Models for the dynamic triggering of volcano seismicity at Sierra Negra, Galápagos Islands.

Abstract

Dynamic earthquake triggering is the process where local earthquakes are triggered by dynamic stress perturbations often from teleseismic earthquakes. Dynamic triggering from regional earthquakes can also trigger local volcanic seismicity. An understanding of dynamic triggering on volcanoes offers a window into volcano stress state and seismicity initiation, in general. Repeated episodes of dynamic triggering have been recorded at Sierra Negra, a large basaltic shield volcano on Isabela Island, Galápagos. Sierra Negra is a large elliptical summit caldera with a trap-door fault system and a 2km deep sill-like magma reservoir below the caldera. Sierra Negra erupted in June 2018, as part of a cycle of pre-eruption inflation, co-eruption deflation, and renewed post-eruption inflation. Dynamic earthquake triggering was observed at Sierra Negra following high magnitude teleseismic events that occurred from 2010-2018, with the number of dynamically triggered earthquakes increasing with increasing inflation of the magma reservoir. However, the locations and mechanisms of this dynamic triggering have not been determined. In this study, we aim to answer two questions: 1) is it possible to successfully interpret dynamic triggering on Sierra Negra and, 2) can dynamic triggering in Sierra Negra be used as a stress gauge? To interpret dynamic triggering, an STA/LTA detection algorithm has been designed which detects when a dynamically triggered event has happened. This provides insight into how regularly dynamic triggering occurs on Sierra Negra and how, if at all, it is related to teleseismic events. The detection algorithm has also been used to compare the dynamic triggering rate to the local seismic rate on Sierra Negra. We have also located where dynamic triggering occurs on Sierra Negra. To address question two; Peak Dynamic Strain (PDS) has been used as a threshold to detect when dynamic triggering occurs, PDS can be used to understand the stress state of Sierra Negra pre-, co- and post-2018 eruption. 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.