Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

Geological effects recognized as EAE are referred a fault
planes crossing the ancient town, soil displacements due
to seismic shaking, city damage for landslides and
rockfalls, liquefaction generating tilted monuments,
remains buried by seismites and tsunamites within the
archaeological stratigraphic sections. This kind of effects
is the traditionally cited ones in the still young
archaeoseismological literature.
Building fabric coseismic effects
These effects indicate direct damage on buildings
associated to ground deformation and/or seismic waves.
First of all, a complete understanding of the type of
construction (masonry, mortar type, column type, arch
construction etc.) used by ancient cultures in each
particular archaeological site is needed. This architectural
knowledge allows recognizing those potential buildings to
find archaeoseismic effects and discards those other ones
damaged by secondary effects.
Fig. 2: Kink folds in an irregular pavement. Decumanus Maximum
in the Roman City of Baelo Claudia (Cadiz, Spain)
In this case the differentiation of damage produced by (a)
natural ground instability or (b) seismic ground shacking is
a starting key point. Some good examples of probable
ground shacking deformations are folded pavements (Fig.
2), shocks and oriented fracturing in pavement flagstones,
folded mortar floors, pop up‐like structures on
pavements, titled and folded masonry walls, etc. (Fig. 3).
Other deformations exclusively affecting to the building
fabrics will be: penetrative fractures on masonry blocks,
displaced or differentially rotated arches (Fig. 4) or,
broken pottery found in fallen position, etc.
SECONDARY POSTSEISMIC EFFECTS
Other common deformations observed in present
damaged archaeological remains are typically linked to
the abandonment and eventual ruin of the site. Roof and
vault collapses of habited houses commonly trigger fires
by the burning of the wood fabric of most of the pre‐
modern roofs. Flash flood events triggered by earthquake
severe damage on ancient earth dams is also a source of
information but, complicate for decoding and
interpretation. Eventually the development of ancient
antiseismic either structures or building designs (Fig. 5)
are a key feature talking about previous strong seismic
shaking events. Destructive horizons within the
geoarchaeological record tell about histories of city
reconstruction. In this case the identification of
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology)
116
demolition horizons linked to city reconstruction after
severe earthquake damage is also a common feature in
archaeoseismological sites. In the same way the analyses
of recycled and reutilized architectural elements also can
indicate the nature and age of city reconstruction.
Fig. 3: Tilted wall in the defensive wall of the Roman City of Baelo
Claudia (Cadiz, Spain)
Eventually, the burial history of the archaeological site
can introduce complementary deformation and/or
amplify the existing ones. An analysis of the present
geomorphology of the area, operating geomorphic
process during burial and the urban geology of the
studied site are necessary to understand the existing
deformation, if deformation is coeval and finite.
All the aforementioned structures of deformation are
listed in the table of Figure 1, which illustrate most of the
deformational structures we can see today in
archaeological sites formerly affected by at least one
earthquake. As in classical palaeoseismic analysis the
building fabric coseismic effects has to be ascribed to a
particular geoarchaeological horizon, which will be the
earthquake horizon. In this sense before the structural
analysis of building deformation we have to collect the
complete geoarchaeological history of the city.
The simplest analysis of orientation of many of the
building fabric effects can help to put constrains on the
directivity or not of related ground deformation (e.g. Silva
et al., 2009). Consistently oriented building deformation
will indicate the sense of ground movement, but also to
differentiate it for other non‐oriented deformations
caused by other phenomena. The determination of the
geographical quadrant in which presumably a seismic
source (NE, SW, etc.) was located for an historic, non‐
documented, event is a quality step provided by the
archaeoseismology.
CONCLUSIONS: THE GEOLOGICAL STRUCTURAL ANALYSIS
OF DAMAGED STRUCTURES
The application of classical techniques on geological
structural analysis for ductile and brittle deformations
affecting buildings is a second quality step. The analysis of
the strain may allow the calculation of the strain ellipsoid
associated with the arrival of seismic wave. For the
interpretation of the results derived from structural
analysis is important the previous classification of the
structures following the aforementioned EAE guidelines.