Abstract

Archaeological excavations carried out from 2001 to 2003 at the vicus Tolbiacum site, located in the present day city of Zülpich, Germany, unearthed heavily damaged and destroyed late Roman fortification works. Damage includes 0.17-m-wide tensile cracks in a 3.1-m-wide wall, tilted walls and fortification towers, horizontal displacement of wall sections of 0.95 m, and rotation of wall fragments. Engineering seismological models are used to test the hypothesis that the observed building damage is of seismogenic nature. The site is located at the western edge of the Lower Rhine Embayment (lre), in the northern part of the Rhine rift valley. More than 20 damaging earthquakes occurred in the area over the past 300 years. Several paleoseismic events with magnitudes of at least 6.4 occurred during the Holocene. Site-specific strong motion seismograms are modeled for assumed large earthquakes at active faults in the lre using stochastic methods and calibrated Q models and duration models. Nonlinear site amplification is calculated based on an S-velocity model from refraction seismic experiments and downhole measurements. Finite element simulation of the dynamic behavior of a fortification tower shows a natural period of the soil-building system of about 3.0 sec. Horizontal and vertical displacements at the top of the 8-m-tall tower reached 0.12 m and 0.06 m, respectively, for the simulated event. No indications of man-made activity exist that could explain the damage. Running water can also be excluded as a threat to the buildings due to the orographic situation, and slow-acting gravitational causes can be excluded due to the damage structure. The destruction and damage is regarded as most probably seismogenic, and site intensity is assessed at IX on the European Macroseismic Scale (ems98) (Grünthal, 1998). This result marks the first time an intensity above VIII is described for a tectonic earthquake in Germany. Preliminary dating results from three carbon-14 (14C) accelerator mass spectometry (ams) samples lead to the second half of the fourth century a.d.

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