Laboratoire de Géologie, ENS, Paris

Along the Ecuadorian margin, oblique subduction induces deformation of the overriding continental plate. For the last 15 Ma, both exhumation and tectonic history of Ecuador suggest that the northeastward motion of the North Andean Sliver (NAS) was accompanied by an eastward migration of its eastern boundary and successive progressively narrowing restraining bends. Here we present geologic data, earthquake epicenters, focal mechanisms, GPS results, and a revised active fault map consistent with this new kinematic model. All data sets concur to demonstrate that active continental deformation is presently localized along a single major fault system, connecting fault segments from the Gulf of Guayaquil to the eastern Andean Cordillera. Although secondary faults are recognized within the Cordillera, they accommodate a negligible fraction of relative motion compared to the main fault system. The eastern limit is then concentrated rather than distributed as first proposed, marking a sharp boundary between the NAS, the Inca sliver, and the Subandean domain overthrusting the South American craton. The NAS limit follows a northeast striking right-lateral transpressional strike-slip system from the Gulf of Guayaquil (Isla Puná) to the Andean Cordillera and with the north-south striking transpressive faults along the eastern Andes. Eastward migration of the restraining belt since the Pliocene, abandonment of the sutures and reactivation of north-south striking ancient fault zones lead to the final development of a major tectonic boundary south and east of the NAS, favoring its extrusion as a continental sliver, accommodating the oblique convergence of the Nazca oceanic plate toward South America.

Magma movement or reservoir pressurisation can drive swarms of low-magnitude volcano-tectonic 31 earthquakes, as well as occasional larger earthquakes (> M 5) on local tectonic faults. Earthquakes > 32 M 5 near volcanoes are challenging to interpret in terms of evolving volcanic hazard, but are often 33 associated with eruptions, and in some cases enhance the ascent of magma. We present geodetic 34 observations from the first episode of unrest known to have occurred near Chiles and Cerro Negro de 35 Mayasquer volcanoes on the Ecuador-Colombian border. A swarm of volcano-tectonic seismicity in 36 October 2014 culminated in a M w 5.6 earthquake south of the volcanoes. Satellite radar data spanning 37 this earthquake detects displacements that are consistent with dextral oblique slip on a reverse fault at 38 depths of 1.4-3.4 km within a SSW-NNE trending fault zone that last ruptured in 1886. GPS station 39 measurements capture ~ 20 days of uplift before the earthquake, probably originating from a pressure 40 source ~10-15km south of Volcán Chiles, at depths exceeding 13 km. After the M w 5.6 earthquake, 41 uplift ceased and the rate of seismicity began to decrease. Potential mechanisms for this decline in 42 activity include a decrease in the rate of movement of magma into the shallow crust, possibly caused 43 by the restriction of fluid pathways. Our observations demonstrate that an earthquake triggered during 44 volcanic unrest can inhibit magmatic processes, and have implications for the hazard interpretation of 45 the interactions between earthquakes and volcanoes. 46 47 Highlights (3-5, 85 characters) 48 49 • 2014 unrest at Chiles-Cerro Negro culminated in M w 5.6 earthquake 50 • Slip was shallow and consistent with El Angel fault zone 51 • Earthquake was preceded, and potentially triggered, by mid-crustal pressurisation 52 • The M w 5.6 earthquake coincided with cessation of uplift and start of fall in seismicity 53 • Earthquakes during volcanic unrest may inhibit the driving magmatic process 54 55 57 Volcano Tectonic (VT) seismicity is common during volcanic unrest and eruption (e.g. Roman & 58 Power, 2011) and usually consists of low-magnitude (<M4) earthquakes (e.g., Benoit & McNutt, 1996).

At subduction zones, transient aseismic slip occurs either as afterslip following a large earthquake or as episodic slow slip events during the interseismic period. Afterslip and slow slip events are usually considered as distinct processes occurring on separate fault areas governed by different frictional properties. Continuous GPS (Global Positioning System) measurements following the 2016 M w (moment magnitude) 7.8 Ecuador earthquake reveal that large and rapid afterslip developed at discrete areas of the megathrust that had previously hosted slow slip events. Regardless of whether they were locked or not before the earthquake, these areas appear to persistently release stress by aseismic slip throughout the earthquake cycle and outline the seismic rupture, an observation potentially leading to a better anticipation of future large earthquakes.

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.

The authors omitted to cite two papers 31,32 that suggest active fault systems in northwest South America are partly controlled by the obliquity of convergence between the subducting Nazca oceanic plate and South America, as well as two papers 33,34 that discuss aspects of active crustal fault maps from the northern Andes.

A dense GPS network deployed in Ecuador reveals a highly heterogeneous pattern of interseismic coupling confined in the first 35 km depth of the contact between the subducting oceanic Nazca plate and the North Andean Sliver. Interseismic models indicate that the coupling is weak and very shallow (0-15 km) in south Ecuador and increases northward, with maximum found in the rupture areas of large (Mw > 7.0) megathrust earthquakes that occurred during the 20th century. Since the great 1906 Mw = 8.8 Colombia-Ecuador earthquake may have involved the simultaneous rupture of three to six asperities, only one or two asperities were reactivated during the large seismic sequence of 1942

Cotopaxi volcano, Ecuador, experienced notable restlessness in 2015 that was a major deviation from its normal background activity. Starting in April and continuing through November 2015 strong seismic activity, infrasound registry, hikes in SO 2 degassing and flank deformation with small displacements were some of the geophysical anomalies that were registered. Obvious superficial changes, such as small hydromagmatic eruptions, emission of vapor and ash columns, thermal hotspots around the crater and in nearby orifices and exacerbated glacier melting were also observed. Our contribution provides an overview of the 2015 Cotopaxi unrest by presenting the patterns of geophysical data and the sequence of events produced by the volcano. Cotopaxi's last important VEI 4 eruption was in 1877. Then it had devastating effects because of the transit of huge lahars down 3 major drainages. Comparatively, the 2015 activity never surpassed a magnitude VEI 2 and principally produced limited hydromagmatic explosions and semi-continuous low energy emissions and light ashfalls. Given the potential of major destruction from a large Cotopaxi eruption it is important to understand the geophysical fingerprints that characterized the 2015 episode with an eye to identifying onset of future restless periods. Overall, the monitoring activities, the data interpretation, formulation of reasonable eruptive scenarios, and finally, the preparation of a stream of constant information being relayed to concerned authorities and the public, was a real test of the IGEPN's capacity to deal with a complicated eruption situation whose outcome was not apparent at the beginning, but which concluded in a very small eruptive episode.

Displacement waveforms derived from Global Navigation Satellite System (GNSS) data have become more commonly used by seismologists in the past 15 yrs. Unlike strong-motion accelerometer recordings that are affected by baseline offsets during very strong shaking, GNSS data record displacement with fidelity down to 0 Hz. Unfortunately, fully processed GNSS waveform data are still scarce because of limited public availability and the highly technical nature of GNSS processing. In an effort to further the use and adoption of high-rate (HR) GNSS for earthquake seismology, ground-motion studies, and structural monitoring applications, we describe and make available a database of fully curated HR-GNSS displacement waveforms for significant earthquakes. We include data from HR-GNSS networks at near-source to regional distances (1-1000 km) for 29 earthquakes between M w 6.0 and 9.0 worldwide. As a demonstration of the utility of this dataset, we model the magnitude scaling properties of peak ground displacements (PGDs) for these events. In addition to tripling the number of earthquakes used in previous PGD scaling studies, the number of data points over a range of distances and magnitudes is dramatically increased. The data are made available as a compressed archive with the article.

Introduccion El proyecto OSISEQ nace como una iniciativa del UMR Geoazur (Francia) para continuar y afinar los resultados arrojados por otros proyectos como las campanas geofisicas marinas: Sisteur 2000, Salieri, 2001, Amadeus y Esmeraldas, 2005 y de estudios de la deformacion a largo y corto-tiempo conocida en la zona, ADN 2008-2011. Este proyecto se enmarca a su vez dentro de un proyecto bi-nacional de creacion de un Laboratorio Mixto Internacional (LMI) del Instituto de Investigacion para el Desarrollo (IRD) en el cual colaboran los laboratorios de Geoazur, Magmas et Volcans, IS Terre y el socio local ecuatoriano, el Instituto Geofisico de la Escuela Politecnica Nacional. Este proyecto tiene un componente sismico muy importante: terrestre y marino para lo cual otra de las contrapartes nacionales es el INOCAR (Instituto Oceanografico de la Armada).

The North Andean Sliver (hereinafter NAS) lies at the northwestern end of the South American plate (hereinafter SOAM). This extensive area exhibits a complex deformation process controlled by the interactions of Nazca, Caribbean, South America plates, and Panama block, producing crustal seismicity, arc-continental collision, and subduction processes. Previous models based on partial GPS data sets have estimated the NAS kinematics as a single rigid block moving towards northeast at 8-10 mm/yr (Nocquet et al. 2014, Mora-Paez et al 2019). By contrary, geologic interpretations as well as seismotectonic data propose more complex kinematic models based on the interaction of several blocks (Audemard et al 2014, Alvarado et al 2016). Here, we present an updated and most extensive interseismic horizontal velocity field derived from continuous and episodic GPS data between 1994 and 2019 that encompasses the whole North Andean Sliver. We then interpret it, developing a kinematic elastic block ...

Along the western margin of South America, plate convergence is accommodated by slip on the subduction interface and deformation of the overriding continent 1-6. In Chile 1-4 , Bolivia 6 , Ecuador and Colombia 5,7 , continental deformation occurs mostly through the motion of discrete domains, hundreds to thousands of kilometres in scale. These continental slivers are wedged between the Nazca and stable South American plates. Here we use geodetic data to identify another large continental sliver in Peru that is about 300-400 km wide and 1,500 km long, which we call the Inca Sliver. We show that movement of the slivers parallel to the subduction trench is controlled by the obliquity of plate convergence and is linked to prominent features of the Andes Mountains. For example, the Altiplano is located at the boundary of converging slivers at the concave bend of the central Andes, and the extending Gulf of Guayaquil is located at the boundary of diverging slivers at the convex bend of the northern Andes. Motion of a few large continental slivers therefore controls the present-day deformation of nearly the entire Andes mountain range. We also show that a 1,000-km-long section of the plate interface in northern Peru and southern Ecuador slips predominantly aseismically, a behaviour that contrasts with the highly seismic neighbouring segments. The primary characteristics of this low-coupled segment are shared by ∼20% of the subduction zones in the eastern Pacific Rim. Along the western margin of South America, rapid convergence (∼60-70 mm yr −1) of the oceanic Nazca Plate towards South America has produced three of the ten largest subduction earthquakes since 1900. Almost its entire length has been ruptured by M > 8 megathrust earthquakes since the 1500s (ref. 8). Contrasting with this observation, north of the 1746 M w ∼ 8.6 Lima earthquake 9 and south of the 1906 M w 8.8 Ecuador-Colombia event 10 , no M > 8 megathrust earthquake has occurred at least since the Spanish conquest of the Inca Empire in 1532 (Fig. 1a). The only noteworthy historical events occurred in 1619, when the city of Trujillo was destroyed, and in 1912 in the Piura area, but their attribution to subduction events remains uncertain 9. Among the four events with magnitude larger than 7 recorded by seismometers in this region, the 1960 M w 7.6 and 1996 M w 7.5 earthquakes

Along the Ecuadorian margin, oblique subduction induces deformation of the overriding continental plate. For the last 15 Ma, both exhumation and tectonic history of Ecuador suggest that the northeastward motion of the North Andean Sliver (NAS) was accompanied by an eastward migration of its eastern boundary and successive progressively narrowing restraining bends. Here we present geologic data, earthquake epicenters, focal mechanisms, GPS results, and a revised active fault map consistent with this new kinematic model. All data sets concur to demonstrate that active continental deformation is presently localized along a single major fault system, connecting fault segments from the Gulf of Guayaquil to the eastern Andean Cordillera. Although secondary faults are recognized within the Cordillera, they accommodate a negligible fraction of relative motion compared to the main fault system. The eastern limit is then concentrated rather than distributed as first proposed, marking a sharp boundary between the NAS, the Inca sliver, and the Subandean domain overthrusting the South American craton. The NAS limit follows a northeast striking right-lateral transpressional strike-slip system from the Gulf of Guayaquil (Isla Puná) to the Andean Cordillera and with the north-south striking transpressive faults along the eastern Andes. Eastward migration of the restraining belt since the Pliocene, abandonment of the sutures and reactivation of north-south striking ancient fault zones lead to the final development of a major tectonic boundary south and east of the NAS, favoring its extrusion as a continental sliver, accommodating the oblique convergence of the Nazca oceanic plate toward South America.