My work focused on the development of improved methodologies for the evaluation of seismic hazard and its related uncertainties, based on the study of active faults systems and dynamic modelling of the seismic cycle. I worked in particular on the probabilistic estimate of a fault's maximum magnitude earthquake and of its return period. Those parameters are indeed crucial to estimate seismic hazard. Seismicity can be viewed as a stochastic process which is constrained by the principle of moment conservation: seismic ruptures must in principle rupture fault portions which had accumulated a deficit of slip, in view of their long term slip rate, during the interseismic period. In Chapter 1, I explain how we implemented those constraints in the evaluation of the probability distribution describing the magnitude and return period of the largest earthquake, propagating the geodetic uncertainties up to the hazard calculation. We applied this methodology to the Parkfield Segment of the San Andreas Fault, where the seismic cycle is particularly well documented. Our study implies potential maximum magnitude between 6.5 and 7.5, with a return period of 140 to 300 years. In Chapter 2, we applied the same methodology to the Cascadia subduction zone, known to have produced a M~9 earthquake in 1700 but where the seismic hazard remains poorly constrained. As part of this study we determined a model of interseismic coupling and of fault slip due to Slow Slip Events using an Independent Component Analysis-based inversion method. Finally, in Chapter 3, I use dynamic modelling to tackle the problem of partial ruptures. Large earthquakes tend to be confined to fault areas locked in the interseismic period but they often rupture them only partially. For example, during the 2015 M7.8 Gorkha earthquake, Nepal, a slip pulse propagating along-strike unzipped the bottom edge of the locked portion of the Main Himalayan Thrust. The lower edge of the rupture produced dominant high-frequency (>1 Hz) radiation of seismic waves. We showed that similar partial ruptures occur spontaneously in a simple dynamic model of earthquake sequences on a planar fault without mechanical, frictional and geometrical heterogeneities.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21965号 / 工博第4620号 / 新制||工||1720(附属図書館) / 京都大学大学院工学研究科都市環境工学専攻 / (主査)教授 酒井 伸一, 教授 勝見 武, 准教授 平井 康宏 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Chemical weathering of silicate rock consumes atmospheric CO2 and supplies the oceans with cations, thereby controlling both seawater chemistry and climate. The rate of CO2 consumption is closely linked to the rate of CO2 outgassing from the planetary interior, providing a negative feedback loop essential to maintaining an equable climate on Earth. Reconstruction of past global temperatures indicates that a pronounced episode of global cooling began ~50 million years ago, coincident with the collision of India and Asia, and the subsequent exhumation of the Himalayas and Tibet. This has drawn attention to the possible links between exhumation, erosion, changes in silicate weathering rates, and climate. However, many of the present-day weathering processes operating on the continents remain debated and poorly constrained, hampering our interpretations of marine geochemical archives and past climatic shifts. To constrain the controls on silicate weathering, this thesis investigates the lithium (Li) isotopic composition of river waters, suspended sediments and bed load sediments in the Alaknanda river basin, forming the headwaters of the Ganges. Due to the large fractionation of Li isotopes in the Earth’s surface environment, Li is sensitive to small changes in silicate weathering processes. As a consequence of the pronounced gradients in climate (rainfall and temperature) and erosion across the basin, the river waters show large variations in their Li isotopic composition (δ7Li), ranging from +7.4 to +35.4‰, covering much of the observed global variation. This allows a detailed investigation of the controls on Li isotope fractionation, and by extension silicate weathering. The Li isotopic composition is modelled using a one-dimensional reactive transport model. The model incorporates the continuous input of Li from rock dissolution, removal due to secondary mineral formation, and hydrology along subsurface flow paths. Modelling shows that the Li isotopic variations can be described by two dimensionless variables; (1) the Damköhler number, ND, which relates the silicate dissolution rate to the fluid transit time, and (2) the net partition coefficient of Li during weathering, kp, describing the partitioning of Li between secondary clay minerals and water, which is primarily controlled by the stoichiometry of the weathering reactions. The derived values of the controlling parameters ND and kp, are investigated over a range of climatic conditions and on a seasonal basis, shedding light onto variations in the silicate weathering cycle. In a kinetically limited weathering regime such as the Himalayan Mountains, both climate and erosion exert critical controls the weathering intensity (the fraction of eroded rock which is dissolved) and the weathering progression (which minerals that are being weathered), and consequently the fractionation of Li isotopes and silicate weathering in general. Modelling of the Li isotopic composition provides an independent estimate of the parameters which control silicate weathering. These estimates are then used to constrain variables such as subsurface fluid flux, silicate dissolution rates, fluid transit times and the fraction of rock which is weathered to form secondary clay minerals. The simple one-dimensional reactive transport model therefore provides a powerful tool to investigate the minimum controls on silicate weathering on the continents.

This study aims to examine the motivation, experience and benefits of Nepalese domestic tourists visiting the seismic memorial sites after the 25 April 2015 earthquake (known as Gorkha earthquake). A total of 403 surveys was gathered from seismic sites of Nepal (Kathmandu, Bhaktapur and Patan). Data were tested to analyze why the tourists are interested in disaster sites and how their experience during their visit impact the benefits of the visits. Additionally, partial least square structural equation modeling (PLS-SEM) was employed to test the relationships among tourist motivations, experiences, and perceived benefits at the dark tourism sites in Nepal. Among the five motivational factors discovered, the empirical results depict that emotional reaction is the strongest factor of the dark tourism motivation, affecting both cognitive and affective experiences. Additionally, this study confirms that cognitive experience is more influenced by dark tourism motivations than affective experience. Among the four experience factors examined in the study, self-reflection is found to have the strongest impact of three aspects of perceived dark tourism benefits, such as knowledge gain, fulfillment, and appreciation. Overall, the findings of the study provide important implications to the management sectors of dark tourism sites, enhancing the importance of providing cognitive experiences (i.e. distributing the educational materials about the dark tourism events and offering the knowledgeable tour guide who can guide the sites) and affective experience of the tourists (storytelling about the events, organizing educational and volunteering programs at the sites). Further, this study contributes to the limited literature in the context of dark tourism and provide important managerial and practical implications based on the case of Nepal earthquake in 2015.