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

Summary: The Mw 5.4 Roermond earthquake of 1992 April 13 was one of the strongest events during the last 500 years in Central Europe. For the period March-May 1992, we collected records of 194 continuously operating well-level sensors, mostly located within 120 km of the epicentre. Nearly all wells penetrate unconfined or poorly confined Quaternary deposits with high hydraulic conductivities. 81 out of 194 raw data sets show a significant dynamic or step-like response of centimetre amplitude to the passage of seismic waves. Precursory anomalies are not obvious in these records. Coseismic well-level fluctuations may improve our knowledge of the role of pore fluids in crustal rheology and earthquake mechanics. The rather high number of individual observational records for a single event allows a regional correlation of the signs and amplitudes of the coseismic steps to changes in volume strain caused by the earthquake. The coseismic strain field at the surface was calculated for a homogeneous and a layered half-space. The results show reasonable agreement in the sign of the 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.