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Late Quaternary Seismicity and Climate in the Western Nepal : Himalaya

Abstract : The Himalayan collision, in which India underthrusts below Tibet, regularly produces major destructive earthquakes in Nepal and its neighboring countries, most of which are fatal to nearby communities. As a wall dividing the Indian plains and the Tibetan plateau, the Himalaya also significantly modifies the atmospheric circulation, affecting both the local and global climate. This thesis explores the poorly known Quaternary history and evolution of Himalayan climate and seismicity, more particularly in the least populated and most remote region of Western Nepal. In terms of climate and environmental change, one of the least understood aspects of Himalayan history during the late Quaternary is the extension of glaciers and their impacts on landscape evolution. Based on field observations, cosmogenic nuclide dating (10Be) and satellite observations, we estimated the maximum extent of glaciers during the Last Glacial Maximum, which supports the hypothesis of a relatively large glacier cover, but not of an extended ice cap, at the scale of Western Nepal. In terms of seismology, the social, economic and political implications of the occurrence of an earthquake of higher magnitude than the recent earthquake of 2015, whose epicenter is located near the city of Gorkha, is a major concern and largely motivates this thesis. The last major earthquake of magnitude greater than 8 (Mb) took place on 6 June 1505 and had a profound impact on the Nepalese population and the surrounding area. In Western Nepal the 1505 event was the last earthquake that ruptured the Main Frontal Thrust according to historical archives and paleoseismological studies, which gave rise to the concept of a seismic gap in western Nepal and adjacent areas in northern India. With this in mind, this thesis addresses two major issues on the Himalayan seismic behavior: on the first hand is the hypothesis of a seismic gap in the central Himalaya and on the second the temporal distribution of earthquakes during the late Quaternary. For this purpose, a new research approach independent of paleoseismic trenches was applied in the Himalaya. By using lakes as paleoseismometers, we were able to refine the temporal resolution and identify earthquakes that had not yet been documented in the accessible databases on a 700-year scale. Our results from Lake Rara highlight significant previously-unknown earthquakes (Mw>6.5) and they reveal that Western Nepal is seismically as active as central Nepal. Furthermore, they call into question the hypothesis of a seismic gap in the central Himalaya. Based on a longer sediment core from the same lake, we studied the temporal distribution of earthquakes over a period of 6000 years, which has highlighted the random nature of the occurrence of earthquakes, constituting a paradigm shift where the notion of seismic cycle is still prevalent. The random nature of the occurrence of earthquakes both on short (instrumental) and Quaternary time scales disproves the hypothesis of the seismic gap in the central Himalaya and underlines the permanent risk for the million people of concern. The final part of this thesis addresses the possible global relationship between seismic rate fluctuations and climate change during the Holocene. Our results show that the global seismicity clustered over 7000 years and appears to be synchronous with the sum of glacial advances through the Mid and Late Holocene.
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Submitted on : Monday, November 26, 2018 - 4:23:09 PM
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Zakaria Ghazoui. Late Quaternary Seismicity and Climate in the Western Nepal : Himalaya. Applied geology. Université Grenoble Alpes; Rijksuniversiteit te Gent. Sectie Mariene Biologie, 2018. English. ⟨NNT : 2018GREAU026⟩. ⟨tel-01935412⟩



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