Nature of the recent vertical ground movements inferred from high-precision leveling data in an intraplate setting: NE Ardenne, Belgium

Yearly high-precision leveling surveys have been performed from 1993 to 1998 along a 48-km-long local network centered on Gileppe Lake, at the foot of the Ardenne massif, Belgium. Institut Geographique Militaire and Institut Geographique National (IGM-IGN) levelings of 1948 and 1974 yielded additional information. The data were corrected for collimation and thermal errors. Other sources of error (Earth tides, atmospheric refraction, rod miscalibration) were estimated, and corrections were made when necessary. Random errors of individual surveys range from 0.15 to 0.74 mm/root km. The recorded elevation changes indicate the existence of tilted segments some 5 to 10 km in length, with yearly tilt values amounting up to 2 mu rads. Fault movements of a few mm/yr are also demonstrated. The segment limits are spatially stable from one year to the next, but very frequent sense reversals of the vertical motions are observed. This explains why the total movement over a 20- to 30-year period is barely higher than the yearly displacements. Various nontectonic causes of vertical ground movement (e.g., monument instability, varying water loading of the lake, sediment compaction, etc.) have been considered, yielding little satisfactory explanation of the patterns of observed elevation changes. We propose that the recorded motions, which we call reversible fault creep and reversible tilt, respectively, are a true tectonic signal, reflecting the "tectonic noise" which accompanies the long-term deformation of uplifting areas in intraplate settings. Such movements are characteristic of fault segments which may occasionally generate earthquakes.