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Despitetheavailabilityofstudiesonthefrequencydensityoflandslideareasinmountainousregions,frequency–areadistributions of historical landslide inventories in populated hilly regions are absent. This study revealed that the frequency–area distribution derived from a detailed landslide inventory of the Flemish Ardennes (Belgium) is significantly different from distributions usually obtainedinmountainousareaswherelandslidesaretriggeredbylarge-scalenaturalcausalfactorssuchasrainfall,earthquakesorrapid snowmelt. Instead, the landslide inventory consists of the superposition of two populations, i.e. (i) small (b1–2·10 −2 km 2 ), shallow complexearthslides that areat most30yrold, and(ii) large(N1–2·10 −2 km 2 ), deep-seatedlandslides that areolder than100yr. Both subpopulations are best represented by a negative power–law relation with exponents of −0.58 and −2.31 respectively. This study focused on the negative power–law relation obtained for recent, small landslides, and contributes to the understanding of frequency distributions of landslide areas by presenting a conceptual model explaining this negative power–law relation for small landslides in populatedhilly regions.According to the model hilly regions canberelatively stable under the present-dayenvironmental conditions, andlandslidesaremainlytriggeredbyhumanactivitiesthathaveonlyalocalimpactonslopestability.Therefore,landslidescausedby anthropogenic triggers are limited in size, and the number of landslides decreases with landslide area. The frequency density of landslide areas for old landslides is similar to those obtained for historical inventories compiled in mountainous areas, as apart from the negative power–law relation with exponent −2.31 for large landslides, a positive power–law relation followed by a rollover is observed for smaller landslides. However, when analysing the old landslides together with the more recent ones, the present-day higher temporal frequency of small landslides compared to large landslides, obscures the positive power–law relation and rollover. © 2007 Elsevier B.V. All rights reserved.

The Feldbiss fault is the main active fault bounding to the southwest the Roer Valley Graben of NW Europe. In order to investigate the reasons of the discrepancy between its short- and long-term slip rate estimates, we have repeatedly surveyed a 2.3 km-long line by levelling at Sittard (The Netherlands) from April 2001 to December 2004. Simultaneously, three superposed aquifers were monitored on both sides of the fault. The across-fault differential groundwater variations are shown to be responsible for a seasonal 0.87 mm/m vertical motion of the fault, mainly through the effect of pore pressure changes in confined aquifers. After removal of this effect, I get a residual trend of − 0.60 ± 0.11 mm/yr for the relative motion of the hangingwall. This trend was disturbed in the summer of 2002 by transient millimetre-level up and down motions temporally associated with the small Eschweiler earthquake that occurred on July 22, 2002 on the Feldbiss fault at a distance of 35 km from Sittard. I explain this as a creep event triggered by the passage of trapped seismic waves. The subsisting discrepancy between the long-term slip rate of 0.06 mm/yr and short-term rates of ∼ 1.5 mm/yr (from 1962 to 2001) and 0.6 mm/yr (from 2001 to 2004) is tentatively interpreted as resulting from the current occurrence of a decade-long episode of aseismic slip involving a large part of the fault plane. Occasionally associated with minor seismicity, this aseismic slip event could betray a thickening of the transitional region located below the upper crustal zone of stable sliding. It causes a total fault slip of 5–10 cm in a few tens of years, releasing smoothly all, or a great part of, the strain accumulated during the previous 1–2 ky of fault quiescence.

Tectonic motion rates of individual faults and regional deformation in an intraplate setting are estimated, based on the analysis of ten yearly surveys of a regional levelling network in NE Ardenne (W Europe). Owing to the frequency and number of measurement epochs, much greater than in classical comparisons of general surveys, the tectonic and near-surface components of ground motion are clearly separated. The marked temporal variability in ground motion strongly depends on the amount of precipitation fallen during the six months preceding each survey and the subsequent drying off of the subsoil at the time of the surveys. Moreover, the ground response to this influence varies from place to place, leading to high differential movements at the local scale. Taking into account the percentage of surveyed faults which moved tectonically during the time of the study, I calculate fault motion rates of 0.06–0.09 mm/yr, similar to geological rates. Moreover, the data indicate that one way for intraplate normal faults to accommodate tectonic strain aseismically is intermittent fault creep, with short episodes of a few mm displacement separated by many year-long time intervals of fault quiescence. A flexural deformation shows the superposition of a tilt event on a trend corresponding to a tilt rate of 0.16 μrad/yr for a 2.8-km-long segment.

In order to get a better insight into the temporal pattern of height change variations in a moderately seismic intraplate area, a 1 km long section has been weekly leveled from April, 21, 1997 to January, 12, 1998 in NE Ardenne (Belgium). As this section is located very close to a small reservoir of 25U10 6 m 3 , it also allowed the influence of lake level variations on the nearby ground motion to be studied. The measured height difference variations show a maximum amplitude of 3.45 mm, with weekly values not exceeding 1 mm. A two-dimensional (2D) finite element modeling confirms that waterload variations in the lake are the primary cause of movement of the section, inducing maximum ground subsidence of about 2 cm and seasonal tilting of 3^4 Wrads within a range of 2^3 km. We also show that a fault passively alters the spatial distribution of the waterload-dependent movements. The removal of this component from the observed ground motion leaves oscillating residual displacements characterized by an amplitude of 1^1.5 mm and a ‘period’ of about 2 months. We demonstrate that these displacements are independent of rainfall and probably of groundwater fluctuation too. Although most of the residual motions take place on the fault straddled by the leveling section, no credible mechanism can be found to ascribe these motions to a tectonic process. fl 2000 Elsevier Science B.V. All rights reserved.