Neotectonics and late Holocene paleoseismic evidence in the Plio-Quaternary Daroca Half-graben, Iberian Chain, NE Spain. Implications for fault source characterization

The official seismic hazard models in Spain used in the seismic building codes do not incorporate Quaternary faults, largely due to insufficient data for their proper characterization. There is an obvious need to conduct investigations in most of the recognised Quaternary faults to unambiguously demonstrate their Quaternary tectonic activity and assess their seismogenic potential. This work illustrates the integration of cartographic, tectonic, geomorphological, paleoseismological and geophysical methods for the characterization of the slow-moving extensional Daroca Fault, related to the negative inversion of the Alpine Daroca Thrust, Iberian Chain, NE Spain. Cartographic data indicate that the 27 km long Daroca Fault and the 17 km long Calamocha Fault, separated by a stepover 1.9 km wide, can be considered as segments of the same structure that might rupture jointly and generate Mw7 earthquakes. A long-term slip rate of 0.06–0.02 mm/yr has been estimated for the 27 km long Daroca Fault using an ESR-dated (Electro Spin Resonance) offset pediment. The work discusses why this slip rate is significantly lower than those estimated in nearby normal faults using OSL ages (Optically Stimulated Luminiscence), but comparable with those derived from offset early Pliocene limestones. A trench excavated across the Daroca Fault exposed evidence of the MRE (most recent event) on the fault, with bracketing ages of 2354–1544 cal yr BP (404 BC – 386 AD). This event likely caused the destruction and abandonment of Roman cities in the vicinity of the fault. Several explanations are proposed for the anomalously low vertical displacement of this surface faulting event recorded in the central sector of a 27 km long fault segment: multi-strand rupture, full-segment rupture, partial segment rupture, spillover rupture, and secondary sympathetic rupture.