Datensatz

Zusammenfassung

Adapting the magnetotelluric (MT) method for monitoring the dynamic behaviour of the Andean subduction system in Northern Chile is focus of this thesis. Electromagnetic fields, sampled at nine permanent MT stations which cover an area of approximately 250 x 100 km² in the Andean fore-arc, are evaluated to monitor the electrical resistivity structure associated with the deep hydraulic system of the subduction zone. The long term monitoring of geo-electromagnetic fields reveals different types of temporal variations of vertical magnetic transfer functions (VTF) in different period ranges which are evaluated and interpreted. Computation of time series of daily VTFs of an overall length of 4 years exhibit seasonal variations with amplitudes of more than 100% of their absolute values for different components at all sites of the array. The observed seasonal variation affects almost exclusively the east-west magnetic field component for periods between 100 and 3000 seconds. These ground-based measurements of magnetic and electric fields exhibit statistically significant coherences with the interplanetary electric field (IEF) derived from solar wind and interplanetary magnetic field data of the Advanced Composition Explorer (ACE) satellite. The IEF penetrates the polar ionosphere from where it propagates towards equatorial latitudes by wave guide transmission, with ionosphere and solid Earth acting as conducting boundaries. Signal coherence between IEF and ground data peaks at periods of approximately 90 min and up to the four harmonics. Coherence values reach 0.4 at these periods and depend on the electromagnetic field component. They vary with season and local time. Transfer functions computed between IEF and ground-based electric and magnetic fields show local maxima at similar periods (90 min and harmonics). The coupling between the east-west magnetic field component and the IEF shows significant seasonal variability, much larger than for the other electromagnetic field components. We conclude that the IEF drives primarily a global circuit of Pedersen currents in the ionosphere. Resulting time-varying magnetic fields induce electric currents in the ground. Related ground-based magnetic (primarily north-south) and electric (primarily east-west) signals vary coherently at all local times and seasons. Conversely, magnetic signals caused by the IEF-driven Hall currents depend much on local time and season. We show for the first time that these ionospheric Hall currents cause no induction in the ground, but they generate magnetic signatures that are confined to the waveguide between ionosphere and Earth's surface. Geo-electromagnetic depth sounding applications as MT assume both spatial and temporal uniform external electromagnetic source fields. The seasonal variation of VTFs exhibits a systematic violation of this basic assumption in Northern Chile. The consequence is a systematic seasonal rotation and length variation of the induction arrows of the period band between 100 and 3000 seconds. If not taken into account, the structure of an electrical resistivity model of the subsurface, obtained by MT inversion, would be distorted. Removing this source field effect with a low-pass filter allows evaluation of residual variations of the VTF time-series which last longer than one year. During 2008 and 2009, I observe a significant variation of the VTFs in the southern part of the network for periods between 1500 and 4000 seconds. To simulate this variation, a 3D reference resistivity model is obtained by inversion of MT and VTF data using eight stations of the network. A region of high conductivity matches spatially with the hydrated mantle wedge. By trial and error, the 3D reference image of the deep electrical resistivity structure is modified and 3D forward modelling is applied to explain temporal variations in the VTFs similar to our observations. That requires modification of the electrical resistivity structure in a region which coincides roughly with the plate interface directly down-dip of the Mw7.7 2007 Tocopilla earthquake. We speculate that the anomalous temporal variations of the VTFs may be caused by large scale fluid relocation in the aftermath of the seismic event.