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.

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We use continuous and campaign measurements from 215 GPS sites in northern Central America and southern Mexico to estimate coseismic and afterslip solutions for the 2009 M w = 7.3 Swan Islands fault strike-slip earthquake and the 2012 M w = 7.3 El Salvador and M w = 7.4 Guatemala thrust-faulting earthquakes on the Middle America trench. Our simultaneous, time-dependent inversion of more than 350 000 daily GPS site positions gives the first jointly consistent estimates of the coseismic slips for all three earthquakes, their combined time-dependent post-seismic effects and secular station velocities corrected for both the coseismic and post-seismic deformation. Our geodetic slip solutions for all three earthquakes agree with previous estimates that were derived via static coseismic-offset modelling. Our time-dependent model, which attributes all transient post-seismic deformation to earthquake afterslip, fits nearly all of the continuous GPS site position time-series within their severalmillimetre position noise. Afterslip moments for the three earthquakes range from 35 to 140 per cent of the geodetic coseismic moments, with the largest afterslip estimated for the 2012 El Salvador earthquake along the weakly coupled El Salvador trench segment. Forward modelling of viscoelastic deformation triggered by all three earthquakes for a range of assumed mantle and lower crustal viscosities suggests that it accounts for under 20 per cent of the observed post-seismic deformation and possibly under 10 per cent. Our results thus point to afterslip as the primary and perhaps dominant mode of post-seismic deformation for these

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

The Jalpatagua fault in Guatemala accommodates dextral movement of the Central America forearc. We present new global positioning system (GPS) data, minor fault analysis, geochronological analyses, and analysis of lineaments to characterize deformation along the fault and near its terminations. Our data indicate that the Jalpatagua fault terminates at both ends into extensional regions. The western termination occurs near the Amatitlan caldera and the southern extension of the Guatemala City graben, as no through-going structures were observed to continue west into the active volcanic arc. Along the Jalpatagua fault, new and updated GPS site velocities are consistent with a slip rate of 7.1 ± 1.8 mm yr−1. Minor faulting along the central section of the fault includes: (1) N-S–striking normal faults accommodating E-W elongation; and (2) four sets of strike-slip faults (oriented 330°, 020°, 055°, and 295°, parallel to the Jalpatagua fault trace). Minor fault arrays support dextral movement along a major fault in the orientation of the Jalpatagua fault. GPS and fault data indicate that the Jalpatagua fault terminates to the east near the Guatemala–El Salvador border. Data delineate a pull-apart basin southeast of the fault termination, which is undergoing transtension as the Jalpatagua fault transitions into the El Salvador fault system to the east. Within the basin, minor faulting and lineations trend to the NW and accommodate NE-directed elongation. This faulting differs from E-W elongation observed along the Jalpatagua fault and is more similar to minor faults within the El Salvador fault system.