Search

We present the HARD project of GPS monitoring of vertical ground motion in NE Ardenne and Eifel (western Europe). Its main purposes are to get a better insight into the present-day rates of vertical ground motion in intraplate settings and to identify the various causes of these motions. Since 1999, we have carried out yearly campaigns of simultaneous GPS measurements at 12 sites situated so as to sample the different tectonic subunits of the study area and especially to record potential displacements across the seismogenic Hockai fault zone. Five campaigns (1999–2003) have been processed currently. Key issues of the data processing with the Gamit software are discussed and first results are presented. Though temporally consistent in many cases, the obtained vertical motion rates are spatially highly variable. They are also much too high (several mm/year) to support a tectonic interpretation, and a long-term influence of groundwater level variations is proposed to account for the observed motions. This influence should be distinguished from seasonal variations and from inter-survey variations linked to the varying degree of soil and subsoil drying off during the successive spring surveys.

Past landslides have been recognized in the Battice area in E-Belgium. In contrast to the other inactive landslides, the Manaihan landslide responded immediately to heavy rainfall events in the last two decades. This study aims to map its spatial extent and the dominant surface features; to measure surface displacement using GPS; to investigate subsurface structure with Cone penetration test (CPT) and corings; and to determine the depth of the shear surface by inclinometers. Results show a partial landslide reactivation. Surface velocities range between 20 and 40 cm/year and are strongly dependent on winter rainfall. CPT results give clear boundaries between the landslide mass and the undisturbed bedrock in the head scarp. Distinct shear surfaces have been determined with displacement rates up to 15.8 mm in 21 days. Further research should apply geophysical methods for two-dimensional information on the ground, investigate geotechnical properties of the landslide mass, model slope instability, and determine the influence of a sewage pipe crossing the central landslide mass as a potential cause for landslide activity.

A series of palaeolandslides is described in eastern Belgium, with geophysical investigations, trench analysis and a number of 14C dates complementing the field description. The proposed sliding mechanism stresses the importance of initial liquefaction of the upper Cretaceous Aachen sands underlying the ~30-m-thick Vaals Clay Formation, in which all landslides are developed. Slope-stability analyses support the hypothesis of sudden landsliding and yield further useful information to discuss their origin. A main difficulty in determining the landslide trigger arises from insufficient absolute dates. Assuming that all landslides occurred simultaneously, we weigh the probabilities, respectively, of a climatic and a seismic trigger. Although the 150 a.d. possible date of landslide initiation falls close to one of the wettest periods of the Holocene, the spatial distribution of the deep slides and their proposed mechanism strongly suggest a seismic origin in connection with a rupture of the nearby Ostend segment of the active Hockai fault zone. However, considering that the nearby 1692, Verviers earthquake apparently caused no ground failure, it is probable that both the climatic and seismic triggers were jointly needed to provoke such deep and extended landsliding on moderate, generally stable slopes.

Precise levelling data are examined in the southern Black Forest massif by comparing them analytically along levelling profiles. Instead of generalizing regional vertical motion trends, the method emphasizes the current activity of accurately located individual structural elements. Measured recent crustal movement rates range from 0.2 to 1.1 mm/year for the 1930–1961 (1972) period, the inferred motions corresponding to aseismic fault creep. Based on the near infrared band image of a Landsat MSS scene, a lineament map of the study area is compared with the recent crustal movement (RCM) map. The Upper Rhine graben eastern border fault between Badenweiler and Freiburg displays the highest RCM: these displacements are consistent with the geological motion and the topographic expression of that structure. Within the massif, both the lineament map and the RCM map demonstrate the predominant role played by WNW/ESE- to NW/SE-striking structures in accommodating the deformation. Moreover, with the seismic dislocations in the southern Black Forest taking place also mainly on N120 and N20 faults, it is most probable that these active structures are deep-reaching zones of crustal weakness. To some extent, seismicity and RCM appear, however, to be mutually exclusive in their spatial distribution, the seismic Black Forest flexure and Dinkelberg block being opposed to the aseismically moving eastern and northern parts of the study area.