Understanding unrest and dynamic triggering processes on Sierra Negra, Galápagos Islands.

Dynamic earthquake triggering refers to the phenomenon where local seismic activity is induced by dynamic stress disturbances, originating from teleseismic earthquakes. An understanding of dynamic triggering on volcanoes offers a window into volcano stress states and seismicity initiation. Sierra Negra, a basaltic shield volcano situated on Isabela Island, Galápagos, has been the site of recurring episodes of dynamic triggering. Sierra Negra features a large elliptical summit caldera with a trap-door fault system and a magma reservoir extending 2km below the surface. Sierra Negra experienced an eruption in June 2018, characterized by a sequence of pre-eruption inflation, co-eruption deflation, and post-eruption inflation. The occurrence of dynamic earthquake triggering at Sierra Negra was observed in response to high magnitude teleseismic events from 2010 to 2018. The frequency of dynamically triggered earthquakes correlates with the increasing inflation of the magma reservoir. In this study, we aim to answer two questions: 1) How confident are we that the seismicity on Sierra Negra is dynamically triggered? And, 2) What is the location of these dynamically triggered events? Random simulations are used to calculate the likelihood that triggered events are related to teleseismic arrivals rather than being representative of local seismic activity. Results show that for the pre-2018 eruption, the likelihood that events are dynamic triggering is very high, compared to post-2018 eruption where events are more likely to be representative of local seismic activity. We only have access to a single station (VCH1) on Sierra Negra meaning the single-station location method must be used to locate all dynamically triggered events. To test and refine this method, 79 known seismic events are located using a full network from April 2018 – December 2018. Rotation of the 3-component VCH1 into the RTZ (radial-transverse-vertical) coordinate system is used to calculate the back-azimuth and the P-wave to S-wave delay is used to calculate the distance between event and station. 21 unknown dynamically triggered events are located in and around the caldera using this method. Looking forward we hope to understand the relationship between the location and timing of dynamic triggering, and its potential use in understanding volcano unrest state.