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International audience; Identifying the circulation of fluids in subduction zone system and understanding their role on the megathrust fault slip modes remains one of the outstanding challenges in Earth Sciences. As these faults have the capacity to generate mega-earthquakes, the associated hazard to the society is significant.In recent years, studies in Japan, Costa Rica and New Zealand try to find the processes that control slip behaviours through marine campaigns and IODP drillings. The Ecuadorian margin is an exceptional laboratory to continue this international effort by adding a subduction zone to the list of those with contrasted slips at shallow depth. In April 2016, a Mw 7.8 earthquake broke the southern part of the 1906 earthquake rupture zone, causing hundreds of deaths and millions of dollars in damages along an increasingly populated coastline. The seismological and geodetic network in place since several years and a dense post-seismic deployment, contributed to observe and define the rupture zone and areas affected by aseismic slip on the shallowest portion of the megathrust fault.Thanks to two marine campaigns (HIPER and SUPER) and other existing onshore/offshore data acquisition, our project Fluid2Slip will participate to determine the exact role of fluids on slip behaviour around the updip part of the seismogenic megathrust fault by localizing fluids and seismicity, imaging fault properties and deformation. Our large experiment allowed a high density onshore/offshore deployment to perform shots and earthquakes FWI (Full Waveform Inversion) and obtain sufficient resolution to tackle the role of fluids with respect to interplate roughness, the nature of sediments, upper plate and lower plate’s structural heterogeneity in seismic/aseismic slip behavior. In addition, subsurface observation and fluid geochemistry will help detect fluid flow and discuss the path they take through the upper plate.

Frequency-domain Full Waveform Inversion (FWI) is potentially amenable to efficient processing of full-azimuth long-offset stationary-recording seabed acquisition carried out with sparse layout of ocean bottom nodes (OBNs) and broadband sources because the inversion can be performed with a few discrete frequencies. However, computing efficiently the solution of the forward (boundary-value) problem in the frequency domain with linear algebra solvers remains a challenge for large computational domains involving tens to hundreds of millions of parameters. We illustrate the feasibility of 3D frequency-domain FWI with the 2015/16 Gorgon OBN case study in the NorthWestern shelf, Australia. We solve the forward problem with the massively-parallel multifrontal direct solver MUMPS, which includes four key features to reach high computational efficiency: An efficient parallelism combining message-passing interface and multithreading, block low-rank compression, mixed precision arithmetic and efficient processing of sparse sources. The Gorgon subdataset involves 650 OBNs that are processed as reciprocal sources and 400,000 sources. Mono-parameter FWI for vertical wavespeed is performed in the visco-acoustic VTI approximation with a classical frequency continuation approach proceeding from a starting frequency of 1.7 Hz to a final frequency of 13 Hz. The target covers an area ranging from 260 km2 (frequency > 8.5 Hz) to 705 km2 (frequency < 8.5 Hz) for a maximum depth of 8 km. Compared to the starting model, FWI dramatically improves the reconstruction of the bounding faults of the Gorgon horst at reservoir depths as well as several intra-horst faults and several horizons of the Mungaroo formation down to a depth of 7 km.

International audience; Identifying the circulation of fluids in subduction zone system and understanding their role on the megathrust fault slip modes remains one of the outstanding challenges in Earth Sciences. As these faults have the capacity to generate mega-earthquakes, the associated hazard to the society is significant.In recent years, studies in Japan, Costa Rica and New Zealand try to find the processes that control slip behaviours through marine campaigns and IODP drillings. The Ecuadorian margin is an exceptional laboratory to continue this international effort by adding a subduction zone to the list of those with contrasted slips at shallow depth. In April 2016, a Mw 7.8 earthquake broke the southern part of the 1906 earthquake rupture zone, causing hundreds of deaths and millions of dollars in damages along an increasingly populated coastline. The seismological and geodetic network in place since several years and a dense post-seismic deployment, contributed to observe and define the rupture zone and areas affected by aseismic slip on the shallowest portion of the megathrust fault.Thanks to two marine campaigns (HIPER and SUPER) and other existing onshore/offshore data acquisition, our project Fluid2Slip will participate to determine the exact role of fluids on slip behaviour around the updip part of the seismogenic megathrust fault by localizing fluids and seismicity, imaging fault properties and deformation. Our large experiment allowed a high density onshore/offshore deployment to perform shots and earthquakes FWI (Full Waveform Inversion) and obtain sufficient resolution to tackle the role of fluids with respect to interplate roughness, the nature of sediments, upper plate and lower plate’s structural heterogeneity in seismic/aseismic slip behavior. In addition, subsurface observation and fluid geochemistry will help detect fluid flow and discuss the path they take through the upper plate.

The Hellenic subduction system exhibits a fairly atypical structure, resulting from the intense radial extension undergone by the Aegean domain from Eocene to Miocene. It features a very wide fore-arc, that itself includes an external non-volcanic arc between southeastern Peloponnesus and southwestern Anatolia, with Crete occupying a frontal position midway along it. South of it, separated from the Aegean domain by a large escarpment, the Hellenic trenches marking the most advanced position of the forearc Alpine nappes and the wide abutting accretionary wedge are found. The Hellenic subduction is widely regarded as poorly coupled from historical seismicity. Yet, a large tsunamigenic destructive event is reported in 365 CE just west of Crete, for which various rupture processes are proposed. In order to gain better insight into the structures of this major event, some deep penetrating seismic acquisition was performed in 2015 during the SISMED project, using the leading-edge equipment of R/V Marcus Langseth along a 210 km profile crossing through the fore-arc; an 8 km-long streamer and a voluminous source were used. Pre-stack depth imaging was applied to the recorded data set, with a strong effort to build a reliable velocity model using common image focusing analysis, both in the time and depth domains. Good imaging conditions thus were reached down to a maximum depth of 25 km. From south to north, our results reveal that: (1) a 6-7 km-thick oceanic crust with hints of compressive deformation enters the subduction, carrying no sediment past the backstop; (2) it crosses the continental Moho as shallow as 13 km depth; (3) above, a set of reactivated antithetic reverse faults controlling the Matapan forearc basin is clearly imaged; (4) no evidence is found supporting a speculated inverse splay fault outcropping at the toe of the Hellenic scarp and tentatively related to the 365 CE event; (5) some steep faulting is observed at mid-escarpment, whose downward continuation coincides with a jump in the forearc Moho’s depth, and that likely accommodates some of the required dextral strike slip partitioning motion; (6) possible clues are found for some compressive reactivation of the Maleas basin, north of the external arc.

International audience; The north Ligurian margin is a complex geological area in many ways. It has witnessedseveral phases of highly contrasting deformation styles, at both crustal scale and that of shallower covertectonics, simultaneously or in quick succession, and with significant spatial variability. This complexinterplay is mirrored in the resulting intricate structures that make it hard to identify active faultsresponsible for both, the significant seismicity observed, and the tectonic inversion undergone bythe margin, identified at longer time scales on morphostructural grounds. We present here the firstpreliminary results of the leg 1 of SEFASILS cruise, conducted in 2018 offshore Monaco, in an effort toanswer these questions by means of modern deep seismic acquisitions, using multichannel reflectionand wide-angle sea-bottom records. Some first interpretations are provided and point towards anactive basement deformation that focuses at the limits between main crustal domains.

International audience; In the last decade, seismic imaging below salt/basaltic bodies proved beyond doubt to be the most challenging task using conventional strategies in terms of surveying, processing and inversion. Recent innovations across the whole workflow pushed further the resolution limit in such complex environments for resource exploration purposes. On the other hand, for instance, the western Mediterranean basin, a fairly studied seismogenic zone, still raises some questions due to the lack of clear-cut imaging results. The latter is caused by the presence of the ill-famed Messinian salt at shallow depth in this region which generates a wavefield dominated by backscattering and diffractions due to the abrupt high velocity contrast and structural complexity.We take a fresh look at a new batch of towed-streamer data acquired in the Ligurian basin during the SEFASILS campaign. During this study, we examine preliminary results using a conventional velocity model building strategy based on velocity analysis techniques and another based on a state of the art tomographic method.We review the theory behind slope tomography, a reflection/diffraction tomography method based upon inversion of locally-coherent events. A locally-coherent seismic event, described by its two-way traveltimes and the slopes, in a common-shot or common-receiver gather can be interpreted as a reflection/diffraction from a small reflector segment or diffractor in depth. A key advantage of slope tomography methods in such settings is the automatic densely picked data cube of locally-coherent events opposed to a very challenging tracking of continuous events. We also explain the advantages of our parsimonious formulation in tackling the velocity-position trade-off.We illustrate the resolution power of slope tomography compared to more conventional techniques. We point out the main challenges and insurmountable obstacles encountered while assessing the quality of the reconstructed models and depth migrated images.

Summary Ultra-long offset stationary-receiver acquisitions implemented with multi-component Ocean Bottom Nodes (OBN) are emerging as a suitable technology for deep offshore subsalt imaging by full waveform inversion (FWI). Ultra-long offsets allow to undershoot the deepest targeted structures with diving waves. In this framework, first-arrival traveltime + slope tomography (FASTT) provides a simple and efficient tool to build kinematically-accurate initial velocity model for full waveform inversion (FWI). In particular, the incorporation of slopes in the first-arrival tomography in addition to traveltimes removes artifacts in poorly illuminated areas and better reconstruct structural dips. Either the FASTT or the FWI velocity models can be used as reliable background models for prestack depth migration of co-incident towed-streamer data. However, the high resolution of the FWI velocity model facilitates the integrated interpretation of the reflectivity mapped by migration and the intermediate-scale velocity variations reconstructed by waveform inversion. We illustrate the workflow combining first-arrival tomography + FWI of ultra-long offset OBN data and migration of co-incident towed-streamer data with a challenging deep crustal case study in the eastern Nankai subduction zone.

Whether subducted oceanic reliefs such as seamounts promote seismic rupture or aseismic slip remains controversial. Here we use swath bathymetry, prestack depth-migrated multichannel seismic reflection lines, and wide-angle seismic data collected across the central Ecuador subduction segment to reveal a broad similar to 55kmx50km, similar to 1.5-2.0km high, low height-to-width ratio, multipeaked, sediment-bare, shallow subducted oceanic relief. Owing to La Plata Island and the coastline being located, respectively, similar to 35km and similar to 50-60km from the trench, GPS measurements allow us to demonstrate that the subducted oceanic relief spatially correlates to a shallow, similar to 80kmx55km locked interplate asperity within a dominantly creeping subduction segment. The oceanic relief geometrical anomaly together with its highly jagged topography, the absence of a subduction channel, and a stiff erosive oceanic margin are found to be long-term geological characteristics associated with the shallow locking of the megathrust. Although the size and level of locking observed at the subducted relief scale could produce an M-w >7+ event, no large earthquakes are known to have happened for several centuries. On the contrary, frequent slow slip events have been recorded since 2010 within the locked patch, and regular seismic swarms have occurred in this area during the last 40years. These transient processes, together with the rough subducted oceanic topography, suggest that interplate friction might actually be heterogeneous within the locked patch. Additionally, we find that the subducted relief undergoes internal shearing and produces a permanent flexural bulge of the margin, which uplifted La Plata Island. Plain Language Summary Subducted seamounts play an important but still uncertain role in earthquake rupture processes, as seamounts are considered to subduct either aseismically or seismically. We use marine geophysical data across the central Ecuador convergent margin to reveal a broad similar to 55kmx50km, similar to 1.5-2.0km high, shallow subducted oceanic relief of the Carnegie Ridge. New GPS measurements collected on La Plata Island and along the Ecuador coast allow us to demonstrate that the subducted oceanic relief spatially correlates to a shallow, similar to 80kmx55km locked interplate asperity within a dominantly creeping subduction segment. The oceanic relief geometrical anomaly together with its rough topography and the stiff oceanic Ecuador margin are found to be long-term geological characteristics associated with the shallow locking of the plate interface. Although the size and level of locking observed at the subducted relief scale could produce an M-w >7+ event, no large earthquake but frequent slow slip events and associated seismic swarms occurred within the locked patch. These transient processes together with the rough subducted oceanic topography support the view of a heterogeneous interplate frictional pattern within the locked patch. On a regional scale, the subduction of the oceanic relief has deformed the Ecuador margin and uplifted La Plata Island.

International audience; Tectonic processes that control the transition from poorly consolidated sediment entering the subduction channel (SC) to the seismogenic zone are documented using seismic imaging. We applied pre-stack depth migration and a post-processing sequence to a seismic reflection line acquired across the Ecuador convergent margin to obtain a 2D-quantitative image of the first ∼24 km of the SC. Structural interpretation shows that the SC consists of a 630-1150-m-thick, low-velocity, continuous sheet of sediment that dips ∼6° landward and undergoes shear deformation. The long sheet is bounded at top and bottom by décollement thrusts, and developed over time Riedel shears and basal thrust faulting and folding downdip, pointing to a dynamic mega-shear zone. Modeling the strong uppermost and basal SC reflectors reveals that they are associated with 40-80-m-thick, 50-350 m/s, low-velocity perturbations layers inferred to be fluid rich and mechanically weak. A fine-scale velocity model shows two anomalously low-Vp areas in the long sheet, advocating patches of over-pressured fluids. Evidence for Vp variations along the upper-plate foundation suggests either underplated bodies or a fluid-damage zone. A simple temporal reconstruction indicates that underthrusting the long sheet initiated >450 kyr ago and interrupted ∼54 ± 13 kyr ago, when frontal accretion resumed. During this transient evolution, the SC boundaries revealed highly unstable as most of the SC was underplated while down going plate material may have been sheared off and incorporated to the SC.

A seismic processing workflow based on iterative ray + Born migration/inversion and target-oriented postprocessing of the migrated image is developed for fine-scale quantitative characterization of reflectors. The first step of the workflow involves linear iterations of the ray + Born migration/inversion. The output of the first step is a true-amplitude migrated image parameterized by velocity perturbations. In a second step, postprocessing of the migrated image is performed through a random search with a very-fast simulated annealing (VFSA) algorithm. The forward problem of the global optimization is a simple convolutional model that linearly relates a vertical profile of the band-limited migrated image after depth-to-time conversion to a 1D velocity model composed of a stack of homogeneous layers of arbitrary velocity and thickness. The aim of the postprocessing is to eliminate the limited bandwidth effects of the source from the migrated image for resolution improvement and enhanced geological interpretation of selected targets. The global optimization approach allows for uncertainty analysis required by the intrinsic nonuniqueness of the velocity model output by the postprocessing. The relevance of the convolutional model when applied to the output of the ray + Born migrated inversion is first illustrated with a one-layer model. The accuracy and the robustness of the workflow to image geologically complicated models are then illustrated with an application to the synthetic Marmousi model. Some practical issues (e. g., the source wavelet estimate and the scaling of the migrated image required by the VFSA optimization) are discussed with an application to a 2D real seismic multichannel reflection data set collected in the Gulf of Guayaquil (Ecuador). The postprocessing is applied to derive the fine-scale velocity structure of a decollement zone on top of the subduction channel. The postprocessing allows for mapping structural variations along different segments of the decollement, which can be associated with changes in fluid content and porosity.