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Abstract

This thesis presents studies which address the regional-scale distribution of fluids during the seismic cycle in the subduction zone of Northern Chile. To infer the distribution of fluids, this work investigated the regional-scale electrical resistivity structure of the subduction zone. The resistivity structure was imaged and monitored using the magnetotelluric (MT) geophysical method. The analysed MT data were collected at nine permanent sites of the Integrated Plate Boundary Observatory Chile (IPOC) located along the forearc region in Northern Chile (21°-23.5°S, 69°-70°W). The regional-scale resistivity structure was derived by modelling long-period MT data using 3-D inversion approach. This model exhibits a complex resistivity structure, suggesting significant spatial variability in the fluid content at different regions of the subduction zone. The model shows that the continental South American plate within the forearc region is mainly characterized by high resistivities (>1000 Ωm) between the coast and 69°W. The model shows additionally a number of regional low resistivity zones (LRZs, <10 Ωm) within the continental crust. Long-active fault zones and clusters of seismicity are observed within these LRZs, suggesting that these resistivity anomalies are relatively weak and permeable regions of the continental crust, where deformation processes and circulation of fluids have been focused over geologic time. Some of these LRZs reach the continental lower crust above the intraplate seismogenic zone, coinciding spatially with boundaries of the rupture area of large earthquakes and regional NW-SE structural lineaments. In the forearc mantle wedge the model exhibits a 150 km-long trench-parallel LRZ above the intermediate-depth seismogenic zone, suggesting significant along-strike variability in the amount of fluids released from the oceanic Nazca plate. The temporal evolution of the resistivity structure was monitored by analysing the IPOC MT data during 2007-2014. This analysis reveals anomalous temporal variations of the vertical magnetic transfer function (VTF) at one IPOC site. These anomalous variations were reproduced by modelling a decrease in resistivity within a seismically active region of the continental crust located above the interplate seismogenic zone. The spatiotemporal distribution of seismicity suggests that the inferred changes in resistivity were associated with episodes of upward migration of fluids generated at the plate interface. The sensitivity of MT data to changes in the resistivity structure and in the geomagnetic activity was evaluated. Possible regional-scale resistivity changes above the interplate seismogenic zone were simulated, obtaining that they can generate measurable variations in MT responses (TFs) recorded at the IPOC sites. However, the modelled variations in IPOC TFs can be masked by unwanted effects correlated with changes in magnetic fields generated in the atmosphere (source effects). The analysis of 18 years of geomagnetic data recorded at mid-latitude sites reveals that globally the VTF exhibits patterns related to source effects. Main patterns identified were periodical seasonal variations and a long-term trend correlated with the 11-year solar cycle. Such source effects can be identified as patterns observed synchronously in temporal variations of VTFs recorded at neighbouring sites, and which additionally show significant correlation with fluctuations of the geomagnetic activity.