One of the main results of the project has been development of an approach to on-fault identification of paleoearthquakes related to thrust faults (Fig.1). This approach has been used to determine the paleoearthquakes occurred in different active faults in the Ventura basin (California margin). This zone has been the focus of different studies, but during PALEOSEISQUAKE we have mapped for the first time with high accuracy the offshore active structures and have produce the first onshore-offshore structures correlation. Based on the results of the paleoseismological study, we have determined and characterized the occurrence of 3 to 4 large earthquakes during the Holocene related to these structures. In addition, the estimated surface vertical displacements (1-10 m) show the potential of these faults to also generate a tsunami during the earthquake rupture.
The Newport-Inglewood-Rose Canyon (NIRC) fault system, in the California Borderlands, has been studied to reveal the relationships between faults and folds in the area. To this aim, we have interpreted a 3D sparker seismic dataset. The results show a complex structural pattern with two main fault systems, almost perpendicular between them, and the formation of folds usually in zones where these two fault systems intersect. This points that this structures may be related to a larger shear zone produced between the NIRC and the San Marcos faults. This information allows understanding better complex fault systems and possible future earthquake ruptures, a topic of interest in the earthquakes community after the Kaikoura 2016 (New Zealand) and the Ridgcrest 2019 (California) earthquakes.
The other focus area of the PALEOSEISQUAKE project has been the Alboran Sea and specifically the Yusuf fault. A paleoseismological study has been carried out using ultra-high-resolution bathymetric and seismic data acquired with an Autonomous Underwater Vehicle (AUV). The analysis of the dataset has shown the presence of a fault scarp offseting the seafloor and has allowed characterizing the earthquake potential of the fault. The results reveal that the fault has produce at least 8 surface rupture earthquakes in the last 200 ka, which could have magnitudes larger than 7.0. The observed vertical displacements (0.5-1 m) may not have the potential to generate large tsunamis. We have also observed the presence of submarine landslides in the seismic data. The comparison between the occurrence of the paleoearthquakes and the paleolandslides might point that it is not a direct relationship between the occurrence of an earthquake and the triggering of the landslide. However, this needs further research and will be the focus of future research projects.
The results obtained during the PALEOSEISQUAKE have shown the importance of understanding different submarine fault systems and characterize their seismic potential. A better knowledge about this faults and the potential threaten that they represent may lead to an improvement of the seismic hazard estimations to the coastal areas and, thus, a better understanding about the socio-economic impact of the occurrence of a large earthquake in one of these faults.
