abstract (from Grecksch, G.; Fischer, K. D. & Kümpel, H.-J. (1997). Z. dt. geol. Ges., 148/3-4, 341-355)

Seismotectonically induced well level changes in the Lower Rhine Embayment

Well level anomalies due to tectonic earthquakes have been observed in many earthquake regions. In the Lower Rhine Embayment numerous coseismic well level changes were observed following the M5.9 Roermond-Earthquake of April 13, 1992. This event was one of the strongest earthquakes during the last five hundred years in Mid-Europe. Through questionnaires we collected recordings of altogether 194 continuously operating well level sensors. Nearly all data are from shallow wells penetrating unconfined aquifers. About 40% show a significant dynamic or step-like response of cm amplitude to the event. A precursory anomaly was in no case evident. This data set is one of the most extensive collections of coseismic hydrological signals for a single earthquake.

Coseismic well level fluctuations are believed to reflect sudden pore pressure changes associated to in-situ volume strain and the redistribution of stress in the brittle crust. Systematic analysis of such fluctuations may improve the knowledge of the role of pore fluids in crustal rheology. The apparently random distribution of wells with significant or no anomalies lead us to conduct slug tests in more than 60 different wells in order to check for irregularities in transmissivity. A clear relationship between the amplitudes of anomalies and hydraulic conductivities of the connected aquifers was not obvious. The earthquake's static coseismic strain field, derived from analytical model calculations for a homogeneous half-space, is in reasonable agreement with the sign of the observed well level steps but their amplitudes are about two orders of magnitudes larger than those predicted from the wells' volumetric strain sensitivities. The observed irregular amplitude distribution of the well level anomalies is possibly due to a spatially heterogeneous distribution of pore pressure, which to some extent is equilibrated during the passage of seismic waves. This is the target of ongoing numerical poroelastic modelling.