TS4.4

For many countries, the methodology for offshore geohazards mitigation lags far behind the well-established onshore methodology. Particularly complicated is the mapping of active faults. One possibility is to follow the onshore practice, i.e., identifying a sub-seabed Holocene horizon and determining whether it displaces this horizon for each fault. In practice, such an analysis requires numerous coring and often ends without an answer.   

Here we suggest a new approach aimed for master planning. Based on high-quality seismic data, we measure for each fault the amount of its recent (in our specific case 350 ky) displacement and the size of its plane. According to these two independently measured quantities, we classify the faults into three hazard levels, highlighting the “green” and “red” zone for planning.

Our case study is the Israeli continental slope, where numerous salt-related, thin-skinned, normal faults dissect the seabed, forming tens of meters high scarp, which are crossed by gas pipelines. A particular red zone is the upper slope south of the Dor disturbance, where a series of big listric faults rupture the seabed in an area where the sedimentation rate is four times faster than the displacement rate. We suggest that this indicates seismic rupture rather than creep.

How to cite: Laor, M. and Gvirtzman, Z.: Classifying offshore faults for hazard assessment: A new approach based on fault size and vertical displacement, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7112, https://doi.org/10.5194/egusphere-egu22-7112, 2022.

Whenever sedimentation exceeds the tectonic rate, the detection and investigation of active faults become challenging, especially when the investigated area is offshore. The coastal area of the central Adriatic is characterized by the presence of Plio-Pleistocene thrusts, which strongly controlled the evolution of the Apennines foredeep. Apart from the significant exception of the Conero promontory, these thrusts are all blind and have no significant signature in the bathymetry. Nonetheless, the coastal and offshore central Adriatic has experienced some moderate-magnitude seismic sequences related to the frontal thrusts on either side, belonging to the Apennines and the Dinarides chains.

In the last years, multiple studies conducted along the Apennine orogeny assessed the Plio-Pleistocene slip rates using different approaches and methodologies. Fault plane dimensions and attitude are key parameters for seismotectonic information fed into seismic and tsunami hazard analyses. In this work, we present the interpretation of two regional seismic reflection profiles across the central Adriatic, calibrated with the available well-logs, which show the evolution of the thrust system in space and time and their influence on the development of the Apennines foredeep and help to put some constraints to understand the most recent tectonic history of the region.

How to cite: Maesano, F. E., Toscani, G., Panara, Y., and Basili, R.: Structural reconstruction and Quaternary evolution of the buried thrust in the central Adriatic Sea (Italy)., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12537, https://doi.org/10.5194/egusphere-egu22-12537, 2022.

The Calabrian accretionary prism is the result of a complex interaction between subduction-related tectonics and sedimentation, active since the Eocene. The limited seismicity recorded in recent years in the area appears mostly associated to the subduction interface and could reflect either a weak subduction coupling or a slow subduction rate. Nevertheless, recent intense deformation and uplift of the seafloor has been observed within the accretionary prism.

The analysis of multichannel 2D and high-quality 3D seismic data, morphobathymetric data and instrumental seismicity, allows defining and characterizing both the deeper and shallower tectonic deformation affecting the northeastern sector of the Calabrian accretionary prism. 

Besides the uppermost thrust fault of the Calabrian accretionary prism, that outlines the Crotone promontory, the shallow tectonic pattern of the prism is characterized by a belt of broad flat-topped anticlines, and a set of minor narrow structures, mainly NNW-SSE to N-S oriented, that present a variable relationship with the underlying main thrust faults. The uppermost sedimentary strata within the anticlines are affected by numerous small-scale extensional faults, not rooted at depth, likely due to outer-arc extension above uplifted depocenters. In places, the inversion of basin-bounding faults is also visible. More regularly spaced and cylindrical NW-SE anticlines are also observed in the Gulf of Taranto, in the outer sector of the accretionary prism, where a thrust/back-thrust tectonic style is present. The origin of the anticlines varies within the overall set and reflects the long-term tectonic evolution of the accretionary prism, with the oblique docking of the Calabrian accretionary prism onto the Apulian Escarpment as a key feature.

How to cite: Lipparini, L., Argnani, A., Sgattoni, G., Pellegrini, C., Rovere, M., and Molinari, I.: Structural setting, active tectonics and seafloor morphology of the northeastern Calabria accretionary prism (Ionian Sea, Italy), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-932, https://doi.org/10.5194/egusphere-egu22-932, 2022.

The identification and seismic characterization of the active structures in the Alboran Sea (westernmost Mediterranean) are essential to evaluate better the exposure of the South Iberian Peninsula and Maghreb coasts to different natural hazards. The Alboran Sea accommodates part of the present-day crustal deformation related to the NW-SE convergence (4-5 mm/yr) between the African and Eurasian plates. The area is characterized by low to moderate magnitude instrumental seismicity. However, large earthquakes (I > IX and M > 6.0) have occurred in this region in historical and recent times (i.e., 1522 Almeria, 1790 Oran, 1910 Adra, 1994 and 2004 Al-Hoceima or 2016 Al-Idrissi earthquakes). The dextral strike-slip Yusuf Fault System (YFS) is one of the largest active faults in the Alboran Sea and its seismogenic and tsunamigenic hazard needs to be characterized. The fault system trends WNW-ESE and has a length of ~150 km. Using multi-scale bathymetric (ranging from m to cm) and seismic data and different morphological and seismic analysis tools (i.e., slope or relief image maps), we have imaged and characterized the fault system. The analysis of this dataset reveals that the YFS is a complex structure composed of an array of strike-slip faults. The 3D structural model shows that most of the identified faults reach up and offset the seafloor and the Upper Quaternary sedimentary units. The current morphology of the seafloor is a consequence of the Plio-Quaternary tectonic evolution that have resulted in the formation of a large pull-apart basin, which is deeper than the surrounding areas, a topographic ridge, an elongated depression and morphologic lineaments following its trend. The dataset also images several submarine landslides scars, mainly on the steeper slopes surrounding the pull-apart basin. In addition, the analysis of ultra-high resolution data acquired along the Yusuf lineament with AUV has revealed the presence of a series of en-echelon scarps with heights ranging from few centimeters to less than 10 meter. Seismic profiles across these scarps show that they are related to different fault strands of the YFS that are offsetting the seafloor, possibly because of an earthquake occurred in historical times.

How to cite: Perea, H., Martínez-Loriente, S., Llopart, J., Canari, A., Gómez de la Peña, L., Bartolomé, R., and Gràcia, E.: The Plio-Quaternary activity of the Yusuf Fault System (Alboran Sea; Westernmost Mediterranean): From 3D deep structure to seafloor geomorphology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2942, https://doi.org/10.5194/egusphere-egu22-2942, 2022.

The Iberian Peninsula is surrounded to the north by the convergence margin between Eurasia and the former Iberian plates (North and Northwest Iberian margin), and to the south by a transform plate boundary between Eurasia and Nubia (Gulf of Cádiz) to a shear-compressive indentation of Nubia northwards in the Alborán Sea. These margins are affected by historic and present-day seismicity, which are linked to active tectonic structures deforming the seafloor of the margins. The main objective is to better understand their development in the framework of the present plate organization and thus evaluate the seismic hazard around Iberia. Therefore, we carried out an extensive geophysical characterization of submarine faults, focusing on those that show seabed morphological expressions, by mapping them with high-resolution swath bathymetry data, high-resolution parametric sub-bottom profiles and multichannel 2D seismic profiles. Their activity and distribution are in good agreement with the geodetic and seismological observations.

Our results show that the present-day active tectonics and its related deformation, including seismicity and tsunami-affected coastal areas, are mainly located in the south Iberian margin, around the boundary between the Eurasian and Nubia tectonic plates. The submarine active faults are represented in this margin by a large strike-slip fault system and fold-thrust systems, in response to the NW-SE convergence between the aforementioned tectonic plates. The different orientation and distribution of submarine faults, and the fault type from focal mechanism of seismic events, led us to identify simple and pure shear zones from the Alborán Sea to the east, to the Gibraltar Arc and Gulf of Cadiz to the west. This suggests a strain partitioning model along the plate boundary in response to the present-day shear stress orientation.

Deformation is also documented in the NW Iberian margin. Thrust fault systems with high seismic activity were identified and mapped along Iberian ocean-continent transition around the Galician and Portuguese margins, reflecting the re-activation of former Cenozoic faults. Deformation in this margin is also derived from the westward motion of the Iberian oceanic domain and the clockwise rotation of the Iberian continental domain with respect to the Eurasian plate.

How to cite: Ramos, A., Somoza, L., Medialdea, T., Terrinha, P., and Vázquez, J.-T.: Submarine active tectonics in the south and northwest Iberian margins, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11506, https://doi.org/10.5194/egusphere-egu22-11506, 2022.

In southwestern Japan, the northwestward subduction of the Philippine Sea plate beneath the Eurasian plate results in large magnitude (>8) earthquakes and tsunamis (e.g. 1944 Tonankai and 1946 Nankaido earthquakes) and slow earthquakes at the Nankai margin. As part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), Long Term Borehole Monitoring Systems (LTBMSs), a suite of high-sensitivity borehole sensors providing real-time observations of hydrologic processes and crustal deformation, were installed from 2010 at 3 boreholes of the International Ocean Discovery Program (IODP).  

The pore pressure recorded by the LTBMSs, used as a proxy for volumetric strain, shows transient variations associated with slow slip events (Araki et al., 2017). Similar observations have been made at other subduction zones, like the north Hikurangi margin (e.g. Wallace et al., 2016), highlighting the key role of hydromechanical properties in fault mechanics and processes. The LTBMSs also capture the pore pressure oscillations arising from Earth tides forcing, with diurnal (~24 h) and semidiurnal (~12 h) periods. The phase and amplitude of the tidal signal can be decomposed from the observational data using tidal analysis programs, providing an opportunity to monitor changes related to the hydraulic and poroelastic responses to tectonic loading and transient loading arising from SSEs.

In this study, we use BAYTAP-08 (Tamura and Agnew, 2008), a modified version of the Bayesian Tidal Analysis Program - Grouping Model program of Tamura et al. (1991), to extract the tidal response from the pore pressure recorded at different depth intervals, at three sites: above the updip limit of the locked seismogenic zone at Site C0002 (first-time LTBMS deployment in 2010), at the megasplay fault zone and its footwall at Site C0010 (since 2016) and at the frontal thrust of the accretionary prism at Site C0006 (since 2018).

Tidal amplitudes and phases of semi-diurnal and diurnal tide components were carefully checked for any possible temporal variations, that may be related to subseafloor strain accumulation or coseismic release. We focused on the M2 and O1 canonical components.

Using a 1D poroelastic model, the analytic solution for tidal amplitude and phase was derived and compared with observations. The average amplitude ratio (relative to the seafloor) is 0.62-0.66, which is lower than the theoretical loading efficiency value. The phase lag difference is <1° for all depth intervals, as predicted by the 1D poroelastic theory for the range of permeability values (10^-15 to 10^-19 m²) determined from core samples (e.g. Reuschlé, 2011; Rowe et al., 2011; Tanikawa et al., 2012, 2014; Chen, 2015; Dutilleul, 2021) or drilling data (e.g. Pwavodi and Doan, 2021). This may be caused by the borehole casing or the LTBMS assembly itself. More careful inspection is on the way.

The removal of the tidal signal computed with BAYTAP-08 provides a clearer residual (i.e. non-tidal) pore pressure signal, which seems to have a long-term variation. It may either be the instrumental drift, but may be related to potential subseafloor strain modulations related to plate convergence and seismic activities.

How to cite: Dutilleul, J. and Kinoshita, M.: Tidal analysis of the NanTroSEIZE Long Term Borehole Monitoring System (LTBMS) pore pressure records at the Nankai margin, SW Japan., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4332, https://doi.org/10.5194/egusphere-egu22-4332, 2022.

Short historical and even shorter instrumental records limit our perspective of earthquake maximum magnitude and recurrence, and thus are inadequate to fully characterize Earth’s complex and multiscale seismic behavior and its consequences. Examining prehistoric events preserved in the geological record is essential to reconstruct the long-term history of earthquakes and to deliver observational data that help to reduce uncertainties in seismic hazard assessment for long return periods. Motivated by the mission to fill the gap in long-term records of giant (Mw 9 class) earthquakes such as the Tohoku-Oki earthquake in 2011, International Ocean Discovery Program (IODP) Expedition 386, Japan Trench Paleoseismology, was designed to test and further develop submarine paleoseismology in the Japan Trench.

Earthquake rupture propagation to the trench and sediment remobilization related to the 2011 Mw 9.0 Tohoku-Oki earthquake, and the respective structures and deposits are preserved in trench basins formed by flexural bending of the subducting Pacific Plate. These basins are ideal study areas for testing event deposits for earthquake triggering as they have poorly connected sediment transport pathways from the shelf and experience high sedimentation rates and low benthos activity (and thus high preservation potential) in the ultra-deep water hadal environment. Results from conventional coring covering the last ~1,500 y reveal good agreement between the sedimentary record and historical documents. Subbottom profile data are consistent with basin-fill successions of episodic muddy turbidite deposition and thus define clear targets for paleoseismologic investigations on longer timescales accessible only by deeper coring.

In 2021, IODP Expedition 386 successfully collected 29 Giant Piston cores at 15 sites (1 to 3 holes each; total core recovery 831 meters), recovering 20 to 40-meter-long, continuous, upper Pleistocene to Holocene stratigraphic successions of 11 individual trench-fill basins along an axis-parallel transect from 36°N – 40.4°N, at water depth between 7445-8023 m below sea level. The cores are currently being examined by multimethod applications to characterize and date event deposits for which the detailed stratigraphic expressions and spatiotemporal distribution will be analyzed for proxy evidence of giant versus smaller earthquakes versus other driving mechanisms. Initial preliminary results presented in this EGU presentation reveal event-stratigraphic successions comprising several 10s of potentially giant-earthquake related event beds, revealing a fascinating record that will unravel the earthquake history of the different along-strike segments, that is 10–100 times longer than currently available information. The data set will enable a statistically robust assessment of the recurrence patterns of giant earthquakes as input for improved probabilistic seismic hazard assessment and advanced understanding of earthquake-induced geohazards globally. 

How to cite: Strasser, M., Ikehara, K., Everest, J., and Maeda, L. and the IODP Expedition 386 Science Party: Tracking past earthquakes along the Japan Trench:  Fresh initial results from the IODP Japan Trench Paleoseismology Project, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1588, https://doi.org/10.5194/egusphere-egu22-1588, 2022.