Constraining Late Pleistocene to Holocene seismic fault activity in NE Iberia: The value of integrating complementary techniques in a low-strain region

The northeastern margin of the Iberian Peninsula, extending from the Vallès-Penedès Graben to the Valencia Depression, constitutes a passive margin related to the opening of the Valencia Trough during the Neogene. It comprises a series of extensive basins bounded by several mountain ranges, where numerous NNE–SSW-oriented normal faults are present, commonly associated with mountain fronts. Previous studies show that some of these faults, such as the El Camp fault, remain active, although at very low slip rates, as low as 0.02 m/kyr for Late Pleistocene.
Recent analyses of high-resolution Digital Elevation Models (DEMs) have revealed several additional morphological escarpments crosscutting different Quaternary alluvial fan systems across the region. These scarps, found from the Sant Jordi Plain to the La Salzadella Basin, share the same orientation as the Neogene faults, suggesting a common tectonic origin. They are discontinuous, arranged in a right-stepped pattern, and locally display vertical offsets of up to 8 m. Furthermore, several geomorphic features indicate recent tectonic activity, including aligned fissures within the fans and entrenched channels developed on the upthrown block that fade out after crossing the escarpments. Each family of escarpments exceeds 10 km in length, with important implications for the seismic hazard of the region.
Geophysical surveys (ERT, GPR, SRT, HVSR, and MT), validated with borehole data, confirm the presence of faults beneath each analysed escarpment system. At the Vinaixarop escarpment, for instance, ERT profiles revealed a steeply dipping discontinuity plane with a vertical offset of approximately 40 m affecting upper Pliocene sediments.
The faulted alluvial fans are interpreted as Lower to Middle Pleistocene in age and are mainly composed of carbonate gravels. Paleoseismological trenches excavated at four sites (L’Ampolla, Vinaixarop, and two at Sant Rafael del Riu) on different scarps revealed consistent evidence of late Quaternary faulting, such as ruptured strata. Additionally, ground-based hyperspectral cameras (400–1700 nm) were deployed on trench walls as an ancillary tool to map faulted
stratigraphic layers and to detect subtle coseismic deformation. As sedimentation on the fans ceased by the Middle Pleistocene, subsequent activity has been primarily recorded through the deformation of pedogenic features (calcretes), commonly found in the study area. To constrain the timing of this activity, U–Th dating was performed on fault-related carbonates and deformed calcretes. Preliminary results indicate that tectonic activity along these faults persisted at least until the Late Pleistocene. However, a deformed colluvial wedge and the presence of open fissures observed on trench walls suggest Holocene activity.
Additionally, three cosmonuclide depth-profiles were sampled to date displaced geomorphic surfaces, and hence estimating the long-term slip rates of the faults. Ongoing analyses aim to demonstrate Holocene seismic activity and to further characterize paleoearthquakes and their seismic parameters.
In summary, studies of active tectonics in regions of very low strain are inherently challenging, especially when recent sedimentation is scarce and pedogenic processes are intense. Nevertheless, this work highlights that integrating multiple complementary techniques is the most effective approach to address such settings.