The influence of topographic and lithologic features on horizontal deformations

Abstract

SUMMARY A number of studies have been carried out in order to determine the magnitude of effects originating from the surroundings of an observation site which influence seismometer records or deformation observations with tilt- or strainmeters. The investigations generally comprise thermal, atmospheric, topographic, and cavity effects, as well as ocean loading. Recently, these studies have been continued and extended using Finite Element (FE) modeling. The effects are investigated exemplarily using air pressure variations as loading signal. The observatory sites differ in their local surroundings. Based on this the influence on topographic and lithologic features is investigated with respect to air pressure changes. The models developed reflect different topographies around stations: wide and narrow valleys, hill flanks, and flat areas. At the foot of the hill flank, galleries are integrated into the models. The topographic and the gallery geometry influence are systematically investigated with regard to e.g. rock coverage above an observation site, changes in the hill slope, width of a valley, and lithology. The deformations are only caused by air pressure changes and not associated with gravity changes due to an advancing pressure front. For loading different air pressure scenarios are simulated with an uniform load and a pressure front which moves over the model in different directions. As an elastic rheology is considered, the effects can be scaled to actual occurring air pressure variations. Initial studies focus on the cavity effect to estimate the order of magnitude to separate the pure cavity effect from other influences. Studies of topographic effects, e.g. an increasing slope angle, yield a non-linear increase in the strain component with amplitudes up to 2 nstrain and tilt changes up to 2 nrad for a uniform air pressure load of 1 hPa for PREM parameterized models. Different lithologies can result in five times larger deformations. For moving high pressure areas, changes emerge in strain of up to 2 nstrain and in tilt of up to 4.2 nrad. The results obtained improve the interpretation of deformations observed with regard to fundamental geodynamic processes, and contribute to the selection of future observation sites.