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Enregistrement des évènements extrêmes dans les sédiments, à l'est de Taiwan

Abstract : Taiwan is a young mountain belt, known as one of the most active area in the world. Since the beginning of the 20th century more than twenty _ Mw 7 earthquakes have struck the island. However, the occurrence of larger events (_Mw 8) is still a matter of debate. In this framework it is of key importance to obtain longer record in order to evaluate the occurrence of large past earthquakes. The sub-aqueous paleoseismology, based on the record of the sedimentary gravity deposits, appears as a serious alternative to approach this thematic and is a rapidly advancing field that has the potential to illuminate the long-term history of seismicity.The first part of this work was to investigate the present sedimentary systems off east Taiwan, essential to understand the morphosedimentary features, sedimentary facies and processes governing the evolution of the submarine slope, and the controlling factors of the recent sedimentation. Our results showed that the offshore slope east Taiwan is affected by a variety of sedimentary systems and processes, and that turbidity currents appear as the main erosional processes covering nearly 60% of the sedimentary record. Turbidity currents are generated by distinct controlling factors such as tectonic and climatic activity that enabled us to define twoend-members relative to turbidity currents initiation: Turbidity currents preconditioned by tectonic activity and triggered by earthquakes shaking and likely deposited into intra-slope basin and turbidity currents driven by climatic activity such as extreme floods or typhoons, generated in basin directly connected with onland rivers.The second part consisted to apply a paleoseismic approach, based on turbidites record, at two time-scales. First, we tested and validated the method by correlating turbidites deposits with instrumental earthquakes. Then, once calibrated we extended the time-series back in time. We dated the three most recent turbidites layers circa 2001 ± 3 AD, 1950 ± 5 AD and 1928 ± 10 AD. Using empirical relationship that link peakground acceleration, distance and magnitude to calibrate the seismic sources, we correlate these three turbidites with instrumental earthquakes: the Chengkong Earthquake 12/10/2003 (Mw 6.8), the 11/24/1951 Taitung Earthquake (Mw 7.1) and the 9/4/1935 Lutao Earthquake (Mw 7.0) respectively. Applying criteria to discriminating the different triggering mechanisms for turbidity current generation, we propose that earthquakes are the main triggering mechanisms. Dating and age modeling provided a part of the chronology of extreme events since the last 3000 years and allowed us to estimate return time for earthquakes Mw _ 6.8.This work represents a good starting-point for future investigations in order to better assess Holocene time series of extreme events.
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Rémi Lehu. Enregistrement des évènements extrêmes dans les sédiments, à l'est de Taiwan. Sciences de la Terre. Université Montpellier II - Sciences et Techniques du Languedoc, 2014. Français. ⟨NNT : 2014MON20133⟩. ⟨tel-01661529⟩

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