 'GrANoLA' land and South America, through the Aves Ridge or Lesser Antilles island arc. Our study highlights the how regional geodynamic evolution may have driven paleogeographic change that is still reflected in current biology.
Journal of Seismology, Oct 2, 2007
A worldwide database of liquefactioninduced sand blows has been compiled to generate empirical re... more A worldwide database of liquefactioninduced sand blows has been compiled to generate empirical relationships between earthquake parameters and sand blow geometries. Curves resulting from these empirical relationships can be used afterwards for the study of historic and prehistoric earthquakes that formed sand blows. This database only incorporates instrumental earthquakes inducing sand blows, characterised in terms of magnitude (Mw), hypocentral location and focal mechanism solution. Two relationships are herein presented. The first curve, minimum likelihood of liquefaction occurrence (magnitude/epicentral distance), displays a logarithmic behaviour, as others already proposed. The second curve of sand-blow diameter vs epicentral distance is the first proposal of this kind, whose shape follows a negative power-law. Magnitude estimations of (pre-)historic earthquakes then may be attempted through these empirical relationships. Resulting magnitudes derived from these curves should be mostly considered as underestimated. The curves will reflect actual magnitudes only if, correspondingly, the farthest and largest blow has ever been reported during the after-earthquake survey. Following the same principle, a magnitude estimation resulting from the measuring of a sand blow can only be considered as minimal because it is almost impossible to ascertain that the measured feature is the biggest one produced by the earthquake under evaluation. Finally, these results call for thorough surveys of induced effects after every future moderate-to-large earthquake, as any empirical relationship simply improves by incorporating new data.
AGU Spring Meeting Abstracts, May 1, 2013
Tectonophysics, Apr 1, 2021
Abstract We explore the shear-wave lithospheric velocity structure of the Eastern Caribbean and N... more Abstract We explore the shear-wave lithospheric velocity structure of the Eastern Caribbean and Northern Venezuela using ambient noise tomography with stations deployed around the study area. We construct cross-correlation functions from continuous seismic records, and measure phase velocities of fundamental-mode Rayleigh waves. These velocities are further projected onto 0.6°x0.6° phase velocity grids for each period between 5 s and 50 s. The pseudo-dispersion curve at each grid point is inverted for 1D shear velocity profiles by using a Markov Chain Monte Carlo scheme. The interpolated 3D velocity model shows that the mean shear velocity of the Eastern Caribbean lithospheric mantle is lower than the global average, which is in agreement with values reported in other large igneous provinces. We interpret that low velocities in the lithospheric keel are associated with an anomalous composition and/or an elevated thermal state; this gives the Caribbean plate a high buoyancy that determines the subduction polarities in the region. The results also indicate that: (a) the mantle beneath Northern Venezuela retains compositional anomalies related to extension processes of different ages; (b) the overriding of the Caribbean plate by the Great Antilles arc seems to be much slower than previously suggested; and (c) the localized volcanism in the center of the Lesser Antilles arc is related to asthenospheric flow through the tear induced on the subducted slab by major strike-slip faults.
We use 1.5 years of continuous recordings from an amphibious seismic network deployment in the re... more We use 1.5 years of continuous recordings from an amphibious seismic network deployment in the region of northeast South America and southeast Caribbean to study the crustal and uppermost mantle structure through a joint inversion of surface wave dispersion curves determined from ambient seismic noise and receiver functions. The availability of both ocean bottom seismometers (OBSs) and land stations makes this experiment ideal to determine the best processing methods to extract reliable empirical Green's functions (EGFs) and construct a 3D shear velocity model. Results show EGFs with high signal-to-noise ratio for land-land, land-OBS and OBS-OBS paths from a variety of stacking methods. Using the EGF estimates, we measure phase and group velocity dispersion curves for Rayleigh and Love waves. We complement these observations with receiver functions, which allow us to perform an H-k analysis to obtain Moho depth estimates across the study area. The measured dispersion curves and receiver functions are used in a Bayesian joint inversion to retrieve a series of 1D shear-wave velocity models, which are then interpolated to build a 3D model of the region. Our results display clear contrasts in the oceanic region across the border of the strike-slip fault system San Sebastian-El Pilar as well as a high velocity region that corresponds well with the continental craton of southeastern Venezuela. We resolve known geological features in our new model, including the Espino Graben and the Guiana Shield provinces, and provide new information about their crustal structures. Furthermore, we image the difference in the crust beneath the Maturin and Guárico Sub-Basin.
Journal of South American Earth Sciences, Aug 1, 2021
Journal of Seismology, May 10, 2007
In light of the July 9, 1997, Cariaco earthquake, it is clearly understood now that damage in the... more In light of the July 9, 1997, Cariaco earthquake, it is clearly understood now that damage in the city of Cumanálocated in northeastern Venezuela and frequently destroyed by the largest earthquakes since the first recorded event in 1530is strongly enhanced by poor soil conditions that, in turn, are responsible for site amplification and widespread earthquake-induced effects. Therefore, most previous macroseismic studies of historical earthquakes must be revaluated because those localized high-intensity values at Cumaná surely led to the misestimation of past epicenters. Preliminary paleoseismic results, gathered at three exploratory trenches dug across the surface break of the Cariaco 1997 earthquake in 1998, allow us to associate the 1684 earthquake with this recently ruptured fault segment that extends between the towns of San Antonio del Golfo and Río Casanay (roughly between the two gulfs of Cariaco and Paria, state of Sucre). Other major results from the reassessment of the seismic history of this fault are: (a) the 1766 event seems to have generated in a different source to the El Pilar fault because the size of the felt area suggests that it is an intermediate-depth earthquake; (b) damage to Cumaná produced by the 1797 event suggests that this was a local earthquake, perhaps equivalent to the 1929 earthquake, which ruptured for some 30 km just east of Cumaná into the Gulf of Cariaco; and (c) seismogenic association of the 1530 and 1853 earthquakes still remains unclear but it is very likely that these ruptures occurred offshore, as suggested by the rather large tsunami waves that both events have generated, placing their hypocenters west of Cumaná in the Cariaco Trough. This reassessment also sheds light into the El Pilar fault segmentation and the behavior of its seismogenic barriers through time.
Quaternary International, 1996
Four marine terraces are identified on Paraguana at +80 m, +40-50 m, +15-20 m and +6 m, skirting ... more Four marine terraces are identified on Paraguana at +80 m, +40-50 m, +15-20 m and +6 m, skirting the 'inselberg' of cerro Santa Ana. Besides the Holocene marine deposits, patches of correlative sediments are only associated with the two youngest terraces within a narrow strip along the seashore because older Quaternary marine terraces are mainly erosional surfaces that truncate the uppermost limestone of the Pliocene Paraguana formation. Diverse marine deposits are observed between seashore and +5-6 m: sand barriers, mud flats, salt flats, stromatolites mats, coquinas and coral reefs in living position. Some of ~ facies are undoubtfully Holocene, but some others proved to be older than the J4C dating limit. Therefore, the latter ones should be ascribed to isotope-stage 5, but further sampling and dating are required to confirm their chronology. It is worth mentioning that probable stage-5 facies of Paraguana are as high as its worldwide undeformed equivalents, implying a certain tectonic stability during late Pleistocene. However, the assumption of a stable Par~uana block has to be modified since the existence of staircased marine terraces implies a certain amount of uplift, even if it is rather slow (< 0.8 mm/year).
Tectonophysics, Sep 1, 2006
This paper discusses the surface rupture of the Cariaco July 09, 1997 Ms 6.8 earthquake in northe... more This paper discusses the surface rupture of the Cariaco July 09, 1997 Ms 6.8 earthquake in northeastern Venezuelalocated at 10.545°N and 63.515°W and about 10 km deep. The field reconnaissance of the ground breaks confirms that this event took place on the ENE-WSW trending onshore portion of the dextral El Pilar fault (between the Gulfs of Cariaco and Paria), which is part of the major wrenching system within the Caribbean-South America plate boundary zone. Dextral slip along this fault was further supported by the structural style of this rupture (en echelon right-lateral R shears connected by mole tracks at restraining stepovers) and by larger geometric complexities (pop-ups at Las Manoas and Guarapiche), as well as by the focal mechanism solutions determined for the event by several authors. This 1997 surface ruptre comprised two distinct sections, from west to east: (a) a main very conspicuous, continuous, 30-km-long, rather straight, 075°N-trending alignment of en echelon surface breaks, with a rather constant, purely dextral coseismic slip of about 25 cm, but reaching a maximum value of 40 cm slightly northwest of Pantoño; and (b) a secondary discontinuous, 10-km-long, boomerang-shaped rupture, with a maximum coseismic slip of 20 cm at Guarapiche. The onshore extent of the surface rupture totalled 36 km, but may continue westward underwater, as suggested by the very shallow aftershock seismicity. This aftershock activity also clearly defined the steep north dip of the fault plane along the western rupture, suggesting tectonic inheritance on this major fault. From many locals' accounts, the rupture seems to have propagated from Pantoño to the west (highly asymmetric bidirectionality). This suggests that earthquake nucleation happened at or near the Casanay-Guarapiche restraining bend and rupture quickly propagated westward, allowing only a small fraction to progress eastwards beyond the bend. Additionally, the large fraction of after-slip (or creep) released is to be related to such restraining bend, which seems to have partly locked slip during rupture.
Geophysical Journal International, Oct 1, 2018
We study the isostatic behaviour of major structures in the Eastern Caribbean, namely the Aves Ri... more We study the isostatic behaviour of major structures in the Eastern Caribbean, namely the Aves Ridge, the Venezuela Basin, the Grenada Basin and the Lesser Antilles Arc. Based on isostatic regional and residual anomalies, gravitational admittance and flexural modelling, we analyse topographic, gravity and crustal thickness information. We fiend that the Airy-Heiskanen model better explains mass distribution of the Eastern Caribbean crust, except for the Grenada Basin, where the Pratt-Hayford model fits data better. Elastic thickness in the region varies between 5 and 70 km with most typical values ranging between 17 and 24 km. The Aves Ridge is close to the Airy-type isostasy, where deviations are explained by low elastic thickness values beneath the structure (∼5 km) suggesting that its load is mostly supported by the lithospheric mantle. The Venezuela Basin is out of isostatic compensation, and is most likely still in subsidence due to the extension suffered in the Late Cretaceous. The Grenada Basin is a Wernicke-type extensional basin, while the Tobago Basin is a forearc-type basin closed by a remanent piece of the old South American passive margin. Lastly, the crust to the northeast of the Aves Ridge (referred in this paper as the 'modified Saba crust') has more affinity to the ridge than to the Grenada Basin, as suggested by some authors. We hypothesize that its thick crust is the result of the underplating of material derived from a subduction with a slow rollback velocity.
Revista de la Facultad de Ingeniería Universidad Central de Venezuela, Jul 24, 2020
La tectonica activa en Venezuela ha sido largamente trabajada por varios autores durante decadas.... more La tectonica activa en Venezuela ha sido largamente trabajada por varios autores durante decadas. Con el proposito de estudiar la cinematica inter-sismica de fallas geologicas en el occidente venezolano, se llevaron a cabo campanas de adquisicion de datos empleando instrumental del Global Navigation Satellite System (GNSS) en los anos 2011, 2013 y 2016. La medicion con la tecnica GNSS aporta datos precisos, los cuales proporcionan una gran informacion en la medicion de los desplazamientos tectonicos, y permite determinar los campos de velocidades geodesicos en un corto periodo. Con los datos obtenidos se obtuvieron los modelos: elastico simple y asimetrico para la falla de Oca-Ancon; un modelo de dos fallas que incluye la falla de Oca-Ancon y las secciones mas septentrionales de la falla de Bocono; y un modelo elastico simple para la zona norte de la falla de Bocono. Los mejores modelos indican una velocidad de campo lejano de 1,50 ± 0,05 mm a -1 y una profundidad de bloqueo de 16,5 ± 3,0 km para la falla Oca-Ancon, y para la zona norte de la falla de Bocono, se estimo una velocidad de 15-16 mm a -1 con una profundidad de bloqueo de 12,0 km. El sistema de fallas Oca-Ancon acomoda aproximadamente un 7-8 por ciento del desplazamiento lateral derecho entre las placas Caribe y Suramerica, mientras que la falla de Bocono acomoda aproximadamente 80 por ciento de los 20 mm a -1 .
Boletin de Geología, Dec 1, 2014
En el presente trabajo de investigación se realiza un estudio geológico del complejo lagunar Los ... more En el presente trabajo de investigación se realiza un estudio geológico del complejo lagunar Los Patos, en un ambiente próximo costero y se busca verificar la existencia de paleotsunamis en el oriente del país, mediante estudios sedimentológicos, geoquímicos y paleontológicos a través de núcleos no perturbados tomados en dicha albufera en la ciudad de Cumaná. Fueron analizados los núcleos L-Patos 09-01 y L-Patos 09-02, donde la geoquímica se aplicó al núcleo L-Patos 09-01 mediante difracción de rayos X, análisis de elementos mayoritarios (fluorescencia de rayos X-FRX-) y dataciones 14 C, mientras el núcleo L-Patos 09-02 se utilizó para la realización de análisis de susceptibilidad magnética. Los resultados mostraron una secuencia cuaternaria de sedimentación típica de ambientes lagunares costeros, con un predominio de facies arcillosas y limosas, con escasos intervalos delgados de arenas muy finas. Mineralógicamente, la fracción detrítica se encuentra representada por el cuarzo y minerales de arcilla (ilita y caolinita); ambos se presentan como constituyentes esenciales de los sedimentos. Así mismo, se presentan minerales evaporíticos como yeso y halita. Se establecieron cinco facies sedimentarias que mostraron características texturales químicas y mineralógicas diferentes. Así mismo, se demostró que las arenas limpias ubicadas en el tope de la secuencia, presentaron características sedimentológica (bases erosivas, geoquímica (aumento de Ca y S) y paleontológicas (restos de plantas y raíces), que permiten evidenciar la ocurrencia de paleotsunamis en la laguna Los Patos, Venezuela.
, and the GARANTI cruise team Deep Structure of the Grenada Basin From Wide-angle Seismic, Bathym... more , and the GARANTI cruise team Deep Structure of the Grenada Basin From Wide-angle Seismic, Bathymetric and Gravity Data The Grenada back-arc basin is located between the Aves Ridge, which hosted the remnant Early Paleogene "Great Caribbean Arc," and the Eocene to Present Lesser Antilles Arc. The main aim of this study is to constrain the circumstances leading to the opening of the basin. Three combined wide-angle and reflection seismic profiles were acquired in the Grenada basin. 39 OBS were deployed along the profil GA01, 40 along the profil GA02, and 37 along the profil GA03. The wide-angle seismic data were modeled with the "rayinvr" software of Colin Zelt (Zelt & Smith, 1992) and its combined inverse and forward approach. The velocity-depth curves from the Aves and active LAA region show that seismic velocities in both structures are similar indicating that the composition of rocks at Aves Ridge is similar to that of the active arc Ridge and the modern arc with other regions shows similarities with other volcanic arcs, such as the Izu-Bonin or Aleutians. We propose that the Grenada Basin opened in an oblique way and under influence of moderate volcanism.
La Falla de Ibagué presenta rasgos contundentes de actividad neotectónica asociados con su trazo,... more La Falla de Ibagué presenta rasgos contundentes de actividad neotectónica asociados con su trazo, los cuales la definen como una estructura potencialmente productora de grandes sismos. En el campo cercano existen importantes centros poblados del país con cerca de 13 millones de habitantes (Ibagué, Bogotá, Armenia, Pereira y Manizales son los más importantes). Por esta razón, el panorama de la amenaza sísmica asociada a esta estructura es evaluado por medio de una combinación de diferentes técnicas para conocer su verdadero potencial sismogénico. Las disciplinas de la neotectónica, como la morfotectónica y la paleosismología, son imprescindibles para ampliar el tiempo de observación y para determinar el grado de actividad sísmica derivada de una estructura geológica. La morfotectónica estudia las geoformas creadas por las fallas activas, particularmente las fallas sismogénicas; la paleosismología integra estudios geomorfológicos y estratigráficos por medio de la correlación de las ge...
International Journal of Earth Sciences
The Code described in the manuscript
Journal of South American Earth Sciences, 2016
Preface to the special issue on "Regional moment tensors and stress field in South and Central Am... more Preface to the special issue on "Regional moment tensors and stress field in South and Central America" This special issue follows from the Symposium "Regional Moment Tensor Solutions: advances and new applications" held in Bogot a, Colombia, at the I Regional Assembly of the IASPEI's Latin American and Caribbean Seismological Commission (LACSC) in 2014. Seven papers are presented dealing with determination of moment tensors, focal mechanisms and the stress field in Central and South America. The study areas of each paper are indicated in the index Map of Fig. 1. An interesting analysis of all seismic moment tensors in the Cocos plate subduction zone by Guzm an-Speziale and Zúñiga shows important differences between the sections underneath the North American and the Caribbean plates. Well-located hypocenters show the Cocos plate has a shallower dip beneath the North American and a steeper dip beneath the Caribbean plate. Beneath the Contents lists available at ScienceDirect
Journal of Geophysical Research: Solid Earth, 2016
In eastern Venezuela, the Caribbean‐South American plate boundary follows the El Pilar fault syst... more In eastern Venezuela, the Caribbean‐South American plate boundary follows the El Pilar fault system. Previous studies based on three GPS campaigns (2003–2005–2013) demonstrated that the El Pilar fault accommodates the whole relative displacement between the two tectonic plates (20 mm/yr) and proposed that 50–60% of the slip is aseismic. In order to quantify the possible variations of the aseismic creep in time and space, we conducted an interferometric synthetic aperture radar (InSAR) time series analysis, using the (NSBAS) New Small BAseline Subset method, on 18 images from the Advanced Land Observing Satellite (ALOS‐1) satellite spanning the 2007–2011 period. During this 3.5 year period, InSAR observations show that aseismic slip decreases eastward along the fault: the creep rate of the western segment reaches 25.3 ± 9.4 mm/yr on average, compared to 13.4 ± 6.9 mm/yr on average for the eastern segment. This is interpreted, through slip distribution models, as being related to coup...
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Papers by Franck Audemard