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Étude pétrophysique et hydrogéologique des sédiments entrant dans les zones de subduction

Abstract : This study explores how earthquake and tsunami risks are controlled by compositional, petrophysical and hydrogeological properties of sediment inputs to subduction zones and how these properties evolve based on samples and logging data from recent expeditions of the International Ocean Discovery Program (IODP) at: 1) north Hikurangi margin hosting moderate seismicity, shallow slow earthquakes and tsunami earthquakes, 2) the Nankai margin where megathrust tsunamigenic earthquakes, slow earthquakes and tsunami earthquakes have been documented, and the 3) north Sumatra margin, characterized by an extremely thick input sedimentary section, where megathrust tsunamigenic earthquakes have been mostly recorded so far. At north Sumatra, we evidence that a clay-rich zone showing high smectite-bound water content will form a weak decollement corresponding to a high-amplitude negative polarity seismic reflection seaward of the trench that traps fluids produced by pre-subduction dehydration of the bottom of the thick input section. The strengthened thick input section and splay faults draining fluids from the decollement beneath the prism both promote shallow co-seismic slip propagation, thus mega-earthquake and catastrophic tsunami. In contrast, at north Hikurangi, the decollement may form in fluid-rich pelagic carbonates or altered volcaniclastics above a rough altered oceanic crust. There, the heterogeneous compositional, hydrogeological and petrophysical properties of the input section trigger contrasted fluid distribution and potential for excess pore pressure generation along the plate boundary that might be exacerbated in the vicinity of a SSE source zone landward of a seamount subducted beneath the outer wedge. The combination of these heterogeneities likely induces heterogeneous frictional properties at the plate boundary fault favorable to the occurrence of shallow slow and tsunami earthquakes. Slow slip is suspected to 1) propagate all the way the trench along the plate boundary fault or alternatively along splay faults and 2) to cyclically release part of the strain accumulated at the plate boundary resulting in moderate seismicity in the region. This study also provides 1) essential quantitative inputs for advanced fluid-budget and seismo-mechanical modeling that are essential steps toward earthquake and tsunami hazard assessment and 2) methodological improvements including a workflow for porosity and compaction state characterization in clay-rich sediments
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Submitted on : Thursday, October 14, 2021 - 4:17:23 PM
Last modification on : Tuesday, October 19, 2021 - 7:01:30 PM

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  • HAL Id : tel-03378509, version 1

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Jade Dutilleul. Étude pétrophysique et hydrogéologique des sédiments entrant dans les zones de subduction. Sciences de la Terre. Université de Lorraine, 2021. Français. ⟨NNT : 2021LORR0120⟩. ⟨tel-03378509⟩

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