Aquisgrani terrae motus factus est (part 2): Evidence for medieval earthquake damage in the Aachen Cathedral (Germany)

During archaeological and geotechnical investigations, the Carolingian part of the Aachen Cathedral (Germany) was excavated. The Aachen Cathedral was the first building in Germany to join the UNESCO World Heritage list. In the rock walls of the fundament and medieval floors, open cracks were found. Subsidence of the mortar floor of about 50 cm with lateral dragging, which was reconstructed during the construction of the Cathedral, can be observed. The main building is founded on Devonian limestones and shales, Pleistocene loess and Roman urban debris. The loess and loamy sediments show angular cracks and fragments, which are not related to permafrost features but are interpreted as shear and extensional fractures. Some of the cracks are filled with black clay from below as they open downwards. These structures are interpreted as injection features caused by thixotropy of clays. Open cracks, lateral spread and injections structures occur usually during earthquakes with a magnitude higher than 5.5.The historical German earthquake catalogues provide three important passages for historical earthquakes in the region of Aachen. Both fall into a relatively short time interval. One occurred in winter 803 AD, another one in 823 AD, and the last one in 829 AD between Ash Wednesday and Easter. Both notes tell of earthquake shaking and significant structural damage and fatalities. This and the constructional history allowed dating of the event, favouring the 803 AD event as the causative damaging earthquake observed in the Aachen Cathedral.