The impact of topographic and geological features on deformations of the upper crust
Deformations are observable by various kinds of instruments, e.g. strainmeter, tiltmeter, seismometer, GPS-permanent station. The observed deformations contain signals of different amplitudes and greatly diverse origin: periodic (tides, …) and aperiodic signals (tectonics, …), and local effects (cavity effect, topographic effect, …). Investigations show that many effects caused by local surroundings of the instrument can be some orders of magnitude larger than the signals of interest. Reductions of the residual variations are indispensable for the extraction of e.g. the pure tectonical signal. One of the challenges is to understand transfer mechanisms between loading and local conditions such as topography, cavity, and geological features, which lead to additional or modified deformations. In this study numerical modeling using the Finite-Element method with elastic rheology is carried out to estimate these effects. Barometric pressure changes are used as an example for the load in order to analyze the additional deformations. Different scenarios are simulated: a uniform load, a pressure front which moves over the model and meteorological events. To understand the different effects and sources related to the additionally produced deformations, the study is divided into three parts: (1) a principle systematic study to understand the general phenomena using box models, (2) a study of deformations at four selected broadband observatories, and (3) investigations related to effects on regional scales for the example of Central Europe, where the observatory sites considered under (2) are located. Significant strain and tilt signals are found for each of the effects studied under (1) and (2), which have to be considered on the reduction. The overall consequence of the investigations is that in order to provide reductions of disturbing effects, in geodynamic studies, the load fields in their spatial and temporal variations need to be known in much more detail.
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