Marco Naujoks

Hydrological signals in gravity — foe or friend?

Although hydrological effects on gravity are known nearly as long as the influence of barometric pressure, they are not as well understood as the latter. The improvement of gravity data quality during the last years adds weight to the importance of understanding the hydrological influence. Moxa observatory is one station at which studies regarding hydrological effects are carried out. From soil moisture, water level and meteorological observations the effects of different hydrological contributors including snow can be modelled and compared to the gravity residuals of the superconducting gravimeter (SG). The total peak-to-peak amplitude amounts to 35 nm/s2. Contributions from the various areas around the observatory partly compensate due to the hilly morphology. The comparison between residuals and computed total hydrological effect yields a good agreement, but also shows that not all hydrological influences have been taken into account. A significant additional hydrological influence is due to the hill flank near the SG.

Combined geological and gravimetric mapping and modelling for an improved understanding of observed high-resolution gravity variations: a case study for the Global Geodynamics Project (GGP) station Moxa, Germany

Geodynamic observatories around the globe continuously monitor signals like gravity, tilt and strain as a function of time. However, global signals are often masked by local effects, caused by the direct surroundings of the station, including the local geological setting. This link is well established for superconducting gravimeters (SG) that observe the gravity field variations at very high resolution. An enhancement of the SG time series by the application of local correction is exemplarily shown here in a very practicable procedure for the Geodynamic Observatory Moxa, Germany. We show how the combination of geological and gravimetrical mapping and modelling around Moxa results in a significant correction of the original gravity data, as can be proven by comparison with the satellite-derived gravity field. Detailed geological mapping of the observatory surroundings, including measurements of fold axes, foliations and joints, was the basis of the present study. The fold structure in the area of interest was interpreted by geometrical 3D modelling. The complete representation of different rock types in space also provides a better understanding of the local hydro-geological situation. Using a combination of the 3D geological model with the high-resolution Bouguer map of the observatory surroundings, we developed a 3D density model. This model enables the correction of small gravity effects caused by mass variations close to the SG, in particular local hydro-geological mass changes. Thus, the procedure presented here, based on a close connection of geology and gravimetry, is a powerful tool for the reduction of local gravity effects on SG recordings. It should be applicable to SG stations worldwide, where similar hydrologically driven mass changes can be assumed.

Investigations about earthquake swarm areas and processes

Earthquake swarms are observed worldwide, especially in connection with fluid movement and volcanism. Two regions are compared by numerical investigations using the finite element method: The Vogtland/NW-Bohemia area situated at the border between Germany and the Czech Republic and the Magadi region in the Kenya Rift. For the Vogt-land area a high-precision three-dimensional gravity model was developed. That modelling shows an interaction between geometries of geological structures and geodynamic processes and yields strong indications that a magmatic system at the crust mantle boundary is much more probable than an upwelling mantle as a source of the earthquake swarms. The geodynamic models for the two regions under investigation take into account the regional stress field and thermal stresses as well as creep and plasticity with a porous elastic rheology. The investigations are fo-cussed on the interaction between pore pressure variations, temperature changes, fluid movements, stress accumulation and deformations. It is suspected that these processes play an essential role in the generation of earthquake swarms. An essential result of the modelling is that the existence of the regional stress field alone neither explains the occurrence of the earthquake swarms in the Vogtland area nor in the Magadi area. Temperature changes and periodic pore pressure variations in the earth’s crust are most important for the geodynamic processes, although they are weighed differently in each focal area.