| Summary: | The main goal of this Ph.D. thesis is to improve locations of earthquake hypocenters and to resolve heterogeneous crustal structure and its effects on travel times. The data and case studies are drawn from the Icelandic national SIL network and the temporary NICE project deploy-ment in the Tjörnes Fracture Zone. Iceland presents complex tectonics and active volcanism, consequences of its position astride the Mid-Atlantic Ridge between the European and North American plates and on top of a melting anomaly in the mantle below. Studies focused on characterizing the seismicity and the crustal structure are prerequisites for further seismologi-cal studies in Iceland, e.g., on seismic sources, the evolution of volcanic systems, activity on seismic faults and seismic hazard, among others. Different methods have been explored. First, we estimated empirically travel-time functions of seismic waves and their uncertainties for 65 stations in the Icelandic permanent network (SIL) using arrival times. The estimated travel-time functions and uncertainties were used to relocate the complete catalog applying a nested-search algorithm to this non-linear problem. The clearest changes in locations compared to the SIL solutions were obtained in the peripheral areas of the network, in particular in the Tjörnes Fracture Zone (North Iceland) and on the Reykjanes Peninsula (South Iceland). Relocations with empirical travel times were used complementary with constrained earth-quake relocation and the collapsing methods of Li et al. [2016] to study the seismicity in the Hengill area (South Iceland). Patterns in the seismicity in the final locations reproduce lin-eations previously found in relative relocations in the area. The brittle-ductile transition was estimated, obtaining a smaller depth in the northern part of the region, dominated by volcanic processes, compared with the south, controlled by tectonic processes. Furthermore, the Hengill fissure swarm that hosts two large geothermal power plants, was found to have deeper penetrat-ing ...
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