La zone de subduction centrale en Equateur est caractérisée par un patch fortement couplé, pas de grands séismes connus et de fréquents essaims sismiques, dont certains associés à des épisodes de glissements lents (SSE). Les hypocentres déterminés sur un réseau temporaire dense onshore-offshore image la sismicité de fond et plusieurs essaims (01/2013), synchrones d’un SSE (Mw´ 6.3). Une sismicité permanente se produit à 20-30 km de profondeur, proche et sous la zone de contact interplaque (ZCI), bordant la partie profonde de la zone couplée. Les essaims superficiels (10 km) ont lieu sur des zones de failles crustales de la plaque plongeante (ZFC-Nazca), inverses, sub-verticales et qui bordent un massif océanique en subduction. Le SSE 2012-2013 est un événement composite se développant sur 2 patchs distincts. Le premier patch (P1), plus petit, se localise sur une partie peu couplée de la ZCI et le second (P2), plus superficiel, sur une zone fortement bloquée (>70%). Depuis le 25/11 et durant ~1,5 mois, sans sismicité, P1 se déverrouille progressivement, indiqué par un glissement discontinu, lent et faible. Le 13/01, l'accélération soudaine du glissement sur P1 réactive une ZFC-Nazca (1er essaim), située immédiatement updip P1 et downdip P2, ce qui favorise le début d´un glissement rapide, fort et continu sur P2 (et sur P1). Nous proposons que les fluides, expulsés par l'activité sur la ZFC-Nazca et injectés au niveau de la ZCI, contribuent à modifier le comportement de stabilité des matériaux, favorisant ce SSE inattendu sur une zone fortement bloquée. Environ 80% du moment asismique total sont libérés sur P2 en une semaine, concomitant d´essaims sur différentes ZFC-Nazca / The central subduction zone of Ecuador is characterized by a highly coupled patch, no known large earthquakes and frequent seismic swarms, some of them associated to slow slip events (SSE). The earthquakes recorded on a temporal onshore-offshore network show an unprecedented image of the background seismicity and of several intense swarms in early 2013, concomitant of a SSE (6.3 Mw). The 20-30 km deep permanent seismic clusters develop near and below the interface contact zone (ICZ), contouring the downdip limit of the locked area. The shallower swarms (10 km depth) occur on sub-vertical inverse crustal fault zones of the Nazca plate that seem to bound a known oceanic massive in subduction. The 2012-2013 SSE is a composite event developing on 2 distinct patches. The first and smaller patch (P1) is sited on an intermediate coupled portion of the ICZ than the shallower second patch (P2) that lies on a highly locked area (> 70%). Since 2012 Nov. 25 and during ~1.5 month, without seismicity, P1 progressively unlocks as revealed by the slow, low and intermittent slip. On 2013 Jan. 16, the sudden slip acceleration on P1 activates a Nazca crustal fault zone (first swarm) located immediately updip P1 and downdip P2, which in turn favors the onset of the faster, higher and continuous slip on P2 (and on P1). We propose that the fluids expelled by the fault activity and injected above, within the ICZ, contribute to modify the material stability behavior and favor the unexpected SSE on a highly locked area. About 80% of the total aseismic moment is released during one week on P2, at the same time than intense seismic swarms on distinct Nazca crustal fault zones

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Eastern North America experiences large intracratonic earthquakes that are not well understood but pose a risk to urban centers and other infrastructure. Compilation of regional earthquake epicentres for south-central Ontario and western Quebec demonstrate a close association with sutures and failed rifts (the St. Lawrence Rift) recording the formation and breakup respectively of successive supercontinents Rodinia and Pangea. Where seismic potential could be underestimated through lack of historical seismicity or where little is known about active faults, lake deposits can provide a valuable record of past seismic shaking events in the form of sediment deformation structures (i.e. ‘seismites’). In central Canada, the lacustrine seismographic record began approximately 10,000 years before present with the retreat of the Laurentide Ice Sheet, older records having been removed by glacial erosion. Most bedrock lake basins are structurally-controlled and underlain by the same Precambrian basement structures (shear zones, terrane boundaries and other lineaments) implicated as the source of ongoing mid-plate earthquake activity. High resolution seismo-stratigraphic data presented here supports the model that ongoing mid-plate earthquake activity is a consequence of brittle deformation of the upper crust of the North American plate. Such activity appears to have been greatest during deglaciation but continues today. The detailed geophysical and sedimentary studies, as shown here, have major societal relevance in areas of eastern North America affected by intraplate earthquakes. The recognition and mapping of earthquake related features in lakes for seismic risk analysis is a means of constraining seismic recurrence intervals and more realistically assess seismic risk across the populated area of Ontario and Quebec where events occur on time scales much longer than recorded history. / Dissertation / Doctor of Philosophy (PhD)

Planet earth is restless and one cannot control its inside activities and vibrations those leading to natural hazards. Earthquake is one of such natural hazards that have affected the mankind most. Most of the causalities due to earthquakes happened not because of earthquakes as such, but because of poorly designed structures which could not withstand the earthquake forces. The improper building construction techniques adopted and the high population density are the major causes of the heavy damage due to earthquakes. The damage due to earthquakes can be reduced by following proper construction techniques, taking into consideration of appropriate forces on the structure that can be caused due to future earthquakes. The steps towards seismic hazard evaluation are very essential to estimate an optimal and reliable value of possible earthquake ground motion during a specific time period. These predicted values can be an input to assess the seismic vulnerability of an area based on which new construction and the restoration works of existing structures can be carried out. A large number of devastating earthquakes have occurred in India in the past. The northern region of India, which is along the plate boundary of the Indian plate with the Eurasian plate, is seismically very active. The north eastern movement of Indian plate has caused deformation in the Himalayan region, Tibet and the North Eastern India. Along the Himalayan belt, the Indian and Eurasian plates converge at the rate of about 50 mm/year (Bilham 2004; Jade 2004). The North East Indian (NEI) region is known as one of the most seismically active regions in the world. However the peninsular India, which is far away from the plate boundary, is a stable continental region, which is considered to be of moderate seismic activity. Even though, the activity is considered to be moderate in the Peninsular India, world’s deadliest earthquake occurred in this region (Bhuj earthquake 2001). The rapid drifting of Indian plate towards Himalayas in the north east direction with a high velocity along with its low plate thickness might be the cause of high seismicity of the Indian region. Bureau of Indian Standard has published a seismic zonation map in 1962 and revised it in 1966, 1970, 1984 and 2002. The latest version of the seismic zoning map of India assigns four levels of seismicity for the entire Country in terms of different zone factors. The main drawback of the seismic zonation code of India (BIS-1893, 2002) is that, it is based on the past seismic activity and not based on a scientific seismic hazard analysis. Several seismic hazard studies, which were taken up in the recent years, have shown that the hazard values given by BIS-1893 (2002) need to be revised (Raghu Kanth and Iyengar 2006; Vipin et al. 2009; Mahajan et al. 2009 etc.). These facts necessitate a comprehensive study for evaluating the seismic hazard of India and development of a seismic zonation map of India based on the Peak Ground Acceleration (PGA) values. The objective of this thesis is to estimate the seismic hazard of entire India using updated seismicity data based on the latest and different methodologies. The major outcomes of the thesis can be summarized as follows. An updated earthquake catalog that is uniform in moment magnitude, has been prepared for India and adjoining areas for the period till 2010. Region specific magnitude scaling relations have been established for the study region, which facilitated the generation of a homogenous earthquake catalog. By carefully converting the original magnitudes to unified MW magnitudes, we have removed a major obstacle for consistent assessment of seismic hazards in India. The earthquake catalog was declustered to remove the aftershocks and foreshocks. Out of 203448 events in the raw catalog, 75.3% were found to be dependent events and remaining 50317 events were identified as main shocks of which 27146 events were of MW ≥ 4. The completeness analysis of the catalog was carried out to estimate completeness periods of different magnitude ranges. The earthquake catalog containing the details of the earthquake events until 2010 is uploaded in the website the catalog was carried out to estimate completeness periods of different magnitude ranges. The earthquake catalog containing the details of the earthquake events until 2010 is uploaded in the website the catalog was carried out to estimate completeness periods of different magnitude ranges. The earthquake catalog containing the details of the earthquake events until 2010 is uploaded in the website A quantitative study of the spatial distribution of the seismicity rate across India and its vicinity has been performed. The lower b values obtained in shield regions imply that the energy released in these regions is mostly from large magnitude events. The b value of northeast India and Andaman Nicobar region is around unity which implies that the energy released is compatible for both smaller and larger events. The effect of aftershocks in the seismicity parameters was also studied. Maximum likelihood estimations of the b value from the raw and declustered earthquake catalogs show significant changes leading to a larger proportion of low magnitude events as foreshocks and aftershocks. The inclusions of dependent events in the catalog affect the relative abundance of low and high magnitude earthquakes. Thus, greater inclusion of dependent events leads to higher b values and higher activity rate. Hence, the seismicity parameters obtained from the declustered catalog is valid as they tend to follow a Poisson distribution. Mmax does not significantly change, since it depends on the largest observed magnitude rather than the inclusion of dependent events (foreshocks and aftershocks). The spatial variation of the seismicity parameters can be used as a base to identify regions of similar characteristics and to delineate regional seismic source zones. Further, Regions of similar seismicity characteristics were identified based on fault alignment, earthquake event distribution and spatial variation of seismicity parameters. 104 regional seismic source zones were delineated which are inevitable input to seismic hazard analysis. Separate subsets of the catalog were created for each of these zones and seismicity analysis was done for each zone after estimating the cutoff magnitude. The frequency magnitude distribution plots of all the source zones can be found at http://civil.iisc.ernet.in/~sitharam . There is considerable variation in seismicity parameters and magnitude of completeness across the study area. The b values for various regions vary from a lower value of 0.5 to a higher value of 1.5. The a value for different zones vary from a lower value of 2 to a higher value of 10. The analysis of seismicity parameters shows that there is considerable difference in the earthquake recurrence rate and Mmax in India. The coordinates of these source zones and the seismicity parameters a, b & Mmax estimated can be directly input into the Probabilistic seismic hazard analysis. The seismic hazard evaluation of the Indian landmass based on a state-of-the art Probabilistic Seismic Hazard Analysis (PSHA) study has been performed using the classical Cornell–McGuire approach with different source models and attenuation relations. The most recent knowledge of seismic activity in the region has been used to evaluate the hazard incorporating uncertainty associated with different modeling parameters as well as spatial and temporal uncertainties. The PSHA has been performed with currently available data and their best possible scientific interpretation using an appropriate instrument such as the logic tree to explicitly account for epistemic uncertainty by considering alternative models (source models, maximum magnitude in hazard computations, and ground-motion attenuation relationships). The hazard maps have been produced for horizontal ground motion at bedrock level (Shear wave velocity ≥ 3.6 km/s) and compared with the earlier studies like Bhatia et al., 1999 (India and adjoining areas); Seeber et al, 1999 (Maharashtra state); Jaiswal and Sinha, 2007 (Peninsular India); Sitharam and Vipin, 2011 (South India); Menon et al., 2010 (Tamilnadu). It was observed that the seismic hazard is moderate in Peninsular shield (except the Kutch region of Gujarat), but the hazard in the North and Northeast India and Andaman-Nicobar region is very high. The ground motion predicted from the present study will not only give hazard values for design of structures, but also will help in deciding the locations of important structures such as nuclear power plants. The evaluation of surface level PGA values is of very high importance in the engineering design. The surface level PGA values were evaluated for the entire study area for four NEHRP site classes using appropriate amplification factors. If the site class at any location in the study area is known, then the ground level PGA values can be obtained from the respective map. In the absence of VS30 values, the site classes can be identified based on local geological conditions. Thus this method provides a simplified methodology for evaluating the surface level PGA values. The evaluation of PGA values for different site classes were evaluated based on the PGA values obtained from the DSHA and PSHA. This thesis also presents VS30 characterization of entire country based on the topographic gradient using existing correlations. Further, surface level PGA contour map was developed based on the same. Liquefaction is the conversion of formally stable cohesionless soils to a fluid mass, due to increase in pore pressure and is prominent in areas that have groundwater near the surface and sandy soil. Soil liquefaction has been observed during the earthquakes because of the sudden dynamic earthquake load, which in turn increases the pore pressure. The evaluation of liquefaction potential involves evaluation of earthquake loading and evaluation of soil resistance to liquefaction. In the present work, the spatial variation of the SPT value required to prevent liquefaction has been estimated using a probabilistic methodology, for entire India. To summarize, the major contribution of this thesis are the development of region specific magnitude correlations suitable for Indian subcontinent and an updated homogeneous earthquake catalog for India that is uniform in moment magnitude scale. The delineation and characterization of regional seismic source zones for a vast country like India is a unique contribution, which requires reasonable observation and engineering judgement. Considering complex seismotectonic set up of the country, the present work employed numerous methodologies (DSHA and PSHA) in analyzing the seismic hazard using appropriate instrument such as the logic tree to explicitly account for epistemic uncertainties considering alternative models (For Source model, Mmax estimation and Ground motion prediction equations) to estimate the PGA value at bedrock level. Further, VS30 characterization of India was done based on the topographic gradient, as a first level approach, which facilitated the development of surface level PGA map for entire country using appropriate amplification factors. Above factors make the present work very unique and comprehensive touching various aspects of seismic hazard. It is hoped that the methodology and outcomes presented in this thesis will be beneficial to practicing engineers and researchers working in the area of seismology and geotechnical engineering in particular and to the society as a whole.

Induced seismicity (earthquakes caused by injection or extraction of fluids in Earth's subsurface) is a major, new hazard in the United States, the Netherlands, and other countries, with vast economic consequences if not properly managed. Addressing this problem requires development of predictive simulations of how fluid-saturated solids containing frictional faults respond to fluid injection/extraction. Here we present a numerical method for linear poroelasticity with rate-and-state friction faults. A numerical method for approximating the fully coupled linear poroelastic equations is derived using the summation-by-parts-simultaneous-approximation-term (SBP-SAT) framework. Well-posedness is shown for a set of physical boundary conditions in 1D and in 2D. The SBP-SAT technique is used to discretize the governing equations and show semi-discrete stability and the correctness of the implementation is verified by rigorous convergence tests using the method of manufactured solutions, which shows that the expected convergence rates are obtained for a problem with spatially variable material parameters. Mandel's problem and a line source problem are studied, where simulation results and convergence studies show satisfactory numerical properties. Furthermore, two problem setups involving fault dynamics and slip on faults triggered by fluid injection are studied, where the simulation results show that fluid injection can trigger earthquakes, having implications for induced seismicity. In addition, the results show that the scheme used for solving the fully coupled problem, captures dynamics that would not be seen in an uncoupled model. Future improvements involve imposing Dirichlet boundary conditions using a different technique, extending the scheme to handle curvilinear coordinates and three spatial dimensions, as well as improving the high-performance code and extending the study of the fault dynamics.