GPS constraints on deformation in northern Central America from 1999 to 2017, Part 1 – Time-dependent modelling of large regional earthquakes and their post-seismic effects

Abstract

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

GPS constraints on deformation in northern Central America from 1999 to 2017, Part 2: Block rotations and fault slip rates, fault locking and distributed deformation

“…Supporting Information Table S1 gives the locations, velocities and other relevant information for these sites. Other useful information, including the data sources for all the GPS sites, can be found in Ellis et al (2018) and their Supporting Information. Ellis et al (2018) describe the methods that we use to process all the raw GPS data for our two-stage analysis, and the corrections to all the GPS station position time-series for the coseismic and postseismic effects of the earthquakes in 2009 and 2012.…”

“…Locations of GPS stations used in this study. The velocities of all GPS sites used in this study were corrected by Ellis et al (2018) for the transient effects of the three post-2009 earthquakes whose rupture areas are depicted by the light green patches in the figure. EQ; earthquake.…”

“…EQ; earthquake. This figure is adapted from Ellis et al (2018). The boxes identify regions that are shown in later figures.…”

“…Reduced chi-square (χ 2 ν ) for the model is 2.84 implying that the average data misfit is 1.7 times larger than the estimated uncertainty. Factors that may contribute to the higher-than-expected misfits include the usual random data errors, errors due to imperfect coseismic and time-dependent post-seismic corrections from the first stage of this analysis (Ellis et al 2018) and model-related misfits such as errors in our assigned block geometries, distributed deformation within blocks that are assumed to be rigid, imprecise a priori constraints on the inversion and viscoelastic deformation not accounted for by our elastic model (e.g. Li et al 2015).…”

“…This is the second stage of a two-part seismotectonic study of northern Central America that is based on the many new geodetic data that have become available since the Franco et al (2012) regional-scale geodetic study of northern Central America. In the first stage of our analysis, Ellis et al (2018) inverted the daily position time-series for more than 200 regional GPS sites to estimate time-dependent coseismic slip and post-seismic afterslip solutions for the 2009 May M w = 7.3 Swan Islands, 2012 August M w = 7.3 El Salvador and 2012 November M w = 7.4 Guatemala (Champerico) earthquakes (Fig. 2), which collectively altered the post-2009 motions of most GPS stations in northern Central America.…”