Mapping the Basement along the Northern Ecuadorian Margin using Refraction and Reflexion Data from the HIPER Experiment.

The 2016 Mw 7.8 Pedernales, Ecuador, earthquake rupture area was accompanied by aseismic slip at shallower depth such as slow earthquake and post-seismic slow slip. In addition, the relatively small forearc and shallow megathrust along the Ecuadorian margin makes it an exceptional natural laboratory to study the link between the ongoing subduction of topographic highs (Carnegie ridge) and seismic/aseismic slip behavior on the plate interface. The HIPER Project (2020-2022) which aims at better characterizing the 3D structure of this forearc domain is based on an international collaboration funded by the French Oceanographic Fleet, the French ANR, Karlsruhe Institute of Technology (KIT, Germany), American NSF and IG-EPN (Ecuador). We successfully deployed a large number of OBS (47), land stations (~200) and nodes (~500) to record both R/V L'Atalante's shots and seismic activity.We collected a grid of five trench-parallel and nine trench-perpendicular profiles that were shot along a 150-km-long margin segment off Esmeraldas and Pedernales. Travel times of first arrivals and oceanic Moho reflections have been picked on these refraction profiles leading to a total of ~33000 picks. The obtained dataset has been used for 2D travel time tomography using the Bayesian approach of the Tomo2D code modified by Korenaga and Sager (2012).P-waves velocity models have been obtained for the fourteen profiles, with a good resolution down to 12 km depth providing new information of the frontal forearc domain. Existing multichannel seismic data along four of our profiles (2 dip-lines and 2 strike-lines) provide the position of the forearc basement acoustic reflector that is used for the interpretation of the tomographic velocity profiles. We identify a high velocity gradient zone matching the forearc basement on the four lines over a distance range of 15-40 km from the trench. Our velocity models highlight a rather low-velocity range for this frontal part of the forearc crust with Vp ranging from 3.8 - 5 km/s which is consistent with the expected crustal scale damaging due to ongoing subduction of Carnegie ridge and seamounts. This work will provide a first map of the forearc basement and its internal seismic characteristics that will be discussed in relation to the spatial distribution of aseismic/seismic areas of the Pedernales segment.