HR: 17:45h
AN: V54B-07    [Abstracts]
TI: Dome surges, long period earthquake generation, and pyroclastic eruptions at Santiaguito Dome, Guatemala
AU: * Johnson, J B
EM: jeff.johnson@ees.nmt.edu
AF: New Mexico Institute of Mining and Technology, Department of Earth and Environmental Science 801 Leroy Place, Socorro, NM 87801,
AU: Lees, J M
EM: jonathan_lees@unc.edu
AF: University of North Carolina, Department of Geological Sciences CB #3315, Mitchell Hall, Chapel Hill, NC 27599-3315,
AU: Sanderson, R
EM: rsanderson@ees.nmt.edu
AF: New Mexico Institute of Mining and Technology, Department of Earth and Environmental Science 801 Leroy Place, Socorro, NM 87801,
AU: Sahagian, D
EM: dork.sahagian@lehigh.edu
AF: Lehigh University, Earth and Environmental Sciences 31 Williams Dr, Bethlehem, PA 18015-3126,
AU: Normand, J A
EM: joshuan@cisunix.unh.edu
AF: University of New Hampshire, Department of Earth Sciences 56 College Road, Durham, NH 03824,
AB: Sudden horizontal surges of the 10⁴ m² Caliente dome at Santiaguito (Guatemala) are likely responsible for the frequent (1-2 per hour) long period (LP) earthquakes that have been consistently observed at this volcano for years. Dramatic dome surface movements, in which portions of the dacite/andesite dome are accelerated from rest to 4 m/s during a few tenths of a second (greater than 30 m/s²), were captured using a high resolution video camera from a vantage point 1200 m above, on the Santa Maria summit. During each event the surge was observed to propagate outward from the central "vent" and reach the crater periphery (more than 100 distant) after 1 to 2 s. This "deformation front" was observed to propagate at a velocity too slow for elastic waves and too quickly for buoyancy waves and appears to represent a static displacement. Assuming a conservatively thin dome thickness of 10¹ m and a laminar horizontal dome flow, the entire surge involves a moment gain of about 10⁹ kg m/s during a time scale of about 1 s. An impulsive force of this magnitude can be expected to impart significant elastic energy to the surrounding country rock. Indeed, this source appears to be the cause of the corresponding LPs, which possess dominant frequencies of 0.5 to 2 Hz, and which were recorded by our temporary local network of six broad-band seismometers. Precursory low-amplitude seismicity (leading up to the LP) and explosive degassing (coincident with the LP) provide additional evidence linking the LPs to the observed acceleration events affecting the dome. We postulate that precursory seismicity reflects incremental failure of the dome and that the main LP and dome surge occurs only when cracks propagate up through the shallow dome itself. Explosive gas venting occurs when pathways have been opened throughout the 10⁴ m² surface extent of the dome, which has been violently accelerated. Resealing of these fissures is suggested by the termination of pyroclastic emissions over the course of a few tens of seconds. The episodic nature of these events, which cycle from quiescence, to precursory seismicity, to surge / LP / pyroclastic emission, to quiescence again on a time scale of 30-60 minutes suggests repeated stress accumulation due to a consistent influx of gas-charged magma of more than 10⁴ m³ per day. This consistent flux of magma is also suggested by the long duration (more than 80 years) of continuous effusion of Santiaguito dome dacite. Further exploration of these relations between observed dome motion, precursory seismicity, LPs, and degassing events in the context of an episodic eruptive sequence may provide new insights regarding the mechanisms of eruption in volcanic domes in general.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8425 Effusive volcanism
DE: 8428 Explosive volcanism
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting