Surface-rupturing paleoearthquakes in a context of slow deforming continental interiors: the Lower Tagus Valley fault, Central Portugal

Estimating the rate and the pattern of active deformation of slow-slip structures in intracontinental regions has always been a challenging task. Central Portugal is one of those intracontinental regions where the convergence of Eurasian and Nubian plates governs its active deformation. The NE-striking Lower Tagus Valley (LTV) is a locus of active deformation and several historical earthquakes. The eastern and western margins of the LTV are fault-controlled zones (Lower Tagus Valley Fault Zone; LTVFZ), characterized by the predominant strike-slip component. The ~80 km long LTVFZ is one of the most significant intraplate structures in mainland Portugal, and its seismic activity may pose a considerable threat in densely populated urban and industrial areas developed along the LTV. However, the spatio-temporal seismic history along the main structures of LTV is still poorly constrained. In this study, we investigate the geomorphologic features along the Eastern LTVFZ using high-resolution digital aerial orthophotos, high-resolution topographic data extracted from airborne Light Detection and Ranging (LiDAR) data sets, drone-derived high-resolution topographic data, and very high-resolution orthophotos acquired by a small unoccupied aerial system. Removing vegetation cover by LiDAR data leads to access to bare earth surface models that are essential to recognize subtle geomorphic features and constrain their offsets. Accordingly, several cumulative left-lateral displacements were measured along a 20-km stretch of the Eastern LTVFZ. The smallest measured offsets range between 2 and 3 meters that may correspond to the coseismic slip during the most recent surface faulting.     

To specify the contribution of the Eastern LTVFZ to the regional seismic hazard, we investigate its seismic history through three paleoseismic trenches excavated across the fault near the city of Almeirim. The stratigraphic units, structural features, and geological relations were first logged in the field and then evaluated using the high-resolution, rectified seamless trench-wall photomosaics. Several vertical to steep fault strands exposed in the trench walls cut through the late Pleistocene and Holocene alluvial deposits, recording the signature of several strong paleoearthquakes. Stratigraphic analysis and optically stimulated luminescence dating suggest that the most recent surface faulting has occurred sometime in the middle-late Holocene. The horizontal displacement of this earthquake was measured at two localities nearby the trench site, both in the field and on the very high-resolution orthophotos, and amounts to 2 to 3 meters of the on-fault sinistral offset. The evidence of an older earthquake has been preserved in the late marine isotope stage (MIS) 3 deposits, and at least two even older earthquakes recorded in a sequence of alluvial deposits that predate MIS 4. Although the Eastern LTVFZ may be characterize by low slip rates and instrumentally a quiescent structure, it remains capable of generating morphogenic large-magnitude earthquakes of Mw 7 to 7.5 with millennial recurrence intervals. Such seismic behavior challenges the reliability of assessing seismic hazard over slow-slipping faults across intraplate settings in the lack of geological information.