3D tomographic constraints on upper-plate elastic structure and inter-plate geometry offshore the Ecuadorian-Colombian margin

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• Manel Prada -- Institute of Marine Sciences, Spanish National Research Council (CSIC)
• Wednesday 05 March 2025, 14:00-15:00
• Wolfson Lecture Theatre.

If you have a question about this talk, please contact ChuanChuan Lu.

In subduction zones, upper-plate elastic rock properties play a major role in controlling megathrust fault dynamics (Sallarès & Ranero 2019; Prada et al., 2021; Ulrich et al., 2022). The variations of these properties in the downdip direction are influenced by increasing confining pressure with depth, forming a global depth-dependent pattern (Sallarès & Ranero 2019). However, the understanding of how these properties vary along-strike, especially above the interplate, where elastic energy is stored during interseismic cycle, remains unclear. Here we present 3D tomographic constraints on the velocity structure of the upper plate as well as the inter-plate geometry offshore the Ecuadorian-Colombian margin. The study area has been the locus of large megathrust tsunamigenic earthquakes, including the seventh largest in history (1906 Mw~8.8 Esmeraldas), but elastic properties above the megathrust are not mapped hitherto. Here we use 3D wide-angle seismic (WAS) dataset acquired in 2005 during the ESMERALDAS survey and multichannel seismic (MCS) lines to integrate tectonic information to our tomographic results. WAS data were acquired with 26 ocean bottom seismometers from which we picked travel times of P-waves refracted through the upper and lower plates, as well as P-wave reflections at the interplate. We invert travel times with a 3D joint reflection and refraction travel time method (Melendez et al., 2015) following a Monte Carlo approach to provide uncertainties on model parameters.The resulting model shows the 3D P-wave velocity (Vp) structure of the upper and lower plates as well as the geometry of the interplate reflector. Additional elastic parameters such as rigidity (i.e., shear modulus) were derived using empirical relationships between Vp, Vs, and density. Downdip variations in rigidity align with the inferred global trend. However, rather than being laterally consistent, elastic properties exhibit remarkable variations along-strike within the rupture area of the largest recorded earthquakes. MCS sections in the study area depict a complex interaction between crustal-scale faults. This interaction has been proposed to control the seismogenic behavior of the subduction zone in this region, promoting the occurrence of confined ruptures (Collot et al., 2004). A comparison between MCS reflection lines and tomographic results reveals a correlation between localized low-rigidity upper-plate regions and the interaction of such crustal-scale faulting. The presence of low rigidity areas above the interplate may enhance coseismic slip, while locally damaged regions in the upper-plate may favour inelastic deformation during coseismic events, promoting localised seafloor uplift. In contrast to downdip elastic rock variations above the upper-plate, our results indicate that along-strike variations are dependent on the interaction of crustal-scale structures and, consequently, on local geology.

This talk is part of the Bullard Laboratories Wednesday Seminars series.

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• Bullard Laboratories Wednesday Seminars
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