Trapdoor Fault Activation: A Step Toward Caldera Collapse at Sierra Negra, Galapagos, Ecuador

The 2018 Sierra Negra eruption resulted in meter-scale subsidence due to basaltic magma extraction from a deflating reservoir. The eruption was also characterized by dike intrusions, >4 M-W earthquakes, and sulfur dioxide emissions. We use a combination of Interferometric Synthetic Aperture Radar, Digital Elevation Model, Global Positioning System and seismic data to assess conditions required to trigger episodic caldera collapse at Sierra Negra. The 2018 effusive eruption was mainly sourced from a horizontal sill located at similar to 2 km depth, with a minimum erupted bulk volume of 0.19 km(3). The modeled reservoir is bound by a C-shaped ring of seismicity, suggesting trapdoor fault slip. Two >4.5 M-W earthquakes (5 and 22 July 2018) produced localized subsidence north of the southern trapdoor fault. After removing the modeled subsidence signal, distributed normal trapdoor fault slip explains the location of residual displacement. Furthermore, distributed fault models indicate slip occurred along the northern, central and southern segments of the trapdoor fault during the entire eruption, until 25 August 2018. Theoretical models of caldera collapse estimate that an erupted volume of 0.19 km(3) is too small to trigger full-scale caldera collapse, given the caldera aspect ratio (depth/diameter) of 0.22. Nonetheless, the spatial distribution and duration of seismicity and slip suggest trapdoor fault activation is the initial stage of caldera collapse at Sierra Negra, which may lead to full-scale caldera collapse during larger eruptions. Alternatively, hundreds of medium-sized eruptions, similar to that of 2018, may have triggered fault slip events over the past millennia.