The diversity of slip processes occurring in the megathrust indicates that stress is highly variable in space and time. Based on GNSS and InSAR data, we study in depth the evolution of the interplate slip-rate along the Oaxaca subduction zone, Mexico, from October 2016 through August 2020, including the pre-seismic, coseismic and post-seismic phases associated with the 2020 Mw 7.4 Huatulco earthquake, to understand how different slip processes contribute to the stress accumulation in the region. Our results show that continuous changes in both the aseismic stress-releasing slip and the coupling produced a high stress concentration over the main asperity of the Huatulco earthquake and a stress shadow zone in the adjacent updip region. These findings may explain both the downdip rupture propagation of the Huatulco earthquake and its rupture impediment to shallower, tsunamigenic interface regions, respectively. Time variations of the interplate coupling around the adjacent 1978 Puerto Escondido rupture zone clearly correlate with the occurrence of the last three Slow Slip Events (SSEs) in Oaxaca far downdip of this zone, suggesting that SSEs are systematically accompanied by interplate coupling counterparts in the shallower seismogenic zone. In the same period, the interface region of the 1978 event experienced a remarkably high CFS built-up, imparted by the co-seismic and early post-seismic slip of the Huatulco rupture, indicating large earthquake potential near Puerto Escondido. Continuous monitoring of the interplate slip-rate thus provides a better estimation of the stress accumulation in the seismogenic regions where future earthquakes are likely to occur.

Understanding the interaction between tectonic plates from geodetic data is relevant to the assessment of seismic hazard. To shed light on that prevalently slow aseismic interaction, we developed a new static-slip inversion strategy, the ELADIN (ELastostatic ADjoint INversion) method, that uses the adjoint elastostatic equations to compute the gradient of the cost function. To handle plausible slip constraints, ELADIN is a 2-step inversion algorithm. First it finds the slip that best explains the data without any constraint, and then refines the solution by imposing the constraints through a Gradient Projection Method. To obtain a selfsimilar, physically-consistent slip distribution that accounts for sparsity and uncertainty in the data, ELADIN reduces the model space by using a von Karman regularization function that controls the wavenumber content of the solution, and weights the observations according to their covariance using the data precision matrix. Since crustal deformation is the result of different concomitant interactions at the plate interface, ELADIN simultaneously determines the regions of the interface subject to both stressing (i.e., coupling) and relaxing slip regimes. For estimating the resolution, we introduce a mobile checkerboard that allows to determine lower-bound fault resolution zones for an expected slip-patch size and a given stations array. We systematically test ELADIN with synthetic inversions along the whole Mexican subduction zone and use it to invert the 2006 Guerrero Slow Slip Event (SSE), which is one of the most studied SSEs in Mexico. Since only 12 GPS stations recorded the event, careful regularization is thus required to achieve reliable solutions. We compared our preferred slip solution with two previously published models and found that our solution retains their most reliable features. In addition, although all three SSE models predict an upward slip penetration invading the seismogenic zone of the Guerrero seismic gap, our resolution analysis indicates that this penetration might not be a reliable feature of the 2006 SSE.

The triggering of large earthquakes on a fault hosting aseismic slip or, conversely, the triggering of slow slip events (SSE) by passing seismic waves involves seismological questions with important hazard implications. Just a few observations plausibly suggest that such interactions actually happen in nature. In this study we show that three recent devastating earthquakes in Mexico are likely related to SSEs, describing a cascade of events interacting with each other on a regional scale via quasi-static and/or dynamic perturbations. Such interaction seems to be conditioned by the transient memory of Earth materials subject to the “traumatic” stressing produced by the seismic waves of the great 2017 (Mw8.2) Tehuantepec earthquake, which strongly disturbed the aseismic slip beating over a 650 km long segment of the subduction plate interface. Our results imply that seismic hazard in large populated areas is a short-term evolving function of seismotectonic processes that are often observable.

Accurate kinematic models are fundamental to enhance our knowledge of the seismic cycle as well as to improve surface ground motion prediction. However, the solution of the ill-posed kinematic inverse problem is non-unique (e.g., Cohee & Beroza, 1994; Wald & Heaton, 1994; Cotton & Campillo, 1995 and Minson et al., 2013) and, according to current acquisition systems surrounding active faults, this problem is highly underdetermined, in spite of its rather simple formulation as a linear inverse problem. Non-linear formulations of the problem, based on model reduction strategies, alleviate the underdetermination of the problem. However, non-linear formulations imply drastic assumptions on the rupture history and they complicate the use of linear algebra tools to assess the uncertainties of results. Regardless of the assumed inverse formulation, the incorporation of physical constrains and prior information into the inverse problem is necessary to provide more robust and plausible solutions. In this work (Sanchez-Reyes et al. 2018), we present a new hierarchical linear time domain kinematic source inversion method able to assimilate data traces through evolutive time windows. This progressive approach, both on the data and model spaces, does require mild assumptions based on prior knowledge or preconditioning strategies on the slip rate local gradient estimations. Contrary to similar approaches (Fan et al., 2014), this strategy benefits from the sparsity and causality of the seismic rupture while still ensuring the positivity of the solution. While standard regularization terms are used for stabilizing the inversion, strategies based on parameter reduction leading to a non-linear relationship between the source history and the observed seismograms are avoided. Rise time, rupture velocity and other attributes can be extracted later on from the slip-rate inversion we perform. . Satisfactory results are obtained on synthetic benchmarks proposed by the Source Inversion Validation project (Mai et al. 2016) and for the 2016 M$_w$7.0 Kumamoto mainshock. Our specific formulation combined with simple prior information, as well as numerical results obtained so far, yields interesting perspectives for a quasi-real-time implementation and to ease the uncertainty quantification of such ill-conditioned problem.

On 20 March 2012, a Mw 7.5 thrust earthquake started a series of large events up to magnitude Mw 8.2 (2017) that struck central Mexico during a period of nine years. Before this event, the Mexican subduction zone did not experience subduction earthquakes (Mw > 7.0) for at least 12 years. Most of the events during this highly active period (2012-) occurred in the plate interface, resulting in a significantly larger interplate slip rate in the states of Oaxaca and Guerrero. In this study, we explore how the aseismic slip transient caused by the 2012 Mw 7.5 earthquake affected the region and whether this earthquake had a causal relationship with a nearby similar magnitude event that occurred in 2018 (Mw 7.2). To this end, we identified and analyzed characteristic repeating earthquakes along the Mexican subduction zone for assessing the plate interface slip history and found a notably increase in the aseismic slip rate inferred from repeating earthquake activity following the 2012 mainshock, which suggests a long-standing slip perturbation in Oaxaca near the trench that continued until the 2018 Mw 7.2 Pinotepa Nacional earthquake.