201. The Baelo Claudia earthquake problem
- Author
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Grützner, Christoph and Reicherter, Klaus Rudolf
- Subjects
Angewandte Geophysik ,geophysics ,Baelo Claudia ,Cádiz ,Erdbebengefahr ,Geowissenschaften ,Bucht von Cádiz ,Elektromagnetische Reflexionsmethode ,Erdbebenzone ,Archäoseismologie ,archaeoseismology ,Spain ,Geophysik ,earthquake ,ddc:550 ,Erdbeben ,Paläoseismologie ,paleoseismology ,Spanien - Abstract
The Roman archeological site of Baelo Claudia (Cádiz, south Spain) is located within the Gibraltar Arch, a region with no significant recent or historical seismicity. However, previous studies have emphasized the occurrence of repeated strong archeoseismic damage (intensity >IX MSK) at Baelo Claudia. Identifying the damages in detail and searching for the seismogenic source was the aim of the Baelo Claudia Project - solving the Baelo Claudia earthquake problem. This thesis summarizes the results from the field campaigns in 2005, 2006, 2007, 2008, 2009, and 2010. The text is mainly based on two published and one submitted paper. A multidisciplinary study has been carried out at the archeological site including the detailed mapping of surface deformation and building damage, surface geology and geomorphology, and the collection of structural data. An extensive ground penetrating radar (GPR) survey has been conducted in the ruins for detecting archeoseismologic damages and imaging the subsurface conditions. The obtained data are not conclusive when considered separately, but obvious links between archeoseismic damage, structural and GPR data indicate that the destruction of the city was linked to seismic shaking and that two earthquakes occured within the Roman period. 14C dating results from damaged infrastructure (e.g., the Isis Temple) allow to suppose two devastating events around 40-60AD and 260-290AD. These dates vary from those proposed by earlier studies, which suggested two events in 40-60AD and 350-395AD. The analysis of the pattern and orientation of deformation clearly indicates SW-NE directed compression due to ground shaking. This investigation also focuses on localized landslides and liquefaction processes, which appear to be coeval with the earthquakes, but the poor geotechnical parameters of the clayey substratum were determinant to amplify the observed level of destruction. The application of the present Spanish seismic code (NCSE-02) indicates that intensity VIII MSK (0.24-0.26g) can be reached in this zone for 475 year return periods. Despite the earthquake-related damages and the dating of the events seem to be reliable, the search for the local seismogenic fault(s) was problematic. The damage patterns rather indicate a fault in the vicinity than a far-field event (e.g., the 1755 Lisbon seismic source in the Atlantic). High-resolution shallow geophysics were applied to the local faults mapped in order to get information on their activity; marine seismics in the Bolonia Bay were performed in 2006 for imaging the youngest sediments and active faults offshore. Based on the data, trenches were excavated at the Cabo de Gracia fault a few kilometres W of Baelo Claudia. Off-shore highresolution seismic investigations, structural and paleostress data and trenching studies combined with high-resolution GPR and resistivity point to active faulting along N-S trending normal faults. Hence, N-S directed normal faults in the area are claimed as local candidates for moderate earthquake activity. Paleoseismic evidence for a moderate event around 6000-5000BP has been found along the close-by normal Carrizales fault. This points to three local moderate earthquakes in the last 6000 years, and to return periods on the order of 2000-2500 years. In a third step, the observations have been quantified using the two logic trees for paleoseismology (Atakan et al., 2000) and archeoseismology (Sintubin and Stewart, 2008). The results show that a mere paleoseismological classification of the geological features leads to a paleoseismic quality factor (PQF) of 0.03, which is low compared to other studies. Taking into account the additional information from archeoseismological work (archeoseismological quality factor [AQF] is 0.5), it becomes clear that the Baelo Claudia study site provides an opportunity for detailed earthquake investigations. Therefore, it has a high potential for reliable seismic hazard analyses. A complementary application of both logic trees is recommended in future studies if sufficient data are available. The study illustrates that a multi-disciplinary approach in archeo- and paleoseismology is required for reliable data. It is shown that the southernmost part of the Iberian peninsula is characterized by a significant seismic hazard and that the Roman ruins of Baelo Claudia have preserved the earthquake information, even though it was not easy to gather.
- Published
- 2011