## 2 Archaeology and Archaeoseismology of _Baelo Claudia_  
_Baelo Claudia_ is a well-known ancient Roman city located on the edge of the Gibraltar Strait (Cadiz, South Spain) in the Bolonia Bay (Tarifa, Cadiz). The geology of the area is complex, embedded in the Cretaceous to Eocene flysch units of the Betic Cordillera (Figure 1). However, the archaeological site is totally located on marly Cretaceous materials (Bolonia Fm. ), with some more competent interbedded limestone levels originating minor scarps. Towards the littoral sector of the city, some marine terrace formations developed during the middle-late Pleistocene, giving place to the stair-cased coastal landscape on which the old city was built. Geotechnical boreholes in the theatre sector (S2 in Figure 2) identify a buried late Pleistocene terrace at +13-14 m above sea level [9]. Upper terraces at +20-22 m border the lower monumental sector of the city, and more recent terrace levels at +7-8 m constitute this lower plateau of the city. In any case, this lower plateau is buried by thick post-Roman colluvium and aeolian deposits (Figure 2). In particular, the post-Roman dunes buried most of the lower sector of the city after its abandonment [9], preserving the seismic deformations analyzed in this paper. The city developed from the first century BCE to the late fourth century CE. The writings of the Greek geographer _[PERSON]_ (first century CE) indicate that the site was the main harbor in the westernmost Mediterranean during the time of the Roman Empire [8]. _[PERSON]_ described this old settlement (Belon) as an important industrial fishing and \"garum\" center, as well as the main port for reaching the _Mauritania Tingitana_ (North Africa) on the other side of the Strait of Gibraltar (Figure 1). It is important to note that the sole written documents that mention Belon in antiquity come from the work of [PERSON] [8]. The rest of the history of the city has been built from the successive archaeological findings (coins, ceramics, epigraphy, architectural styles, etc.) from excavations undertaken at the site since the early 20 th century [8]. Archaeological excavations in the city evidence three distinct urban periods, which are separated by important discontinuities in the gearchaeological record [9]. Since the early 1980s, these archaeological discontinuities have been linked to the occurrence of seismic events [8; 21; 22]. Later studies [9; 13; 14] clearly indicate the occurrence of two intervening earthquakes in the years 40-60 AD and 365-390 AD as the most decisive events in the urban development of this ancient Roman city. Both earthquakes are catalogued as \"_ancient earthquakes_\" only evidenced by their archaeoseismological record, since there are no historical writings documenting these two earthquakes. Both are presently included in the _Catalogue of Geological Effects of Earthquakes in Spain_[23]. Figure 1: Geological context of the Bolonia Bay showing the most important fault systems in the central area of the Strait of Gibraltar and the most probable seismic sources (CGF and MOF) in the area responsible for the earthquake in the fourth century AD (red stars). Legend: (1) Cretaceous–Eocene Almarchal Fm. (clayey marks and limestones); (2) Paleogene Bolonia Fm. (red clays and limestones); (3) Lower Miocene Facinas Fm. (clayed disthostromic unit); (4) Miocene Alijbe Sandstones and Algeciras Fms. (flysch units); (5) Pliocene (with circles) and Quaternary post-orogenic materials; (6) Major landslides identified in the area. Stratification directions and subcortical folds are identified with dashed lines. Normal faults (with squares) and reverse faults (with triangles) are identified using conventional symology. Modified from [PERSON] et al. [14]. The first earthquake is documented by the large works carried out in the lower sector of the city, which underwent important topographic and levelling works to build the new _Baelo Claudia_, obtaining the status of _\"Roman City\"_ around the year 60 AD [8,21].