<p>The Ecuadorian subduction regularly hosts large earthquakes. Among them, the Mw 8.8 1906 earthquake is the 7th biggest known event. Following the recent 2016 Mw 7.8 Pedernales earthquake, a large deployment of onshore/offshore seismological stations, in addition to the permanent seismological/geodetical network, revealed a complex slip behavior including the presence  of  seismic and aseismic slip.</p><p>During the geophysical experiment HIPER, in march 2020, 47 Ocean Bottom Seismometers (OBS), were densely deployed along a 93-km-long trench-perpendicular profile, recording airgun shots (4990 cu.inch.) performed by R/V Atalante to obtain a high-resolution P-wave velocity image. The profile was located north of the 2016 Pedernales rupture zone passing through an area experiencing aseismic slip and a region of contrasted geodetic interseismic coupling.    </p><p>We used the traveltime tomography code « tomo2d » (Korenaga et al., 2000) to invert first arrivals and reflected phases recorded by our OBS.  A joint 2D-seismic-reflection profile was acquired (abstract by L. Schenini) and provides details on the oceanic basement topography and on Vp velocities in shallow sedimentary layers.</p><p>Regarding the structural complexity in the region, we decided to start the inversion  using an a priori 2D velocity model. Several geophysical experiments have already been conducted offshore-onshore Ecuador (SISTEUR, 2000 ; SALIERI, 2001 and ESMERALDAS, 2005). Compilation of velocity models from tomographic images were used to build two a priori 1D Vp velocity models for both the Nazca oceanic crust and the forearc seismic structure. A 2D a priori Vp velocity model was built by merging the results of the two localized inversions using a selection of OBS on each side of the trench.</p><p>We obtain the crustal structure of the upper and subducting plates down to 20 km depth. Beneath the trench, a ~30-km-wide low-Vp anomaly is observed at lithospheric scale. This velocity is 10% lower than the typical Vp values observed for hydrated Pacific-type oceanic crust near the trench (Grevemeyer et al., 2018). Recorded PmP phases will allow us to further constrain the crustal thickness. While we observe PmP phases in areas of low-Vp, the Moho reflectivity weakens and even disappears from the coincident MCS line. This intriguing observation could highlight processes, such as the presence of fluids or serpentinization, that need to be identified and better understood.</p>

<p>Offshore 2D-Multichannel seismic (MCS)-reflection profiles were acquired in northern Ecuador during the HIPER survey (March/April 2020, R/V L’Atalante) together with one 2D-OBS-seismic-refraction profile (presented in a joint abstract by A. Skrubej). This project (presented in a joint abstract by A. Galve) aims at deciphering the role of lower plate structural heterogeneities and fluids on subduction zone seismogenesis processes within the 2016 Pedernales rupture segment, which is characterized by contrasting slip behaviors. We put a particular emphasis on the segment located at the northern termination of the subducting Carnegie Ridge which was devoid of previous seismic investigations. Three lines of 315-km-long in total, one North of the 2016 Pedernales rupture zone sampling an area experiencing aseismic slip and two lines parallel to the trench, were recorded using an airgun source of 4990 in<sup>3</sup> and a 6-km-long streamer. In this study, we present in detail the seismic processing workflow used to produce an enhanced imaging of the Ecuadorian margin, a prerequisite for tackling the project’s objectives.</p><p>We performed routine MCS data processing onboard to produce post-stack time migrated sections using Geovation<sup>® </sup>CGG’s software. The dip-line collected across the northern Atacames seamounts area provides a detailed image through the whole Nazca oceanic crust down to the Moho, showing a normal crust thickness, at least on the oceanward portion, up to 15 km to the west of the trench. At the trench, we image a horst-like basement topographic high, which outcrops at sea-bottom, offsets the deformation front arcwards, with the outcropping frontal decollement reflector topping this oceanic basement high. Its nature, fluid content potential and lateral extent need to be determined, but its observation at the shallow portion of the interplate megathrust contribute to expand the inventory of subducting rough structures possibly impacting the megathrust frictional slip behavior.</p><p>Further advanced processing include noise attenuation, 2D-SRME multiple attenuation, Kirchhoff pre-stack time migration and preserved amplitude pre-stack depth migration (PSDM) performed in the angle domain. The megathrust fault located at the top of the subducting oceanic crust is imaged down to 7 km depth at a distance of 28 km from the trench which will contribute to complement the high-resolution version of the slab’s top topography close to the trench. A joint analyze of this MCS line and the coincident 2D-OBS-refraction Vp model, reveal that variations in moho acoustic features at 15 km distance to the west of the trench correlates with a 30 km wide and >10-km-thick low Vp anomaly. Nearby previous experiment SISTEUR seismic lines are being reprocessed using the same workflow, in order to further investigate the deep crustal seismic structures over the Pedernales 2016 rupture zone.</p>