Miguel Ángel Rodríguez-Pascua (1)
(1) Geological Survey of Spain
The strain structures observed in archaeological sites affecting man-made constructions like buildings, monuments, pavements, defensive structures, water-supply facilities and ancient harbours, can be assigned to: seismic ground motion, intervening slope process, unstable soils, differential overburden during the burial process, water wave action and simple collapsing due to building abandonment or actions of war, among others. All of these possibilities suggest the necessity of a systematically identification and classification of the trigger mechanism in damaged archaeological remains, especially pointing to their seismic or no-seismic origin. The main question is differentiate the origin of this destruction, for this reason the strain analysis of the ruins can contribute in order to differentiate the origin of the deformations. Before the strain analysis, it is necessary classify the earthquake archaeological effects (EAEs).
The EAEs classification considers both primary and secondary seismic origin (impacts, wall tilting, pavement folding, etc.) and orientation of the damage in relation to the structure of the building. This classification becomes relevant, once a set of EAEs is recognized and classified for a damaged site. This fact implies that both the ductile and brittle structures affecting monuments, walls and buildings can be analysed using the classic structural techniques developed in structural geology. The results obtained from this type of analysis, will allow the reconstruction of the theoretical strain ellipse defined by the deformed fabric of the damaged buildings. The orientation of the strain axes, in particular that defining the maximum horizontal shortening (ey) could be assumed as the orientation of the main seismic ground motion. Its comparison with respect to existing macroseismic information for ancient earthquakes can help to discriminate the causative seismic source. But in those cases in which available macroseismic information is scarce or even null the obtained archaeoseismic strain ellipses can help to identify probable active tectonic structures causing the analysed damage. In conclusion, the systematically orientation of the “ey” can be used like a seismic criteria to discriminate other trigger mechanisms.