Dynamics of explosive volcanism at Fuego volcano imaged with very long period seismicity

The current activity at Fuego volcano, Guatemala, is characterized by frequent explosions, intermittent lava flows, and persistent degassing. Seismic and infrasound data of explosive activity recorded over 19 days in January 2009 on a temporary broadband network show that explosive intensity is highly variable. However, the seismic expression of the largest explosions includes similar very long period (VLP) earthquakes for each explosion. Waveform modeling in the 10–30 s band constrains the source centroid to a position 300 m below and 300 m west of the summit crater. The calculated moment tensor indicates a volumetric source, which is modeled as a near‐vertical dike feeding a SW dipping (35°) sill. The sill is the dominant component, and its projection to the surface nearly intersects the summit crater. The deformation history of the sill is interpreted as (1) an initial inflation that is due to pressurization, followed by (2) a rapid deflation as overpressure is explosively released, and finally (3) a reinflation as fresh magma flows into the sill and degasses. This cycle is attributed to the development of a brittle plug at the top of the magma column and effective sealing of degassing pathways that is due to degassing crystallization as water exsolves and the magma is undercooled. Data from apparent tilt, SO2 concentrations, and infrasound records support this interpretation. The VLP source location suggests the magma pathway may be migrating westward, which has hazard implications for populations living on the flanks of Fuego. Key Points Waveform inversion locates the source of very long period explosion seismicity Conduit geometry is dominated by a shallow sill dipping 35 degrees toward the SW Degassing crystallization causes conduit pressurization, brittle failure & deformation