ALBERT

Description: Large earthquakes are usually assumed to release all of the strain accumulated since the previous event, implying a reduced seismic hazard after them. However, long records of seismic history at several subduction zones suggest supercycle behaviour, where centuries-long accumulated strain is released through clustered large earthquakes, resulting in an extended period of enhanced seismic hazard. Here we combine historical seismology results, present-day geodesy data, and dense local observations of the recent M w 7.8 2016 Pedernales earthquake to reconstruct the strain budget at the Ecuador subduction zone since the great 1906 earthquake. We show that the Pedernales earthquake involved the successive rupture of two patches on the plate interface that were locked prior to the earthquake and most probably overlaps the area already ruptured in 1942 by a similar earthquake. However, we find that coseismic slip in 2016 exceeds the deficit accumulated since 1942. The seismic moment of every large earthquake during the twentieth century further exceeds the moment deficit accumulated since 1906. These results, together with the seismic quiescence before 1906 highlighted by historical records and marine palaeoseismology, argue for an earthquake supercycle at the Ecuador-Colombia margin. This behaviour, which has led to an enhanced seismic hazard for 110 years, is possibly still going on and may apply to other subduction zones that recently experienced a great earthquake. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

Description: The Galápagos volcanic province (GVP) includes several aseismic ridges resulting from the interaction between the Galápagos hotspot (GHS) and the Cocos–Nazca spreading centre (CNSC). The most prominent are the Cocos, Carnegie and Malpelo ridges. In this work, we investigate the seismic structure of the Carnegie ridge along two profiles acquired during the South American Lithospheric Transects Across Volcanic Ridges (SALIERI) 2001 experiment. Maximum crustal thickness is ∼19 km in the central Carnegie profile, located at ∼85°W over a 19–20 Myr old oceanic crust, and only ∼13 km in the eastern Carnegie profile, located at ∼82°W over a 11–12 Myr old oceanic crust. The crustal velocity models are subsequently compared with those obtained in a previous work along three other profiles over the Cocos and Malpelo ridges, two of which are located at the conjugate positions of the Carnegie ones. Oceanic layer 2 thickness is quite uniform along the five profiles regardless of the total crustal thickness variations, hence crustal thickening is mainly accommodated by layer 3. Lower crustal velocities are systematically lower where the crust is thicker, thus contrary to what would be expected from melting of a hotter than normal mantle. The velocity-derived crustal density models account for the gravity and depth anomalies considering uniform and normal mantle densities (3300 kg m−3), which confirms that velocity models are consistent with gravity and topography data, and indicates that the ridges are isostatically compensated at the base of the crust. Finally, a two-dimensional (2-D) steady-state mantle melting model is developed and used to illustrate that the crust of the ridges does not seem to be the product of anomalous mantle temperatures, even if hydrous melting coupled with vigorous subsolidus upwelling is considered in the model. In contrast, we show that upwelling of a normal temperature but fertile mantle source that may result from recycling of oceanic crust prior to melting, accounts more easily for the estimated seismic structure as well as for isotopic, trace element and major element patterns of the GVP basalts.