HR: 0800h
AN: V51D-0780    [Abstracts]
TI: Pre-eruptive seismicity associated with explosive events at Santiaguito volcano, Guatemala
AU: * Sanderson, R W
EM: rsanderson@ees.nmt.edu
AF: New Mexico Institute of Mining and Technology, Dept. of Earth and Environmental Science 801 Leroy Place, Socorro, NM 87801, United States
AU: Johnson, J B
EM: jeff.johnson@ees.nmt.edu
AF: New Mexico Institute of Mining and Technology, Dept. of Earth and Environmental Science 801 Leroy Place, Socorro, NM 87801, United States
AB: We conducted a one-week field campaign to acquire seismo-acoustic data together with other geophysical observations (thermal, radar, video, gas flux) at Santiaguito volcano, Guatemala in January 2007. The project goal was to understand and quantify energy budgets and physical mechanisms associated with explosions and lava effusion at a silica-rich volcano. The Santiaguito volcanic dome complex in western Guatemala provided an ideal laboratory as it has displayed a complex mix of explosive and effusive activity since it began to grow in 1922. In recent years, small discrete explosive events have occurred on the order of once or twice per hour, while a contemporaneous dacitic-andesitic lava flow is extruded to the south/south-east. We are currently seeking an improved understanding of the enigmatic seismicity associated with these explosions, which comprise approximately 85% of the local earthquake catalogue. Rockfalls 10% and other events (regional tectonic, volcano-tectonic and hybrid) 5% comprise the balance of events. During the week-long explosion earthquake catalogue of 300 events (45 per day), we identified ubiquitous "precursory seismicity" of varying length duration (4 - 17 seconds), which preceded visible pyroclastic emissions and had no coincident infrasound. The onset of the comparatively large explosion earthquake (‘primary phase') occurred simultaneously with the generation of an infrasonic pulse and a release of ash and magmatic gas. Precursory seismic signals are emergent, display a spectral peak between 0.5 and 2 Hz, and possess varying degrees of energy at higher frequencies, which is both time-varying and event-dependent. Precursory seismicity typically increases in amplitude leading up to the primary phase, which is typically also peaked below 2 Hz and is classified as a long-period earthquake. Video analysis of the dome surface at the onset of explosions shows a surge, or horizontal displacement, of the dome coincident with the primary phase long period event. Initial analysis of particle motions, wave envelope and frequencies suggests fracturing of the near-vent subsurface. The following long period event suggests an initially slow then rapid vertical motion of conduit material which leads to displacement of the lava flow on the dome's surface. The associated rapid expansion of magmatic gas produces low intensity infrasound and may also contribute to the seismic wavefield.
DE: 7280 Volcano seismology (8419)
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting