Slab top dips resolved by teleseismic converted waves in the Hellenic subduction zone

International audienceThe variations of the arrival times and polarities with backazimuth and distance of teleseismic P-to-S converted waves at interfaces bounding the slab crust under the upper plate mantle are used to constrain the depth, dip angle and azimuth of the slab of the Hellenic subduction zone. A grid search is designed to estimate the model parameters. Dip values of 16-18°, with an azimuth of 20° to 40°, are thus derived at 3 sites aligned over 50 km along the eastern coast of Peloponnesus. They are consistent with the variation from 54 to 61 km of the slab top depths constrained below each receiver. North of the Gulfs of Corinth and Evvia, a similar depth for the top of the slab is found at a distance from the subduction at least 100 km larger. This suggests flatter subduction of a different slab segment. Such a variation in slab attitude at depth across the region from south of the eastern Gulf of Corinth to north of Evvia is a candidate for the control of the recent or active localized crustal thinning of the upper plate we documented in earlier work, and of the surface deformation

The Sedimentary Sequences of the Lesser Antilles Arc South of Guadeloupe FromWide-Angle and Reflection Seismic Data

The Lesser Antilles Island Arc is a European active subduction zone prone to major earthquakes. Huge sedimentary input by the South American rivers, namely Amazon and Orinoco, has formed one of the largest accretionary complexes in the world. In the framework of the THALES project, several coincident wide-angle and multichannel seismic (MCS) profiles (15.5N and 16.5N) have been collected on the accretionary prism beetween the Barracuda and Tiburon Ridges in the Lesser Antilles. We present the analysis results of these data in order to construct a structural model. Preliminary results of 7 different MCS profiles are discussed. The data consist of 1 strike and 6 cross lines. The sedimentary layers imaged on deep-penetrating MCS data were used as a priori information for the wide angle modelling. A total of 16 OBH/OBS (Ocean Bottom Hydrophone/Seismometer) was deployed on a 130 km long wide-angle seismic profile. Seismic velocity models were obtained by a forward modelling of refracted and reflected phases. The final velocity model shows the geometry of the Antilles subduction zone with a sediment thickness of up to 2.6 km. The shallowest layer has a fill velocity of 1.8-2.2 km/s, whereas an older more compacted sediment layer in the deeper portion shows velocities ranged from 2.5 to 3.5 km/s with sediment thickness till 4.3 km. The sedimentary succession marked topographic irregularity and different directional fault system. These layers overlie oceanic crust having velocities in excess 6 km/s with depth of 14-15 km. From the coincident MCS seismic profiles, we incorporated the well resolved sedimentary portions into our model

3D Local Earthquake Tomography of the Ecuadorian Margin in the Source Area of the 2016 Mw 7.8 Pedernales Earthquake

Based on manually analyzed waveforms recorded by the permanent Ecuadorian network and our large aftershock deployment installed after the Pedernales earthquake, we derive three-dimensional Vp and Vp/Vs structures and earthquake locations for central coastal Ecuador using local earthquake tomography. Images highlight the features in the subducting and overriding plates down to 35 km depth. Vp anomalies (∼4.5–7.5 km/s) show the roughness of the incoming oceanic crust (OC). Vp/Vs varies from ∼1.75 to ∼1.94, averaging a value of 1.82 consistent with terranes of oceanic nature. We identify a low Vp (∼5.5 km/s) region extending along strike, in the marine forearc. To the North, we relate this low Vp and Vp/Vs (1.85) which we interpret as deeply fractured, probably hydrated OC caused by the CR being subducted. These features play an important role in controlling the seismic behavior of the margin. While subducted seamounts might contribute to the nucleation of intermediate megathrust earthquakes in the northern segment, the CR seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events offshore that can be linked to the updip limit of large megathrust earthquakes in the northern segment and the absence of them in the southern region over the instrumental period