Reconciling geodetic and geological rates of vertical crustal motion in intraplate regions

Abstract

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 Wrad/yr for a 2.8-km-long segment. ß

Slip rate and mode of the Feldbiss normal fault (Roer Valley Graben) after removal of groundwater effects

“…That this noise is also concentrated across the fault is demonstrated by the 1.13 mm standard deviation of the height change variations of section 57-55, 3 times higher than that of the other sections, despite the proven individual stability of BM 57 and 55. At the local differential scale, the principal noise source has been shown to be the variations of the groundwater level [10,[26][27][28].…”

“…One possible explanation of the problem could lie in that the geodetic rate estimates, in particular for the very low rates of intraplate normal faults, are often biased by groundwater-dependent vertical movements of the ground [10]. Relying on almost four years of repeated precise levelling surveys across the Feldbiss fault at Sittard (Fig.…”

Seasonal height change influence in GPS and gravimetric campaign data

“…with P m = monthly rainfall, is of course conditioned by the absence of groundwater exploitation in the area, and was demonstrated for levelling data in NE Ardenne (Demoulin, 2004). With a predominant loading effect, a decrease in f, supposed to reflect a groundwater lowering, should induce a positive vertical displacement of the ground and vice versa.…”

“…Vertical displacements of the ground are monitored since 1992 in NE Ardenne in order to detect possible tectonic motion either of regional extent or located on particular active faults (Demoulin, 2004;Demoulin et al, 2005). Indeed, this information may significantly contribute to define the regional seismic hazard, provided a clear separation can be made between the reported tectonic displacements and other recorded ground movements caused by near-surface influences.…”

“…The tectonic signal extraction is particularly difficult in the intraplate setting of NE Ardenne, where vertical ground displacements are frequently observed to reach a few mm/year while the expected long-term tectonic motions are at best in the order of 0.1 mm/year. In this respect, groundwater level variations have been demonstrated to play a major role, causing millimetre-scale local differential vertical displacements even when the compared surveys are carried out in the same time of the year (Demoulin, 2004). Groundwater-dependent absolute movements at a particular site are thus expected to be still larger.…”