Uplift and Seismicity Driven by Magmatic Inflation at Sierra Negra Volcano, Galápagos Islands

Andrew Forbes Bell,Mario Calixto Ruiz,Stephen Hernandez,Peter Christopher La Femina

doi:10.1029/2021jb022244

Abstract

AbstractAlthough episodes of surface uplift and elevated seismicity precede many volcanic eruptions, their temporal evolution is often complex, and apparently in contradiction to simple trends predicted by mechanical deformation models. Here, we use continuous global positioning system and seismic data recorded at Sierra Negra volcano, Galápagos Islands, to show how the edifice responded to stress changes driven by magma accumulation in a shallow sill. The rate of uplift varied during the 13 years and 6.5 m of inflation before the 2018 eruption. The number of earthquakes per unit of uplift increased exponentially with total uplift as the differential stress increased. Accordingly, the temporal seismicity rate varied in time as a function of both the total uplift and the uplift rate. The Gutenberg‐Richter b‐value decreased as a function of total uplift. In the final six months before the eruption, a sequence of large (M > 4) earthquakes regulated the state of stress on the fault, each being followed by 2–3 days of postseismic quiescence, and retarding the increase in seismicity rate. These earthquakes did not affect the overall uplift rate. Subsidence of 8.5 m accompanied the 2‐month eruption. On resumption of uplift, the number of earthquakes per unit of uplift was very low, and the b‐value high, reflecting the relaxed stress state of the fault system. These observations show that crustal deformation becomes increasingly brittle at higher stress states, and supports theoretical models based on elastic‐brittle mechanics. They suggest that joint interpretation of deformation and seismicity is key for forecasting future eruptions in similar volcanic settings.

Highlights

Volcanic eruptions are often preceded by surface uplift and elevated seismicity (Amelung et al, 2000; Biggs et al, 2010; Chadwick et al, 2006; Nooner & Chadwick, 2016; Robertson & Kilburn, 2016)
Abstract episodes of surface uplift and elevated seismicity precede many volcanic eruptions, their temporal evolution is often complex, and apparently in contradiction to simple trends predicted by mechanical deformation models
We use continuous global positioning system and seismic data recorded at Sierra Negra volcano, Galápagos Islands, to show how the edifice responded to stress changes driven by magma accumulation in a shallow sill

Summary

Introduction

Volcanic eruptions are often preceded by surface uplift and elevated seismicity (Amelung et al, 2000; Biggs et al, 2010; Chadwick et al, 2006; Nooner & Chadwick, 2016; Robertson & Kilburn, 2016). Such signals can be the first indication of an approaching eruption, and their spatial and temporal patterns form the basis for short-term eruption forecasts (Sparks, 2003; Voight, 1988; White & McCausland, 2019). InSAR observations have confirmed that all of the western Galápagos volcanoes undergo significant amounts of ground deformation (Bagnardi & Amelung, 2012; Galetto et al, 2019; Jónsson et al, 2005)

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