S U M M A R YWe invert GPS velocities from 32 sites in El Salvador, Honduras and Nicaragua to estimate the rate of long-term forearc motion and distributions of interseismic coupling across the Middle America subduction zone offshore from these countries and faults in the Salvadoran and Nicaraguan volcanic arcs. A 3-D finite element model is used to approximate the geometries of the subduction interface and strike-slip faults in the volcanic arc and determine the elastic response to coupling across these faults. The GPS velocities are best fit by a model in which the forearc moves 14-16 mm yr −1 and has coupling of 85-100 per cent across faults in the volcanic arc, in agreement with the high level of historic and recent earthquake activity in the volcanic arc. Our velocity inversion indicates that coupling across the potentially seismogenic areas of the subduction interface is remarkably weak, averaging no more than 3 per cent of the plate convergence rate and with only two poorly resolved patches where coupling might be higher along the 550-km-long segment we modelled. Our geodetic evidence for weak subduction coupling disagrees with a seismically derived coupling estimate of 60 ± 10 per cent from a published analysis of earthquake damage back to 1690, but agrees with three other seismologic studies that infer weak subduction coupling from 20th century earthquakes. Most large historical earthquakes offshore from El Salvador and western Nicaragua may therefore have been intraslab normal faulting events similar to the M w 7.3 1982 and M w 7.7 2001 earthquakes offshore from El Salvador. Alternatively, the degree of coupling might vary with time. The evidence for weak coupling indirectly supports a recently published hypothesis that much of the Middle American forearc is escaping to the west or northwest away from the Cocos Ridge collision zone in Costa Rica. Such a hypothesis is particularly attractive for El Salvador, where there is little or no convergence obliquity to drive the observed trench-parallel forearc motion.

We combine geodetic, structural, and paleomagnetic data from El Salvador with Global Positioning System (GPS) data from southern Honduras and Nicaragua to describe the motions of the Salvadoran and Nicaraguan forearcs and determine the location and style of faulting across the Gulf of Fonseca offset of the volcanic arcs of eastern El Salvador and western Nicaragua. Finite-element modeling of GPS measurements at 35 sites in El Salvador, southern Honduras, and Nicaragua indicates that the Nicaraguan and Salvadoran forearcs both move west to northwest, parallel to their respective trenches, at 15 ± 2 mm yr-1 (95% limit) in a Caribbean plate reference frame. The similar motions of the two forearcs, despite an ~20°-25° difference in the obliquity of subduction beneath them and absence of any signifi cant convergence obliquity offshore from El Salvador, are consistent with a recent hypothesis that the Nicaraguan forearc pushes the Salvadoran forearc to the northwest, possibly driven by northwestward lateral escape of the Central America forearc from its collision zone with the Cocos Ridge offshore from Costa Rica. The Gulf of Fonseca and adjacent eastern El Salvador form an ~60-km-wide extensional zone with E-W elongation, determined by diffuse seismicity, GPS velocities, and numerous young, N-S-striking normal faults mapped with a 10 m digital elevation model (DEM), structural measurements, and Lidar (light detection and ranging). Strike-slip earthquakes in the Fonseca pull-apart structure and evidence for modest (~10°) vertical-axis fault block rotations from paleomagnetic measurements at 33 sites in the Fonseca pull-apart structure both indicate that extension may be accompanied by bookshelf faulting.

In order to analyze the process of subduction of the Nazca and South American plates in the area of the Southern Andes, and its relationship with the tectonic and volcanic regime of the place, magnetotelluric measurements were made through a transversal profile of the Chilean continental margin. The data processing stage included the analysis of dimensional parameters, which as first results showed a three-dimensional environment for periods less than 1s and two-dimensional for periods greater than 10s. In addition, through the geomagnetic transfer function (tipper), the presence of structural electrical anisotropy was identified in the data. After the dimensional analysis, a deep electrical resistivity image was obtained by inverting a 2D and a 3D model. Surface conductive anomalies were obtained beneath the central depression related to the early dehydration of the slab and the serpentinization process of the mantle that coincides in location with a discontinuity in the electrical resistivity of a regional body that we identified as the Nazca plate. A shallow conductive body was located around the Calbuco volcano and was correlated with a magmatic chamber or reservoir which in turn appears to be connected to the Liquiñe Ofqui fault system and the Andean Transverse Fault system. In addition to the serpentinization process, when the oceanic crust reaches a depth of 80-100km, the ascending fluids produced by the dehydration and phase changes of the minerals present in the oceanic plate produce basaltic melts in the wedge of the subcontinental mantle that give rise to an eclogitization process and this explains a large conductivity anomaly present beneath the main mountain range.