ALBERT

Description: A Neolithic wooden well was discovered and excavated between 1989 and 1992 near Erkelenz in the Lower Rhine Embayment. The construction, 3x3 m in size and 13 m deep, was exceptionally large for its time. The larger outer box-frame contained two smaller frames whose construction could be interpreted as an attempt to repair the damaged original well. The outer box was made from 160 oak elements of about 3 m length built in the blockhouse method. The large box is dated to 5090 BC and the two smaller ones to 5057{+/-}5 BC by dendrochronological analysis. At c. 8 m depth several elements of the large box are vertically sheared off and the broken parts moved inward and downward. The cause of this damage has not yet been determined. As the well is located only 3 km from one of the active tectonic faults in the Lower Rhine Embayment, a seismogenic origin of the damage is considered and tested in this paper. This question has relevance for determination of seismic hazard in an area with present-day moderate seismicity but documented occurrence of strong surface-rupturing earthquakes from the palaeoseismic record. First, a geotechnical model for the construction pit with a total volume of c. 540-550 m3 is used to prove the stability of the open pit during well construction and to help explain how the well was built. The seismogenic hypothesis is tested in a deterministic approach using theoretically derived ground motion at the site of the well for two simulated earthquakes with magnitudes 6.2 and 6.8. Ground deformation and relative displacement calculated with a finite element model of the casing are found to be too small to account for the documented damage. Among other potential sources of damage, swelling, shrinking or rotting of the wood elements are possible explanations; however, a conclusive answer to this question remains to be found.