Continental block motion in the Northern Andes from GPS measurements

Nor thwester n South America is a plate boundary zone where the Nazca, Caribbean and South American plates interact to produce a wide area of active continental deformation from the Gulf of Guayaquil (latitude 3 • S) to Venezuela. Previous studies have identified a ∼2000-km-long continental sliver, referred as the North Andean Sliver (NAS), squeezed between the Nazca, Caribbean and South American plates and escaping at ∼1 cm yr −1 northeastward with respect to South America. Subduction of the Nazca Plate beneath the NAS has produced a sequence of large and g reat ear thquakes during the 20th century along the coast of Ecuador and Colombia. Large crustal earthquakes up to magnitude 7.7 have been documented along the proposed easter n boundar y of the N AS. How e ver, acti ve tectonics data, historical and recent earthquakes all indicate active fault systems within the NAS, possibly resulting from the interaction of several tectonic blocks. Here, we derive an extensive horizontal velocity field using continuous and episodic GNSS data from 1994 to 2019.9, covering nor ther n Per u, Ecuador, Colombia, Panama and Venezuela. We model the GNSS velocity field using a kinematic elastic block approach that simultaneously solves for rigid tectonic block rotations and interseismic coupling along the subduction interfaces and along major crustal faults. In contrast to previous results that considered a single rigid NAS, our dense GNSS velocity field demonstrates that the NAS undergoes significant internal deformation and cannot be modelled as single rigid block. We find that block kinematics in the nor ther n Andes are well described by the rotation of 6 tectonic blocks, showing increasing eastward motion from south to north. The Eastern boundary of the sliver is defined by a right-lateral transpressive fault system accommodating 5.6-17 mm yr −1 of motion. Fragmentation of the NAS occurs through several fault systems with slip rates of 2-4 mm yr −1. Slow reverse motion is found across the sub-Andean domain in Ecuador and nor ther n Per u at 2-4 mm yr −1 , marking a transitional area betw een the N AS and stable South America. In contrast, such a transitional sub-Andean domain does not exist in Colombia and western Venezuela. At the northwestern corner of Colombia, fast (∼15 mm yr −1) eastward motion of the Panama block with respect to the NAS results in arc-continent collision. We propose that the Uramita fault and Eastern Panama Deformed Zone define the current Panama/NAS boundary, accommodating 6 and 15 mm yr −1 of relative motion, respectively. A fraction of the Panama motion appears to transfer northeastward throughout the San Jacinto fold belt and as far east as longitude ∼75 • W. Along the Caribbean coast, our model confirms, slow activ

Distribution of discrete seismic asperities and aseismic slip along the Ecuadorian megathrust

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 (Mw = 7.8), 1958 (Mw = 7.7), 1979 (Mw = 8.2) and 1998 (Mw = 7.1). The axis of the Carnegie Ridge, which is entering the subduction zone south of the Equator, coincides well with the location of a 50 km wide creeping corridor that may have acted as persistent barrier to large seismic ruptures. South of this creeping region, a highly locked asperity is found right below La Plata Island. While this asperity may have the potential to generate an Mw similar to 7.0-7.5 earthquake and a local tsunami, until now it is unknown to have produced any similar events. That region is characterized by the presence of slow slip events that may contribute significantly to reduce the long-term moment deficit accumulated there and postpone the failure of that asperity. At the actual accumulation rate, a characteristic recurrence time for events such as those in 1942, 1958 and 1979 is 140 +/- 30 yr, 90 +/- 20 yr, 153 +/- 80 yr respectively. For the great 1906 event, we find a recurrence time of at least 575 +/- 100 yr, making the great 1906 earthquake a rare super cycle event

Motion of continental slivers and creeping subduction in the Northern Andes

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Motion of continental slivers and creeping subduction in the northern Andes

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 similar to 20% of the subduction zones in the eastern Pacific Rim