Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology)
EVIDENCE FOR A MEDIEVAL EARTHQUAKE IN THE AACHEN AREA (GERMANY),
REVEALED BY STRUCTURAL DAMAGE IN THE CATHEDRAL
A. Schaub (1), K. Reicherter (2), C. Grützner (2) and T. Fernández‐Steeger (3)
(1) Archaeologist of Aachen city; Verwaltungsgebäude Am Marschiertor, Lagerhausstraße 20, 52064 Aachen, GERMANY
(2) Neotectonics and Natural Hazards, RWTH Aachen University. Lochnerstr. 4‐20. 52056 Aachen, GERMANY
(3) Chair of Engineering Geology and Hydrogeology, RWTH Aachen University. Lochnerstr. 4‐20. 52056 Aachen, GERMANY
Andreas.Schaub@mail.aachen.de, k.reicherter@nug.rwth‐aachen.de
Abstract: Archaeoseismological and palaeoseismological investigations in the Carolingian Chapel of the Aachen Cathedral proved evidence for
an earthquake with a minimum magnitude of 5.5 in the beginning of the 9 th century AD. Systematic cracks in the stone masonry, ground floor
and foundations, as well as in the cupola of the chapel, point to a high‐energy event. Repair of the damaged structures suggest that those
were carried out during the construction of the chapel in Carolingian times. Furthermore, clay‐filled cracks, so‐called “injection structures”, are
filled from below by squeezing high‐plasticity clay into sharp‐edged broken Pleistocene eolian sediments (Loess), underpinning our findings
with geological observations. Historical documents, the construction history of the chapel and coin findings support the idea of a damaging
earthquake in the Aachen area in 803 AD.
Key words: archaeoseismology, injection structures, structural damage, Lower Rhine Graben
INTRODUCTION
Geological and seismotectonic background
The neotectonic and landscape evolution of the Lower
Rhine Graben (LRG) in western Germany are directly
linked to the Alpine Orogeny and mainly characterized by
subsidence. However, secondary processes (like
earthquakes, extensional tectonics, volcanism, influence
of glaciation/deglaciation of northern Central Europe, and
anthropogenic modifications, i.e. open cast lignite mining)
influence strongly landscape. The geomorphology of the
area is typical for a “seismogenic landscape”, with
pronounced scarps and active faulting, here mainly
normal faults (Fig.2).
Aachen is situated along the northernmost edge and on
the frontal Variscan thrust fault, trending NE‐SW. One of
the major fault lies directly below the plaza between the
palatine and the Carolingian chapel (Figs. 1 and 3).
Younger, NW‐SE trending normal faults cut and displaced
these older structures (Fig.3). Palaeoseismic and historical
earthquake data suggest several major events during
prehistoric times (Camelbeeck and Meghraoui, 1998;
Vanneste et al., 2001, Hinzen and Reamer, 2007),
instrumental data, however, show only minor to
moderate seismicity (with a maximum during the
Roermond earthquake, April 13 th 1992, ML 5.8). Some
well‐preserved remains of structures of Cologne from the
late Roman period (4 th century AD) show severe building
damage (Hinzen and Schütte, 2002). The Praetorium, seat
of the Roman administration and palace, was located on
the banks of a former side arm of the Rhine River. Lack of
any traces of attempted repairs and written sources
which document use of the building for a long time after
the end of the Roman Empire favour a sudden onset of
damage over a gradual process (Hinzen and Schütte,
2002). These authors dated the earthquake activity
observed in Cologne in 800 to 840 AD.
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In contrast to the Dutch and Belgium parts of the LRG, the
German elements of the Lower Rhine Graben are poorly
studied, with the exception of several trench studies by
the Geologischer Dienst of NRW (Skupin et al., 2008).
Here, we report on probable earthquake‐related
structural damage and palaeoearthquakes in the Lower
Rhine Graben area.
Historical background
Charlemagne (Charles the Great) had placed the centre of
his kingdom during Carolingian times in Aachen, Germany
(Aquisgrana). This medieval period and his regency are
regarded to set a benchmark in the birth of Europe, and
are associated with a period of extensive constructions in
Aachen.
Fig. 1: Model of the Carolingian Palatine and octagon Chapel in
Aachen, with Variscan thrust fault