The Polochic‐Motagua strike‐slip fault system in Guatemala marks the on‐land plate boundary between the North American (NA) and the Caribbean (CA) plates. GPS observations in 1999 and 2003 show that the far‐field velocity across the system (NA‐CA relative velocity) is ∼20 mm/yr. This is significantly higher than the NUVEL‐1A velocity but is consistent with the GPS based CA‐NA velocity proposed by DeMets et al. (2000). The observations are modeled by a fault centered on the Motagua fault, locked at a depth of 20 km, with a slip‐rate decreasing from eastern to central Guatemala from 20 to 12 mm/yr towards the NA‐CA‐Cocos triple junction. This decrease is accommodated by ∼8 mm/yr of E‐W extension in the westernmost part of CA south of the Motagua fault. About 10 mm/yr of dextral slip is observed across the Mid‐American Volcanic Arc. The NA‐CA‐Cocos triple junction is thus a complex, ∼400 km‐wide wedge‐shaped area.

International audienceNew GPS measurements in Chiapas (Mexico), Guatemala and El Salvador are used to constrain the fault kinematics in the North America (NA), Caribbean (CA) and Cocos (CO) plates triple junction area. The regional GPS velocity field is first analysed in terms of strain partitioning across the major volcano-tectonic structures, using elastic half-space modelling, then inverted through a block model. We show the dominant role of the Motagua Fault with respect to the Polochic Fault in the accommodation of the present-day deformation associated with the NA and CA relative motion. The NA/CA motion decreases from 18-22 mm yr−1 in eastern Guatemala to 14-20 mm yr−1 in central Guatemala (assuming a uniform locking depth of 14-28 km), down to a few millimetres per year in western Guatemala. As a consequence, the western tip of the CA Plate deforms internally, with ≃9 mm yr−1 of east-west extension (≃5 mm yr−1 across the Guatemala city graben alone). Up to 15 mm yr−1 of dextral motion can be accommodated across the volcanic arc in El Salvador and southeastern Guatemala. The arc seems to mark the northern boundary of an independent forearc sliver (AR), pinned to the NA plate. The inversion of the velocity field shows that a four-block (NA, CA, CO and AR) model, that combines relative block rotations with elastic deformation at the block boundaries, can account for most of the GPS observations and constrain the overall kinematics of the active structures. This regional modelling also evidences lateral variations of coupling at the CO subduction interface, with a fairly high-coupling (≃0.6) offshore Chiapas and low-coupling (≃0.25) offshore Guatemala and El Salvador

Seismological Research Letters, vol. 80, n°6, pp. 977-984, 2009International audienceWe report results from a six-month seismological experiment in the area of the eastern Polochic-Motagua fault system (Guatemala) designed to both characterize the present seismicity and bring some constraints on the lithospheric structure. The seismic activity occurs in the upper 15 km of the crust, on the Polochic and the Motagua faults as well as in a NS-trending graben south of the Motagua fault and within the active folds north of the Polochic fault. From receiver function analysis the Moho discontinuity is found at about 35 km depth north of the Polochic fault and south of the Motagua fault, while the region in between is characterized by a 4-to-6-km thinner crust or by a 6–7% decrease of the Vp/Vs ratio

Microearthquake hypocenters were analyzed in the Krýsuvík geothermal area in SW-Iceland with data taken from two consecutive passive seismic surveys, 2005 and 2009. Five years prior to the 2005 survey, this area was struck by an earthquake initiating a major top-to-bottom fluid migration in the upper crust. We observe from our surveys a complex bottom-to-top migration of seismicity with time following this fluid penetration, suggesting the migration of a pore pressure front controlled by the upper-crust fracture system. We interpret these data as the time and space development of high-temperature hydrothermal cells from a deep upper crustal fluid reservoir in the supercritical field. These results provide an insight into the coupling mechanisms between active tectonics and fluid flow in upper-crustal extensional systems with high thermal flux.