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New approaches in subaqueous PALEOseismology using high‐resolution SEISmics to derive single net paleoearthQUAKEs displacement and to characterize the seismic cycle on active faults

Periodic Reporting for period 2 - PALEOSEISQUAKE (New approaches in subaqueous PALEOseismology using high‐resolution SEISmics to derive single net paleoearthQUAKEs displacement and to characterize the seismic cycle on active faults)

Reporting period: 2018-08-01 to 2019-07-31

PALEOSEISQUAKE: New approaches in subaqueous PALEOseismology using high‐resolution SIESmics to derive single net paleoearthQUAKEs displacement and to characterize the seismic cycle on active faults.

Since the beginning of the XXI Century, our society has witnessed the occurrence of the offshore Sumatra (2004) and Japan (2010) earthquakes and subsequent tsunamis. These events has shown that understanding the earthquake history of active submarine faults is key to comprehend better the related seismic hazard and, thus, to mitigate the consequences of future events. Accordingly, the study of past earthquakes is essential in modern seismic hazard assessment.

Paleoseismology is a field of research that allows to characterize past earthquakes and to determine the seismic potential of the source faults based on the interpretation of the geological record. Thus, its strength is that covers much longer periods than the instrumental or historical seismic catalogues. Subaqueous paleoseismology merge and integrate paleoseismology and marine geology to detect and describe the occurrence of paleoearthquakes on faults located underwater.

The main objective of the PALEOSEISQUAKE project is to implement a new approach to characterize the seismic potential of active faults, with special emphasis on recognizing single event displacements and obtaining precise upper Quaternary slip-rates. To achieve this objective different active fault systems located in the California margin (US) and Alboran Sea (westernmost Mediterranean) have be analyzed using closely-spaced grids (pseudo 3D) or 3D PCable surveys of high-resolution seismic data to derive 3D geological models. The obtained results allow characterizing better the seismic potential of active submarine faults, improve knowledge about their behavior and determine better their seismic and tsunami hazard for the coastal areas.
PALEOSEISQUAKE is a project devoted to understand better the earthquake history and earthquake characteristics of offshore active faults in the California margin and in the Alboran Sea by means of high-resolution seismic data. The project has been conducted at Scripps Institution of Oceanography-University of California San Diego (US) and at the Institute of Marine Sciences-Spanish National Research Council (Spain).

The main objective has been to develop a new approach to characterize the seismic potential of active faults, with special emphasis in obtaining single event displacements and precise slip-rates by using high-resolution seismic and bathymetric data. This approach has been developed in the California margin study area and has been successfully applied in the Alboran Sea. In relation to the consecution of the main objectives other secondary objectives have been attained: 1) obtain the 3D geometry and segmentation model of faults and seismo-stratigraphic units; 2) estimate the parameters that describe the seismic potential of the faults (maximum magnitude, recurrence period, slip rate, displacement per event, and elapsed time since the last event); and 3) determine the tsunamigenic potential of the active faults and recent and buried submarine landslides.

The results of the project have been presented in 17 national and international meetings and workshops and two scientific papers and one book chapter have been already published and three papers are in review or in preparation to be submitted before the end of 2019. In addition, PALEOSEISQUAKE has participated in the organization of the Interact Fault2SHA IV workshop in Barcelona (Spain). Fault2SHA is a European Seismological Commission working group that aims to motivate exchanges between field geologists, fault modellers and seismic hazard practitioners.
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.
Approach developed to identify paleoearthquakes in the HR-CHIRP profiles