Numerical modelling of earthquake induced stress changes and dike migration preceding the 2018 eruption of Sierra Negra Volcano, Galápagos

Studying the interactions between magmatic and volcano-tectonic processes is critical to understanding the behavior of volcanic systems and the triggering of eruptions. Sierra Negra Volcano, Galápagos is one of the largest basaltic calderas on Earth, and its latest eruption in 2018 marked the first time an eruption in the Galápagos was recorded by both GNSS and seismic networks. These unprecedented data sets provide a unique opportunity to investigate the dynamics of eruption triggering and test hypotheses for dike initiation and migration. Observational and modeling studies of Sierra Negra’s previous eruption in 2005 hypothesize that tensile opening induced by a Mw5.3 earthquake on the southern intra-caldera trapdoor fault system resulted in magmatic dike initiation and eruption along the northern rim of the caldera (Gregg et al., GRL, 2018). In the ~13 years following the 2005 eruption, the caldera of Sierra Negra uplifted ~6.5 m (Bell et al., Nat. Comm., 2021). Sierra Negra’s inflation coincided with an increase in Mw > 4 earthquakes located along the intra-caldera fault system from late 2017 to June 2018 and a Mw5.4 earthquake occurred ~9 hours prior to the first eruptive fissure opening on 26 June 2018 (Bell et al., JGR, 2021). The similarities between the 2005 and 2018 pre-eruption sequences provide important clues about the dynamics of eruption triggering at Sierra Negra. In this study, we build upon previous numerical analyses of the 2005 and 2018 eruption (Gregg et al., GRL, 2018; Gregg et al., Sci. Adv., 2020; Bell et al., Nat. Comm., 2021) to model stress changes associated with the 2018 pre-eruptive Mw5.4 earthquake and begin to investigate dike initiation and magma migration leading to the first fissure opening. Specifically, thermomechanical finite element models are implemented in COMSOL Multiphysics to test the hypothesis that the Mw5.4 thrust fault earthquake promoted tensile failure and dike rupture in the northern region of the magma system. Future numerical models will test the hypothesis that the resulting stress field of the Mw5.4 earthquake controlled the pathway of dike migration. Previous numerical deformation models which exclude fault rupture (e.g., Gregg et al., Sci. Adv., 2018) do not result in system failure leading to eruption (i.e., tensile failure in the model space between the magma chamber and surface). Our new numerical model includes a segment of the intra-caldera Trapdoor Fault, and we solve for fault rupture parameters of the 26 June 2018 Mw5.4 earthquake, such as fault geometry and slip, and calculate the resulting stress and strain. The best-fit earthquake parameters are estimated by comparing the observed (cGPS-derived) and modeled co-seismic deformation. The stress field induced by both magma system inflation and fault rupture is evaluated to investigate failure of the volcanic edifice and optimal dike propagation pathways. Preliminary results demonstrate the importance of including fault displacements in model calculations of Sierra Negra’s stress evolution. In future numerical models, we aim to model and constrain magmatic intrusion geometries using pre- and syn-eruptive cGPS data to better understand the impact of static stress transfer in eruption triggering.