A New Fault Branch Revealed by Geology and Archaeology Under the Seismic Lorca Town (Southeast Spain)

Lorca town (southeast Spain) is on the trace of the southwest-northeast Alhama de Murcia fault. This fault splits into several branches in Lorca, which are hidden under the urban area. Most of the branches were identified in excavation sites and deformed houses at the surface. The formerly hypothesized Alburquerque branch is here confirmed, with the discovery of two closely spaced faults in the 6 Selgas street excavation site. This branch, coated with fibrous gypsum, cut and verticalized middle Miocene lutites during the Plio-Quaternary, creating a breccia and fracture cleavage. It also produced surface rupture, disturbing both Late Antiquity burials and a XIII century red gravel unit, most probably accompanied by earthquakes in the town between the 5th century and the house construction date (1775 AD). The preserved southwest Aguado alley facade of that house appears nowadays bent, with the vertical edge of the dihedral angle located directly above the Alburquerque fault. This implied both 2.5° of rotation and 0.2 m of horizontal displacement in the southeast corner of the facade, along the last 250 years. Series disruption, breccia, fracture cleavage, surface rupture, and recent rotation, all features together suggest left-lateral oblique-slip action of the Alburquerque fault.