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.

SUMMARY We describe and model GPS measurements of surface deformation from the Oaxaca segment of the Mexican subduction zone to characterize interseismic strain accumulation and episodic transient slip in this region and test seismologically‐based models of strain accumulation and release along subduction interfaces. Deformation measured from 2001 to 2007 within our dense 31‐station GPS array has consisted of (1) trench‐normal horizontal contraction at rates that decrease monotonically inland from the coast, (2) rapid coastal subsidence that changes gradually to slow uplift at locations more than 100 km inland and (3) periods of transient slip that interrupt the otherwise steady deformation. Inverse modelling of transient station offsets in 2004 and 2006 indicates that transient slip along the subduction interface occurred downdip from the rupture limits of previous large earthquakes in this region in both 2004 and 2006. GPS site velocities that are corrected for the effects of this transient slip vary significantly over distances of only tens of kilometres both along the coastline and inland, implying that similar spatial variations occur in the degree of locking across the subduction interface. Deformation rates measured along the coast reach their maximum above the core of the rupture zone of the 1978 Ms= 7.8 shallow‐thrust earthquake and generally decrease outwards towards the edges of the rupture zone. Bounded‐value, inverse modelling of the interseismic GPS velocity field with a finite element mesh that simulates the study region indicates that much of the rupture zone of the 1978 shallow‐thrust earthquake is fully locked at the plate convergence rate, but that this region is surrounded by weakly locked areas of the subduction interface, which may slow or arrest the propagation of future earthquakes. Much of the deeper region of transient slip, downdip from the seismogenic zone, is also fully locked between the episodes of transient slip; however, the elastic energy that accumulated due to locking of this deeper transitional zone between 2002 and 2006 appears to have been mostly or completed released by the 2004 and early 2006 transient slip events. The approximately balanced energy budget for the deeper zone of transient slip implies that this region is unlikely to contribute significant elastic energy to future earthquakes that originate along the seismogenic zone. Our results support a model in which seismic asperities coincide with regions of strong locking between earthquakes. The potential elastic energy that has re‐accumulated since 1978 in the seismogenic zone is already sufficient to cause a repeat of the 1978 earthquake.

SUMMARY We use data from 12 continuous GPS stations in southern Mexico, including eight new stations, to better characterize transient slip episodes along the Mexican subduction zone. Continuous GPS recording in Oaxaca that began 14 years ago, constituting the longest continuous GPS record in southern Mexico, defines nine distinct episodes of transient slip from 1993 to 2007, including previously unreported transient slip episodes in early 1995 and 2006. All transient slip episodes recorded in Oaxaca City were also recorded at a GPS site ∼400 km away in the state of Guerrero after measurements began there in early 1997, demonstrating that transient slip affects widespread areas of southern Mexico. Well‐recorded transients in 2004 and 2006 appear to have originated along the Oaxaca trench segment, hundreds of kilometres from the potentially hazardous Guerrero seismic gap. During the 2004 slip transient, displacements of more than 20 mm were recorded at stations in the Oaxaca GPS array, a factor of 2–3 larger than at sites in Guerrero. Elastic half‐space modelling of the 2004 displacements at sites throughout southern Mexico indicates that most slip was focused beneath Oaxaca in deeper areas of the subduction interface immediately downdip from previous megathrust rupture zones, but that slip also may have extended significantly updip into a seismic gap along the Oaxaca coast. The best‐fitting model is also able to explain transient vertical offsets of up to 30 mm observed during this event. Shortly before the onset of transient slip in 2004, a pair of moderate (Mw 5.1, 5.5) earthquakes ruptured the downdip end of the seismogenic zone immediately east of the Oaxaca seismic gap, suggesting a possible relationship between the two. In 2006, transient slip began 1–3 months earlier in Oaxaca than in other areas, offering the clearest evidence to date for the existence of one or more source regions for transient slip outside the Guerrero seismic gap, where large amplitude transient slip originated in 2002. The still‐limited data indicate that transient slip that originates elsewhere in Mexico may trigger aseismic slip in Guerrero, and further indicate that transient slip beneath Oaxaca is limited to areas of the subduction interface that surround the rupture zones of previous large shallow‐thrust earthquakes.

We model three slow slip events in 2006 and 2007 recorded by continuous GPS stations in central and southern Mexico to test for overlap between their source regions along the Mexican subduction interface and whether they intrude upward into the rupture zones of previous large earthquakes. Inverse modeling yields source regions beneath central Oaxaca for two of the three slow slip events (SSE), where a previously described SSE occurred in 2004, and beneath Guerrero for the third, where slip events previously occurred in 2001–2002 and possibly 1998. Along with previously published results, our work suggests there are persistent differences between the depths and magnitudes of transient slip beneath Oaxaca and Guerrero. Transient slip beneath Oaxaca in 2004, 2006, and 2007 had a common source region downdip from the seismogenic zone and released elastic strain energy equivalent to Mw∼7.0 earthquakes, equaling most or all energy that accumulated below the seismogenic zone. Transient slip beneath Guerrero in 2006 had a larger moment magnitude (Mw∼7.3) and extended somewhat farther updip, possibly to seismogenic depths. Transient slip thus appears to relieve some elastic strain that accumulates at shallow levels in the Guerrero seismic gap. We find no evidence for spatial or temporal correlations of slow slip along these two widely separated source regions, although better data are needed to test more definitively for any interaction between them.