Subduction of the Nazca plate beneath the Ecuador‐Colombia margin has produced four megathrust earthquakes during the last century. The 500‐km‐long rupture zone of the 1906 (Mw = 8.8) event was partially reactivated by three thrust events, in 1942 (Mw = 7.8), 1958 (Mw = 7.7), and 1979 (Mw = 8.2), whose rupture zones abut one another. Multichannel seismic reflection and bathymetric data acquired during the SISTEUR cruise show evidence that the margin wedge is segmented by transverse crustal faults that potentially correlate with the limits of the earthquake coseismic slip zones. The Paleogene‐Neogene Jama Quininde and Esmeraldas crustal faults define a ∼200‐km‐long margin crustal block that coincides with the 1942 earthquake rupture zone. Subduction of the buoyant Carnegie Ridge is inferred to partially lock the plate interface along central Ecuador. However, coseismic slip during the 1942 and 1906 earthquakes may have terminated against the subducted northern flank of the ridge. We report on a newly identified Manglares crustal fault that cuts transversally through the margin wedge and correlates with the limit between the 1958 and 1979 rupture zones. During the earthquake cycle the fault is associated with high‐stress concentration on the plate interface. An outer basement high, which bounds the margin seaward of the 1958 rupture zone, may act as a deformable buttress to seaward propagation of coseismic slip along a megathrust splay fault. Coseismic uplift of the basement high is interpreted as the cause for the 1958 tsunami. We propose a model of weak transverse faults which reduce coupling between adjacent margin segments, together with a splay fault and an asperity along the plate interface as controlling the seismogenic rupture of the 1958 earthquake.

Tectonics, v. 25, n. 3, p. 22 pp, 2006. http://dx.doi.org/10.1029/2004TC001723International audienc

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[1] The crustal structure of the northern Gulf of California transtensional margin has been investigated by a 280-km-long NW-SE profile, including deep multichannel seismic reflection and densely sampled refraction/wide-angle reflection seismic information combined with gravity modeling. The seismic and gravity modeling constrains two thinned crustal areas, corresponding to the upper Delfín and the upper Tiburón basins. On both sides of the profile, toward the Baja California Peninsula and the Mexico mainland, a progressive thickening of the continental crust is observed. Our results indicate that the crustal thickness is 19 km below the coastline, and it decreases to 14 and 17 km below the upper Delfín and upper Tiburón basins, respectively. In the area between both basins, the crust thickens to 19.5 km. There are significant lateral thickness variations for the different levels of the crust. The interpreted structure is consistent with the existence of an aborted rift below the upper Tiburón basin. Prominent dipping reflections in the multichannel data under upper Tiburón basin and the ridge between upper Tiburón and upper Delfín basins can be explained as a mylonite like zone related to a detachment fault. This interpretation suggests that the structural evolution of upper Tiburón basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust. The mode and locus of extension have evolved from a core complex in upper Tiburón to a narrow rift mode in upper Delfín basin.

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11We characterise the aftershock sequence following the 2016 Mw=7.8 Pedernales earthquake. 12More than 10,000 events were detected and located, with magnitudes up to 6.9. Most of the 13 aftershock seismicity results from interplate thrust faulting, but we also observe a few normal 14 and strike-slip mechanisms. Seismicity extends for more than 300 km along strike, and is 15 constrained between the trench and the maximum depth of the coseismic rupture. The most 16 striking feature is the presence of three seismicity bands, perpendicular to the trench, which 17 are also observed during the interseismic period. Additionally, we observe a linear 18 dependency between the temporal evolution of afterslip and aftershocks. We also find a 19 temporal semi-logarithmic expansion of aftershock seismicity along strike and dip directions, 20 further indicating that their occurrence is modulated by afterslip. Lastly, we observe that the 21 spatial distribution of seismic and aseismic slip processes is correlated to the distribution of 22 bathymetric anomalies associated with the northern flank of the Carnegie Ridge, suggesting 23 that slip in the area could be influenced by the relief of the subducting seafloor. To explain 24 our observations, we propose a conceptual model in which the Ecuadorian margin is subject 25 to a bimodal slip mode, with distributed seismic and aseismic slip mechanically controlled by 26 the subduction of a rough oceanic relief. Our study sheds new light on the mechanics of 27 subduction, relevant for convergent margins with a complex and heterogeneous structure 28 such as the Ecuadorian margin. 29

[1] This paper presents a combined analysis of geological and geophysical data collected both onshore and offshore along the northwestern Peru forearc area (3°30 0 -7°30 0 S), from the coastal plain to the trench axis. Onshore, geomorphic analysis places constraints on the relative importance of eustatic versus tectonic factors in preserving and modifying the uplifted coastal landforms along the coastal plain. Breaking-wave morphologic markers were dated using the in situ produced 10 Be cosmonuclide. The data document a tectonic segmentation, allowing us to differentiate two areas with regard to their evolution through time: the northern Cabo Blanco and the southern Paita-Illesca segments. For the past 200 kyr, both segments uplifted at high rates of 10 to 20 mm yr À1 through tectonic pulses coeval with the eustatic deglacial sea level rises of isotope stage 1 and warm isotope substage 5e, respectively. The uplift and related extensive emersion of the coastal plain require high coupling along the subduction zone and/or underplating at depth. Offshore, industry-acquired reflection seismic lines combined with EM12 bathymetric data allow us to investigate the tectonic regime and deformation of the continental margin and shelf. Major dipping seaward detachments control the long-term subsidence of this area. These main tectonic features define a tectonic segmentation. The Talara, Paita, and Sechura segments are identified from north to south. No clear tectonic correlation in time exists between the onshore and the continental margin segmentations, or in space either. The longterm subsidence of the offshore, indicative of subduction erosion working at depth, requires low coupling along the subduction channel at depth. The distribution of permanent deformation along the northern Peru forearc area includes long-term uplift along the coastal plain and long-term subsidence along the continental margin, the neutral line being located within the 10 km seaward from the Present coastline. An extensive sequence of raised marine cliffs and associated notches evidences that the most recent uplift step (20-23 ka to Present) along the Cabo Blanco segment is related to a sequence of major earthquakes. We infer that eustacy exerts important feedback coupling to the seismogenic behavior of the North Peru subduction zone. We speculate that during sea level fall, pore fluid pressure diminishes along the subduction channel inducing a possible seaward migration of the locked zone (i.e., migration of the updip limit) reaching a maximum by the end of the eustatic low stand. During eustatic sea level rise, pore fluid pressure increases along the subduction channel. This in turn is capable of weakening the previously locked zone along the plate interface beginning an earthquake sequence. Earth's orbital variations are a potential external cause that may control the physical processes at work along plate interface.