Ecole et Observatoire des science de la Terre

Strasbourg, France

Ecole et Observatoire des science de la Terre

Strasbourg, France
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Hofstetter A.,Geophysical Institute of Israel | Dorbath C.,Ecole et Observatoire des science de la Terre | Dorbath L.,Ecole et Observatoire des science de la Terre | Braeuer B.,German Research Center for Geosciences | Weber M.,German Research Center for Geosciences
Journal of Seismology | Year: 2016

We use the recorded seismicity, confined to the Dead Sea basin and its boundaries, by the Dead Sea Integrated Research (DESIRE) portable seismic network and the Israel and Jordan permanent seismic networks for studying the mechanisms of earthquakes in the Dead Sea basin. The observed seismicity in the Dead Sea basin is divided into nine regions according to the spatial distribution of the earthquakes and the known tectonic features. The large number of recording stations and the adequate station distribution allowed the reliable determinations of 494 earthquake focal mechanisms. For each region, based on the inversion of the observed polarities of the earthquakes, we determine the focal mechanisms and the associated stress tensor. For 159 earthquakes, out of the 494 focal mechanisms, we could determine compatible fault planes. On the eastern side, the focal mechanisms are mainly strike-slip mechanism with nodal planes in the N-S and E-W directions. The azimuths of the stress axes are well constrained presenting minimal variability in the inversion of the data, which is in agreement with the Eastern Boundary fault on the east side of the Dead Sea basin and what we had expected from the regional geodynamics. However, larger variabilities of the azimuthal and dip angles are observed on the western side of the basin. Due to the wider range of azimuths of the fault planes, we observe the switching of σ1 and σ2 or the switching of σ2 and σ3 as major horizontal stress directions. This observed switching of stress axes allows having dip-slip and normal mechanisms in a region that is dominated by strike-slip motion. © 2016 Springer Science+Business Media Dordrecht

Kopp J.-B.,ICube | Schmittbuhl J.,Ecole et Observatoire des science de la Terre | Lin J.,ICube | Fond C.,ICube
13th International Conference on Fracture 2013, ICF 2013 | Year: 2013

For rapid crack propagations, two kinds of material behavior have been observed. Typically, most materials show an increase of the fracture energy with the crack tip velocity. However, there do exist a few materials for which the fracture energy tends to decrease with the velocity [1, 2]. They are viscoplastic blend materials like polymers such as rubber toughened polymethylmethacrylate (RT-PMMA). For these materials, crack tips are seen to propagate at the same velocity whatever the loading rate is (or strain energy release rate). This critical velocity has been shown to be the crack branching velocity, at least at a macroscopic scale. Our study shows that the classical approach which considers that the amount of created surface during the propagation can be approximated as the sample thickness multiplied by the crack length is not appropriate. Indeed, this study shows that the exact fracture surface roughness has to be included in the amount of created surface in order to establish an intrinsic material fracture energy GID. As the fracture surface roughness depends on the scale at which the sample is observed, a self-affine model widely used for fracture surfaces is introduced [3, 4]. This statistical geometrical model of the fracture surface with two parameters, a Hurst exponent and a topothesy is shown to be effective and provides a better estimate of the intrinsic surface fracture energy. Copyright © (2013) by International Conference on Fracture.

Deparis V.,77 Clos le Pastoral | Legros H.,Ecole et Observatoire des science de la Terre | Souchay J.,Paris Observatory
Lecture Notes in Physics | Year: 2013

Tidal phenomena along the coasts were known since the prehistoric era, but a long journey of investigations through the centuries was necessary from the Greco-Roman Antiquity to the modern era to unravel in a quasi-definitive way many secrets of the ebb and flow. These investigations occupied the great scholars from Aristotle to Galileo, Newton, Euler, d'Alembert, Laplace, and the list could go on. We will review the historical steps which contributed to an increasing understanding of the tides. © Springer-Verlag 2013.

Hofstetter R.,Geophysical Institute of Israel | Dorbath C.,Ecole et Observatoire des science de la Terre | Dorbath C.,IRD Montpellier | Calo M.,Ecole et Observatoire des science de la Terre
Geophysical Journal International | Year: 2012

New findings of the velocity structure of the crust across the Dead Sea Basin are obtained by applying tomography-based method to local earthquakes. We use P-wave traveltime of 614 earthquakes that occurred in the Dead Sea Basin in 1983-2009. At all depths, the Dead Sea Basin is characterized by lower velocities relative to both the eastern and western sides of the basin. There is significant seismic activity at a depth of about 20 km, mainly in the centre and the northern part of the basin. At shallow depths (<15 km) there is more seismic activity on the eastern side of the basin than on the western side, and the northern basin is generally more active than the southern basin. Asymmetry is also observed in the faults that border the Dead Sea Basin. The Arava Fault on the eastern side, with nearly vertical dip faulting, appears to be a clear boundary at all depths down to about 20 km. The depth extension of the Jericho Fault on the western side of the basin is definitely limited to less than 15 km. At greater depths of 20 km or more, the western side is only partially bounded by a fault. The concentration of earthquakes in the central part of the basin at depths larger than 15 km suggests that the Dead Sea Fault at those depths acts as one single fault that is located in, or near, the central axis of the basin. The existence of a number of clusters of earthquakes that spread from shallow depths of a few kilometres up to a depth of about 22 km, points to several defined faults that traverse the Dead Sea Basin. One such example is the aftershock sequence of the 2004 earthquake that occurred in the northern Dead Sea Basin. Seismic activity near Mt Sodom is relatively low and occurs at shallow depths down to 10 km, whereas at larger depths (≥15 km) it ceases. This implies that the whole structure is relatively shallow with no wide and deeper extension. Seismic activity near and within the Lisan Peninsula extends to somewhat larger depths (∼15 km), and then it ceases. The occurrence of earthquakes at large depths suggests that the upper and the lower crust are relatively cool, as was also suggested by earlier studies, pointing to the fact that the heat flow is significantly below the global average value. © 2012 The Authors Geophysical Journal International © 2012 RAS.

Hibert C.,CNRS Paris Institute of Global Physics | Hibert C.,Bureau de Recherches Géologiques et Minières | Hibert C.,Lamont Doherty Earth Observatory | Mangeney A.,CNRS Paris Institute of Global Physics | And 10 more authors.
Journal of Geophysical Research: Earth Surface | Year: 2014

Since the collapse of the Dolomieu crater floor at Piton de la Fournaise Volcano (la Réunion) in 2007, hundreds of seismic signals generated by rockfalls have been recorded daily at the Observatoire Volcanologique du Piton de la Fournaise (OVPF). To study rockfall activity over a long period of time, automated methods are required to process the available continuous seismic records. We present a set of automated methods designed to identify, locate, and estimate the volume of rockfalls from their seismic signals. The method used to automatically discriminate seismic signals generated by rockfalls from other common events recorded at OVPF is based on fuzzy sets and has a success rate of 92%. A kurtosis-based automated picking method makes it possible to precisely pick the onset time and the final time of the rockfall-generated seismic signals. We present methods to determine rockfall locations based on these accurate pickings and a surface-wave propagation model computed for each station using a Fast Marching Method. These methods have successfully located directly observed rockfalls with an accuracy of about 100 m. They also make it possible to compute the seismic energy generated by rockfalls, which is then used to retrieve their volume. The methods developed were applied to a data set of 12,422 rockfalls that occurred over a period extending from the collapse of the Dolomieu crater floor in April 2007 to the end of the UnderVolc project in May 2011 to identify the most hazardous areas of the Piton de la Fournaise volcano summit. Key Points New seismology-based method for the identification of rockfalls New method for the picking of the rockfall seismic signals emergent onsets New seismology-based method for the location of rockfalls ©2014. American Geophysical Union. All Rights Reserved.

Hofstetter A.,Geophysical Institute of Israel | Dorbath C.,Ecole et Observatoire des science de la Terre
Journal of Geophysical Research B: Solid Earth | Year: 2014

New findings of the structure of the Dead Sea sedimentary basin and its eastern and western bordering regions are obtained by P and PKP wave relative traveltime residuals of 644 teleseisms, as recorded by the Dead Sea Integrated Research portable seismic network in the Dead Sea basin and its neighboring regions. The Lisan Peninsula is characterized by relatively small teleseismic traveltime residuals of about 0.14 s, in the latitude range of 31.22°-31.37° and at the longitude of 35.50°, slowly decreasing toward the west. The largest teleseismic traveltime residuals are in the southern Dead Sea basin, south of the Lisan Peninsula in the latitude range of 31.05°-31.15° and along longitude 35.45° and continuing southward toward the Amaziahu Fault, reaching values of 0.4-0.5 s. There is a small positive residual at the Amaziahu Fault and a small negative residual south of it probably marking the southern end of the Dead Sea basin. East and west of the Dead Sea basin the mean teleseismic traveltime residuals are negative with overall averages of -0.35 s and -0.45 s, respectively. Using the teleseismic residuals, we estimate the horizontal dimensions of the Lisan salt diapir to be 23 km × 13 km at its widest and a maximal thickness of about 7.2 km. The thickness of the Mount Sodom salt diapir is estimated as 6.2 km. ©2014. American Geophysical Union. All Rights Reserved.

Wawrzyniak P.,Bureau de Recherches Géologiques et Minières | Sailhac P.,Ecole et Observatoire des science de la Terre | Marquis G.,Royal Dutch Shell
Geophysical Prospecting | Year: 2013

We propose here a new, robust, methodology to estimate the errors on a magnetotelluric (MT) impedance tensor. This method is developed with the bounded influence remote-reference processing (BIRRP) code in a single site configuration. The error is estimated by reinjecting an electric field residual obtained after the calculation of an impedance tensor into a tensor function calculation procedure. We show using synthetic examples that the error tensor calculated with our method yields a more reliable error estimate than the one calculated from Jackknife statistics. The modulus of realistic error estimates can be used as a quality control and an accurate inversion constraint of MT surveys. Moreover, reliable error estimates are necessary for new applications of MT to dynamic subsurface processes such as reservoir monitoring. © 2013 European Association of Geoscientists & Engineers.

Baillieux P.,University of Neuchatel | Schill E.,GEIE Exploitation Miniere de la Chaleur | Edel J.-B.,Ecole et Observatoire des science de la Terre | Mauri G.,University of Neuchatel
International Geology Review | Year: 2013

The European Cenozoic Rift System hosts major temperature anomalies in Central Europe. In its central segment, the Upper Rhine Graben (URG), temperatures range from 75°C to nearly 150°C at a depth of 2000 m. Different hypotheses have been suggested to explain the localization of these anomalies. Our review and comprehensive interpretation of gravimetric and magnetic data, as well as neotectonic activity patterns, suggests that low-density, mostly magnetic and fractured granitic basement is systematically associated with major temperature anomalies. Further analyses provide insight into different heat transport processes contributing to the localization of these anomalies. Magnetic and gravity anomalies are known to represent lithological variations associated with the pre-Permian.We show their spatial relationship with positive temperature anomalies in the URG. Correlation between magnetics and temperature reveal a mean contribution of heat production to the temperature anomaly of about 10-15°C. A slightly higher mean value is obtained from correlation between gravity and temperature, which may be attributed to effects resulting from fracture porosity. The spatial relationship between temperature anomalies and neotectonic patterns indicates compressional shear and uplift regime for the major anomalies of the central segment of the URG. This is in agreement with different numerical models indicating free convection on fracture zones linked to faults. Our findings show that about 15-25% of the temperature anomaly can be attributed to variation in heat production. Hydrothermal circulation convection along faults, activated by the tectonic context, may explain the remaining 75-85% of the temperature anomalies. © 2013 Taylor & Francis.

Bano M.,Ecole et Observatoire des science de la Terre
Proceedings of 2016 16th International Conference of Ground Penetrating Radar, GPR 2016 | Year: 2016

A GPR system is composed of a transmitter and a receiver antenna placed on the ground surface. The exciting current injected into the transmitter antenna is, in general, unknown. Its knowledge is of great importance in the GPR modeling in time or frequency. In the following analysis we, firstly, identify different types of GPR waves for two parallel dipole antennas located on the ground in a common mid-point (CMP) configuration. Furthermore, we study the polarity of different arrivals from the numerical results calculated in the frequency domain and based on the knowledge of the current injected. A reference radar source can be obtained by taking the first derivative of the direct air wave in real field CMP data. For small offsets the direct waves interfere and the final result approximates the first derivative of the direct air wave. © 2016 IEEE.

Bouche E.,CNRS Paris Institute of Global Physics | Vergniolle S.,CNRS Paris Institute of Global Physics | Staudacher T.,CNRS Paris Institute of Global Physics | Nercessian A.,CNRS Paris Institute of Global Physics | And 3 more authors.
Earth and Planetary Science Letters | Year: 2010

The activity at the surface of the lava lake on Erta 'Ale volcano (Ethiopia) shows that large bubbles are regularly breaking at a fixed position on the lava lake. This is also where the small lava fountains are sometimes produced. Since this location is likely to be directly above the volcanic conduit feeding the lava lake, we have done continuous measurements between March 22 and 26, 2003 to understand the degassing of a volcano in permanent activity. The bubble size has been first estimated from videos, which once combined with the acoustic pressure, can constrain the source of the sound. The gas volume and overpressure stayed roughly constant, between 36-700m3 and 4×103-1.8×104Pa, respectively. Simultaneous thermal measurements showed regular peaks, which occurred when the crust was broken by a large bubble, hence gave a direct indication on the typical return time between the bubbles (1h). These spherical cap bubbles had a high Reynolds number, 4600-20000, therefore a wake, periodically unstable, formed and detached from the bubble bottom. The bubbly wake, if the detachment occurs close to the surface, can explain the duration of lava fountains, measured on the videos. The periodic arrival of bubbly wakes, which mostly detach from the driving spherical cap within the lava lake, could explain the absence of cooling at Erta 'Ale, Erebus (Antartica), Villarica (Chile) and Nyiragongo (Democratic Republic of Congo) without invoking a convective downflow of magma in the conduit, as previously done. © 2010 Elsevier B.V.

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