Julien Gasc
École Normale Supérieure
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Featured researches published by Julien Gasc.
Science | 2013
Alexandre Schubnel; Fabrice Brunet; Nadège Hilairet; Julien Gasc; Yanbin Wang; Harry W. Green
Delineating Deep Faults Most large, damaging earthquakes initiate in Earths crust where friction and brittle fracture control the release of energy. Strong earthquakes can occur in the mantle too, but their rupture dynamics are difficult to determine because higher temperatures and pressures play a more important role. Ye et al. (p. 1380) analyzed seismic P waves generated by the 2013 Mw 8.3 Sea of Okhotsk earthquake—the largest deep earthquake recorded to date—and its associated aftershocks. The earthquake ruptured along a fault over 180-kilometer-long and structural heterogeneity resulted in a massive release of stress from the subducting slab. In a set of complementary laboratory deformation experiments, Schubnel et al. (p. 1377) simulated the nucleation of acoustic emission events that resemble deep earthquakes. These events are caused by an instantaneous phase transition from olivine to spinel, which would occur at the same depth and result in large stress releases observed for other deep earthquakes. Fractures generated by mineral phase transitions in the mantle produce acoustic emissions that resemble deep earthquakes. Phase transformations of metastable olivine might trigger deep-focus earthquakes (400 to 700 kilometers) in cold subducting lithosphere. To explore the feasibility of this mechanism, we performed laboratory deformation experiments on germanium olivine (Mg2GeO4) under differential stress at high pressure (P = 2 to 5 gigapascals) and within a narrow temperature range (T = 1000 to 1250 kelvin). We found that fractures nucleate at the onset of the olivine-to-spinel transition. These fractures propagate dynamically (at a nonnegligible fraction of the shear wave velocity) so that intense acoustic emissions are generated. Similar to deep-focus earthquakes, these acoustic emissions arise from pure shear sources and obey the Gutenberg-Richter law without following Omori’s law. Microstructural observations prove that dynamic weakening likely involves superplasticity of the nanocrystalline spinel reaction product at seismic strain rates.
Nature Communications | 2017
Thomas Ferrand; Nadège Hilairet; Sarah Incel; Damien Deldicque; Loïc Labrousse; Julien Gasc; Joerg Renner; Yanbin Wang; Harry W. Green; Alexandre Schubnel
Intermediate-depth earthquakes (30–300 km) have been extensively documented within subducting oceanic slabs, but their mechanics remains enigmatic. Here we decipher the mechanism of these earthquakes by performing deformation experiments on dehydrating serpentinized peridotites (synthetic antigorite-olivine aggregates, minerals representative of subduction zones lithologies) at upper mantle conditions. At a pressure of 1.1 gigapascals, dehydration of deforming samples containing only 5 vol% of antigorite suffices to trigger acoustic emissions, a laboratory-scale analogue of earthquakes. At 3.5 gigapascals, acoustic emissions are recorded from samples with up to 50 vol% of antigorite. Experimentally produced faults, observed post-mortem, are sealed by fluid-bearing micro-pseudotachylytes. Microstructural observations demonstrate that antigorite dehydration triggered dynamic shear failure of the olivine load-bearing network. These laboratory analogues of intermediate-depth earthquakes demonstrate that little dehydration is required to trigger embrittlement. We propose an alternative model to dehydration-embrittlement in which dehydration-driven stress transfer, rather than fluid overpressure, causes embrittlement.
Science Advances | 2017
Yanbin Wang; Lupei Zhu; Feng Shi; Alexandre Schubnel; Nadège Hilairet; Tony Yu; Mark L. Rivers; Julien Gasc; Ahmed Addad; Damien Deldicque; Ziyu Li; Fabrice Brunet
Nanoseismological analyses on labquakes under controlled conditions shed new lights on mechanisms of deep-focus earthquakes. Global earthquake occurring rate displays an exponential decay down to ~300 km and then peaks around 550 to 600 km before terminating abruptly near 700 km. How fractures initiate, nucleate, and propagate at these depths remains one of the greatest puzzles in earth science, as increasing pressure inhibits fracture propagation. We report nanoseismological analysis on high-resolution acoustic emission (AE) records obtained during ruptures triggered by partial transformation from olivine to spinel in Mg2GeO4, an analog to the dominant mineral (Mg,Fe)2SiO4 olivine in the upper mantle, using state-of-the-art seismological techniques, in the laboratory. AEs’ focal mechanisms, as well as their distribution in both space and time during deformation, are carefully analyzed. Microstructure analysis shows that AEs are produced by the dynamic propagation of shear bands consisting of nanograined spinel. These nanoshear bands have a near constant thickness (~100 nm) but varying lengths and self-organize during deformation. This precursory seismic process leads to ultimate macroscopic failure of the samples. Several source parameters of AE events were extracted from the recorded waveforms, allowing close tracking of event initiation, clustering, and propagation throughout the deformation/transformation process. AEs follow the Gutenberg-Richter statistics with a well-defined b value of 1.5 over three orders of moment magnitudes, suggesting that laboratory failure processes are self-affine. The seismic relation between magnitude and rupture area correctly predicts AE magnitude at millimeter scales. A rupture propagation model based on strain localization theory is proposed. Future numerical analyses may help resolve scaling issues between laboratory AE events and deep-focus earthquakes.
Nature Communications | 2018
Feng Shi; Yanbin Wang; Tony Yu; Lupei Zhu; Junfeng Zhang; Jianguo Wen; Julien Gasc; Sarah Incel; Alexandre Schubnel; Ziyu Li; Tao Chen; Wenlong Liu; Vitali B. Prakapenka; Zhenmin Jin
Southern Tibet is the most active orogenic region on Earth where the Indian Plate thrusts under Eurasia, pushing the seismic discontinuity between the crust and the mantle to an unusual depth of ~80 km. Numerous earthquakes occur in the lower portion of this thickened continental crust, but the triggering mechanisms remain enigmatic. Here we show that dry granulite rocks, the dominant constituent of the subducted Indian crust, become brittle when deformed under conditions corresponding to the eclogite stability field. Microfractures propagate dynamically, producing acoustic emission, a laboratory analog of earthquakes, leading to macroscopic faults. Failed specimens are characterized by weak reaction bands consisting of nanometric products of the metamorphic reaction. Assisted by brittle intra-granular ruptures, the reaction bands develop into shear bands which self-organize to form macroscopic Riedel-like fault zones. These results provide a viable mechanism for deep seismicity with additional constraints on orogenic processes in Tibet.The triggering mechanism of deep seismicity in Tibet remains unclear. Here the authors use experiments to show that granulite when deformed becomes brittle as it passes into the ecologite stability field developing macroscopic riedel fault zones thus providing an explanation for deep seismicity in Southern Tibet.
Physics of the Earth and Planetary Interiors | 2011
Julien Gasc; Alexandre Schubnel; Fabrice Brunet; Sophie Guillon; Hans J. Mueller; Christian Lathe
Physics and Chemistry of Minerals | 2011
Julien Gasc; Fabrice Brunet; Nikolai Bagdassarov
Earth and Planetary Science Letters | 2017
Julien Gasc; Nadège Hilairet; Tony Yu; Thomas Ferrand; Alexandre Schubnel; Yanbin Wang
Journal of Petrology | 2016
Julien Gasc; Fabrice Brunet; Nicolas Brantut; Jérôme Corvisier; Nathaniel Findling; A. Verlaguet; Christian Lathe
Diamond and Related Materials | 2015
Julien Gasc; Yanbin Wang; Tony Yu; Ion C. Benea; Benjamin Rosczyk; Toru Shinmei; Tetsuo Irifune
Japan Geoscience Union | 2017
Yanbin Wang; Feng Shi; Nadège Hilairet; Tony Yu; Julien Gasc; Sébastien Merkel; Norimasa Nishiyama