The sources of the shallow, upper plate earthquakes in Central Mexico, and their possible triggering by the subduction earthquakes, 'normal' and slow

Large earthquakes have broken the Central Mexico upper plate in the historical time (1887, M5.3; 1912, M6.9; 1920, M6.5; 1976, M5.3; 1979, M5.3), but the faults responsible for those earthquakes are not precisely known. Nor is their link with the subduction, especially in terms of possible triggering between subduction and shallow earthquakes. To address those issues, we first identify the major active faults that cut the upper plate, and determine their slip mode and overall organization. For that, we conduct a morphological analysis of the region, based on the use of satellite and topographic images. The upper plate appears dissected by a dense network of hundreds of major active faults, which overall form 2 distinct large-scale systems, named Jalisco and Mexico. The Jalisco system is made of the N-S Colima normal fault system which runs from the coast to the Transmexican Belt (TMB) where it ends in a large fishtail centered on the Chapala Lake. The western branch of the fishtail is made of NW-striking, N-dipping faults that are both normal and left-lateral, while the eastern branch is made of ENE-trending, N-dipping normal faults. The Mexico system resembles a large-scale horsetail. It is made of a major NNW-trending left-lateral strike-slip fault that runs from the coast to the TMB, at the eastern edge of the Oaxaca region. As it enters the TMB, that NNW system connects to a series of E-W, N-dipping, normal-right-lateral faults, while extending further north through a series of NNW, E-dipping normal-left-lateral faults. Together these faults bound to the south and west a series of rhomboidal half-grabens, among those are the basins of Mexico City and Acambay. Mexico City is thus bounded by large active faults but also dissected by smaller ones, both E-W and NNW. The E-W fault system that bounds the Acambay half-graben to the south is likely the one to have ruptured in 1912. Second, we examine whether the subduction interface and the upper plate active faults interact due to static stress transfer. Hypothesizing that the Guerrero gap entirely breaks in a M8.1 earthquake on the subduction interface, we calculate that the static Coulomb stress change that such an earthquake would produce at 12 km depth on the E-W, N60°-dipping Mexico City and Acambay normal faults, would be 0.5-1 bars, hence large enough to trigger the rupturing of those faults. The 1912 Acambay earthquake may thus have been triggered by the 1911 large earthquake that apparently broke the Guerrero gap. Using the slip distribution provided by Cotton et al. (this issue) for the 2006 slow earthquake on the Guerrero subduction interface, we calculate that the static stress change that such a slow event would produce on the same Mexico City and Acambay faults, would be 0.05-0.1 bars. Because slow earthquakes repeat frequently and possibly every 4 yrs in Guerrero (Cotte et al., submitted), only 10 events (over 40 yrs?) similar to the 2006 slow earthquake would be enough to produce a stress change similar to the M8 subduction events, hence possibly play a part in the trigger, and advance the time of occurrence of the crustal Mexico and Acambay faults. Seismic hazard in Central Mexico thus depends on multiple sources, including the slow subduction earthquakes.