Coseismic well-level changes due to the 1992 Roermond earthquake compared to static deformation of half-space solutions

The M-w 5.4 Roermond earthquake of 1993 April 13 was one of the strongest e vents during the last 500 years in Central Europe. For the period March-May 1992, we collected records of 194 continuously operating well-level sensor s, mostly located within 120 km of the epicentre. Nearly all wells penetrat e unconfined or poorly confined Quaternary deposits with high hydraulic con ductivities. 81 out of 194 raw data sets show a significant dynamic or step -like response of centimetre amplitude to the passage of seismic waves. Pre cursory anomalies are not obvious in these records. Coseismic well-level fl uctuations could reflect a redistribution of stress and pore pressure in th e brittle crust. Systematic analyses of such fluctuations may improve our k nowledge of the role of pore fluids in crustal rheology and earthquake mech anics. The rather high number of individual observational records for a sin gle event allows a regional correlation of the signs and amplitudes of the coseismic steps to changes in volume strain caused by the earthquake. The c oseismic strain field at the surface was calculated for a homogeneous and a layered half-space. The results show reasonable agreement in the sign of t he well-level steps but the amplitudes predicted from the wells' volumetric strain responses are much smaller than those that were recorded. Clearly, the coseismic well-level steps cannot be explained by volume strain changes , as derived from linear elastic models.