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Konstantinou K.I.,National Central University | Lee S.-J.,Academia Sinica, Taiwan | Font Y.,Geoazur | Kao H.,Geological Survey of Canada
Physics of the Earth and Planetary Interiors | Year: 2011

The southern Ryukyus represents an area where different tectonic stress regimes result in high seismicity and increased seismic hazard for nearby areas such as Taiwan. On 18 December 2001 at 04:03 (GMT) a strong earthquake (Mw 6.8) occurred in the forearc area of the southern Ryukyu subduction zone. Revised moment tensor solutions published by GCMT and BATS groups show a normal faulting mechanism with some strike-slip component and also point to a shallow focal depth (∼12. km). We use arrival times picked at both Taiwanese and Japanese stations along with a 3D geo-realistic a priori velocity model in order to obtain accurate absolute locations for the mainshock and 153 of its aftershocks. Locations are derived by using the Maximum intersection (MAXI) algorithm which has been used in many previous seismicity studies in the southern Ryukyus. These improved locations indicate that the mainshock was caused by the failure of a NE-SW oriented fault that extends from the edge of the Nanao forearc sedimentary basin to the Ryukyu arc basement. Far-field P and SH waveforms of the mainshock recorded at stations surrounding the source and at distances 30-100°, were inverted for the purpose of investigating its rupture process. A non-negative least-squares inversion technique utilizing multiple time windows was used to derive the spatio-temporal slip distribution. The preferred slip distribution model shows that there is one large area of high slip (∼0.9. m) at 5-15. km depth that essentially represents the crystalline rocks of the Ryukyu arc basement. Another smaller area with lower slip (∼0.4. m) extends at 10-15. km depth beneath the Nanao basin. Most aftershocks are located in areas of low slip (<0.4. m) filling the regions of slip deficit. It is likely that the 18 December 2001 earthquake was caused by a stress field interaction generated by the oblique subduction of the Gagua ridge and the gravitational forces acting at its landward flank. © 2011 Elsevier B.V..

Marot M.,GeoAzur | Monfret T.,GeoAzur | Pardo M.,University of Chile | Ranalli G.,Carleton University | Nolet G.,GeoAzur
Journal of Geophysical Research: Solid Earth | Year: 2013

The region of central Chile offers a unique opportunity to study the links between the subducting Juan Fernandez Ridge, the flat slab, the double seismic zone (DSZ), and the absence of modern volcanism. Here we report the presence and characteristics of the first observed DSZ within the intermediate-depth Nazca slab using two temporary seismic catalogs (Ovalle 1999 and Chile Argentina Seismological Measurement Experiment). The lower plane of seismicity (LP) is located 20-25 km below the upper plane, begins at 50 km depth, and merges with the lower plane at 120 km depth, where the slab becomes horizontal. Focal mechanism analysis and stress tensor calculations indicate that the slab's state of stress is dominantly controlled by plate convergence and overriding crust thickness: Above 60-70 km depth, the slab is in horizontal compression, and below, it is in horizontal extension, parallel to plate convergence, which can be accounted for by vertical loading of the overriding lithosphere. Focal mechanisms below 60-70 km depth are strongly correlated with offshore outer rise bend faults, suggesting the reactivation of preexisting faults below this depth. The large interplane distances for all Nazca DSZs can be related to the slab's unusually cold thermal structure with respect to its age. Since LPs globally seem to mimic mantle mineral dehydration paths, we suggest that fluid migration and dehydration embrittlement provide the mechanism necessary to weaken the rock and that the stress field determines the direction of rupture. ©2013. American Geophysical Union. All Rights Reserved.

Theunissen T.,Montpellier University | Font Y.,Geoazur | Lallemand S.,Montpellier University | Lallemand S.,LIA Associated International Laboratory | Liang W.-T.,Academia Sinica, Taiwan
Geophysical Journal International | Year: 2010

The Ryukyu subduction is known to generate very few earthquakes in its central segment contrarily to its two extremities. We focus in this study on the southernmost part of the Ryukyu subduction zone offshore east Taiwan. Our first task was to build a homogeneous earthquake catalogue for the period 1900-2007. The new catalogue provides homogenized M'W magnitudes and shows that several M'W≥ 7.0 earthquakes occurred offshore Hualien and Suao cities. We then focused on the 1920 June 5 earthquake (reported surface wave magnitude 8.1) previously located beneath the accretionary prism. The revised moment magnitude has been estimated in our catalogue at 7.7 ± 0.2. It is the biggest earthquake ever recorded in the Taiwan area but the fault that has produced this earthquake has not yet been identified with confidence. We relocated this event using regional phases (seismological bulletins archived at the Central Weather Bureau of Taiwan) about 50 km NNE and shallower of its former location, that is, within the Ryukyu Arc basement. According to earthquake bulletin information, revised magnitude, new hypocentral determination and known regional faults, we propose four potential active faults as candidates for the slip associated to this event: (1) the interplate seismogenic zone (ISZ), (2) an out-of-sequence thrust cutting through the forearc and branching on the ISZ, (3) a NS strike-slip fault cutting through the Ryukyu arc and (4) a N-S, westward dipping thrust fault, affecting the Philippine Sea Plate east of the Luzon Arc. The best compromise is to consider a rupture along the ISZ with a shallow nucleation possibly along a splay-fault followed by a downward and lateral propagation of the rupture that would explain the lack of significant seafloor motion and subsequent tsunami. We also estimate the maximum seismic coupling of the ISZ in the region east of Taiwan to about 0.4. In parallel, the evidences of aseismic slip occurring along the ISZ allow us to conclude that this region should only be affected by M < 8 earthquakes. © 2010 The Authors Geophysical Journal International © 2010 RAS.

Camera L.,Geoazur | Mascle J.,French National Center for Scientific Research | Wardell N.,National Institute of Oceanography and Applied Geophysics - OGS | Accettella D.,National Institute of Oceanography and Applied Geophysics - OGS
Bollettino di Geofisica Teorica ed Applicata | Year: 2014

Funded by the EEC Sixth Framework Program, the SEAHELLARC project was aimed to evaluate, and better understand the causes, of the various natural geohazards (chiefly earthquakes and tsunamis), which frequently affect the western Peloponnese area and particularly its coastal domain; this region is one of the most seismically active of Greece and therefore of the Mediterranean Sea. Based on a set of new geophysical data, such as detailed swath bathymetry and high-resolution sub-bottom Chirp, we have distinguished and studied four contrasted domains along this area of the Peloponnese active continental margin underlined by intense crustal seismicity and marked by very contrasted and often sharp continental slopes; from east to west these are: (1) an area including the continental shelf and the upper slope; there sedimentary overload and destabilizations, syn-sedimentary faults, mass transport deposits and active sedimentary by-pass mechanisms are the main risk factors. (2) The middle to lower continental slopes, is mainly expressed by two, N-S trending, faultrelated, depressions, where active deformations, well recorded by actual tilting of the sedimentary blanket, occur. (3) West of this deep structural depressions exists a poorly sedimented ridge area (from which merges the small Strophades Islands) also showing N-S and E-W trending lineaments resulting in a dense network of fractures and scarps and leading too a particularly complex sub-marine morphology; this area, together with the westernmost deep domain (4), which bounds the continental margin, clearly records the effects of significant active tectonic. Our studies of the shallow and recent sedimentary cover of the continental margin off western Peloponnese, confirm that this active margin segment is an area where geohazards can be expected. In addition to fault ruptures, generated at depth by the specific tectonic framework, sedimentary collapses, particularly along the shelf break nearby Cape Katakolo, may trigger significant local tsunamis, which may in turn induce strong damages all along the nearby coasts up to the town of Pylos. © 2014 - OGS.

Papoulia J.,Hellenic Center for Marine Research | Makris J.,GeoPro | Mascle J.,Geoazur | Slejko D.,National Institute of Oceanography and Applied Geophysics - OGS | Yalciner A.,Ankara University
Bollettino di Geofisica Teorica ed Applicata | Year: 2014

The SEAHELLARC project, supported by the European Commission, aimed at evaluating and computing seismic hazard and risk, as well as modelling tsunamis for the town of Pylos, in the western Peloponnese and neighboring region. This paper describes the main scopes and results of the project. © 2014 - OGS.

Mascle J.,French National Center for Scientific Research | Mary F.,Geoazur | Praeg D.,National Institute of Oceanography and Applied Geophysics - OGS | Brosolo L.,French National Center for Scientific Research | And 3 more authors.
Geo-Marine Letters | Year: 2014

Existing knowledge on the distribution of mud volcanoes (MVs) and other significant fluid/free gas-venting features (mud cones, mud pies, mud-brine pools, mud carbonate cones, gas chimneys and, in some cases, pockmark fields) discovered on the seafloor of the Mediterranean Sea and in the nearby Gulf of Cadiz has been compiled using regional geophysical information (including multibeam coverage of most deepwater areas). The resulting dataset comprises both features proven from geological sampling, or in situ observations, and many previously unrecognized MVs inferred from geophysical evidence. The synthesis reveals that MVs clearly have non-random distributions that correspond to two main geodynamic settings: (1) the vast majority occur along the various tectono-sedimentary accretionary wedges of the Africa-Eurasia subduction zone, particularly in the central and eastern Mediterranean basins (external Calabrian Arc, Mediterranean Ridge, Florence Rise) but also along its westernmost boundary in the Gulf of Cadiz; (2) other MVs characterize thick depocentres along parts of the Mesozoic passive continental margins that border Africa from eastern Tunisia to the Levantine coasts, particularly off Egypt and, locally, within some areas of the western Mediterranean back-arc basins. Meaningfully accounting for MV distribution necessitates evidence of overpressured fluids and mud-rich layers. In addition, cross-correlations between MVs and other GIS-based data, such as maps of the Messinian evaporite basins and/or active (or recently active) tectonic trends, stress the importance of assessing geological control in terms of the presence, or not, of thick seals and potential conduits. It is contended that new MV discoveries may be expected in the study region, particularly along the southern Ionian Sea continental margins. © 2014 Springer-Verlag Berlin Heidelberg.

Joanne C.,GeoAzur | Collot J.-Y.,GeoAzur | Lamarche G.,NIWA - National Institute of Water and Atmospheric Research | Migeon S.,GeoAzur
Marine Geology | Year: 2010

During the Pleistocene, the Matakaoa Debris Avalanche (MDA) removed ~ 430 km3 of material and formed a 50 km-long re-entrant in the Matakaoa margin, offshore East Cape, New Zealand. In this study, we examine the post-avalanche processes of continental slope reconstruction via the analysis of bathymetry, seismic-reflection data and one sediment core. The study reconstructs the post-MDA history of the Matakaoa margin and reveals how the giant mass-transport event laid the foundation for present variations in the margin's morphology. After the MDA, 600 ± 150 ka, the high terrigenous discharge of the Raukumara Peninsula rivers (> 38 Mt/yr at present) contributed to the construction of two depositional systems that each are up to 800 m thick: 1) the Pleistocene Sediment Wedge (PSW), resulting from detritic-rich hemipelagic sedimentation, partly reconstructed the slope and healed the western section of the MDA scarp; and 2) the Matakaoa Turbidite System (MTS) developed at the outlet of the Matakaoa Canyon that is fed by the Waiapu River, and nestled against the eastern section of the MDA scarp. In the eastern half of the re-entrant, the topography resulting from the avalanche included a 25 km-wide confined depocentre where the aggradational MTS started to construct through the development of a sediment fan and a turbidite plain. The construction of these two architectural elements at the base of the continental slope lowered the slope angle which restrained further destabilization in the eastern half of the re-entrant. In contrast, in the re-entrant's western half, slope reconstruction (deposition of PSW) has been counteracted by destabilization processes including: localized erosion associated with the headward development of slope canyons, and broad-scale failures resulting from differential compaction and gravity tectonics. Such instability may have contributed to produce the ~ 1000 km3 Matakaoa Debris Flow (38-100 ka). Our analysis points out that a collapsed margin subject to high sedimentation rates is likely to undergo further mass-failures on the long-term (> 300 ka). © 2009 Elsevier B.V. All rights reserved.

Verbeke J.,ETH Zurich | Boschi L.,ETH Zurich | Boschi L.,University of Zürich | Stehly L.,Geoazur | And 2 more authors.
Geophysical Journal International | Year: 2012

We present a new database of surface wave group and phase-velocity dispersion curves derived from seismic ambient noise, cross-correlating continuous seismic recordings from the Swiss Network, the German Regional Seismological Network (GRSN), the Italian national broad-band network operated by the Istituto Nazionale di Geosica e Vulcanologia (INGV). To increase the aperture of the station array, additional measurements from the Mediterranean Very Broad-band Seismographic Network (MedNet), the Austrian Central Institute for Meteorology and Geodynamics (ZAMG), the French, Bulgarian, Hungarian, Romanian and Greek stations obtained through Orfeus are also included. The ambient noise, we are using to assemble our database, was recorded at the above-mentioned stations between 2006 January and 2006 December. Correlating continuous signal recorded at pairs of stations, allows to extract coherent surface wave signal travelling between the two stations. Usually the ambient-noise cross-correlation technique allows to have informations at periods of 30s or shorter. By expanding the database of noise correlations, we seek to increase the resolution of the central Europe crustal model. We invert the resulting data sets of group and phase velocities associated with 8-35s Rayleigh waves, to determine 2-D group and phase-velocity maps of the European region. Inversions are conducted by means of a 2-D linearized tomographic inversion algorithm. The generally good agreement of our models with previous studies and good correlation of well-resolved velocity anomalies with geological features, such as sedimentary basins, crustal roots and mountain ranges, documents the effectiveness of our approach. © 2012 The Authors Geophysical Journal International © 2012 RAS.

Pinzuti P.,CNRS Nantes Laboratory of Planetology and Geodynamics | Humler E.,CNRS Nantes Laboratory of Planetology and Geodynamics | Manighetti I.,Geoazur | Gaudemer Y.,CNRS Paris Institute of Global Physics
Geochemistry, Geophysics, Geosystems | Year: 2013

The temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 56 new lava flows sampled along 10 km of the rift axis and 9 km off-axis (i.e., erupted within the last 620 kyr). Petrological and primary geochemical results show that most of the samples of the inner floor of the Asal Rift are affected by plagioclase accumulation. Trace element ratios and major element compositions corrected for mineral accumulation and crystallization show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. While FeO, Fe8.0, Zr/Y, and (Dy/Yb)N decrease from the rift shoulders to the rift axis, SiO2, Na/Ti, Lu/Hf increase and Na 2O and Na8.0 are constant across the rift. These variations are qualitatively consistent with shallow melting beneath the rift axis and deeper melting for off-axis lava flows. Na8.0 and Fe 8.0 contents show that beneath the rift axis, melting paths are shallow, from 81 ± 4 to 43 ± 5 km. These melting paths are consistent with adiabatic melting in normal-temperature fertile asthenosphere, beneath an extensively thinned mantle lithosphere. On the contrary, melting on the rift shoulders (from 107 ± 7 to 67 ± 8 km) occurred beneath thicker lithosphere, requiring a mantle solidus temperature 100 ± 40°C hotter. In this geodynamic environment, the calculated rate of lithospheric thinning appears to be 4.0 ± 2.0 cm yr-1, a value close to the mean spreading rate (2.9 ± 0.2 cm yr-1) over the last 620 kyr. ©2013. American Geophysical Union. All Rights Reserved.

Moczo P.,Comenius University | Moczo P.,Slovak Academy of Sciences | Kristek J.,Comenius University | Kristek J.,Slovak Academy of Sciences | And 4 more authors.
Geophysical Journal International | Year: 2011

We analyse 13 3-D numerical time-domain explicit schemes for modelling seismic wave propagation and earthquake motion for their behaviour with a varying P-wave to S-wave speed ratio (VP/VS). The second-order schemes include three finite-difference, three finite-element and one discontinuous-Galerkin schemes. The fourth-order schemes include three finite-difference and two spectral-element schemes. All schemes are second-order in time. We assume a uniform cubic grid/mesh and present all schemes in a unified form. We assume plane S-wave propagation in an unbounded homogeneous isotropic elastic medium. We define relative local errors of the schemes in amplitude and the vector difference in one time step and normalize them for a unit time. We also define the equivalent spatial sampling ratio as a ratio at which the maximum relative error is equal to the reference maximum error. We present results of the extensive numerical analysis. We theoretically (i) show how a numerical scheme sees the P and S waves if the VP/VS ratio increases, (ii) show the structure of the errors in amplitude and the vector difference and (iii) compare the schemes in terms of the truncation errors of the discrete approximations to the second mixed and non-mixed spatial derivatives. We find that four of the tested schemes have errors in amplitude almost independent on the VP/VS ratio. The homogeneity of the approximations to the second mixed and non-mixed spatial derivatives in terms of the coefficients of the leading terms of their truncation errors as well as the absolute values of the coefficients are key factors for the behaviour of the schemes with increasing VP/VS ratio. The dependence of the errors in the vector difference on the VP/VS ratio should be accounted for by a proper (sufficiently dense) spatial sampling. © 2011 The Authors Geophysical Journal International © 2011 RAS.

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