HR: 1400h
AN: V33A-01 INVITED     [Abstracts]
TI: Tungurahua Volcano's 1999-2007 Eruptive Process, Monitoring Results and Risk Mitigation
AU: * Mothes, P A
EM: pmothes@igepn.edu.ec
AF: Instituto Geofisico, Escuela Politecnica Nacional, Quito, 1701-2759, Ecuador
AU: Staff, I
EM: ig@igepn.edu.ec
AF: Instituto Geofisico, Escuela Politecnica Nacional, Quito, 1701-2759, Ecuador
AB: The eruptive process at Tungurahua volcano (5023m), in central Ecuador, began in August 1999 with SO2-rich steam emissions. In the previous 8 months, the IG's telemetered net had registered some 1300 volcano-tectonic (VT) seismic events, interpreted to be fracturing by magma ascent. VT's practically ceased in early Sept. 1999, followed by a transition to hybrid and long period (LP) seismic events and continual multispectral tremor and emissions. In October, Strombolian fountaining characterized the activity. This onset of magmatic activity prompted Ecuador's president to evacuate inhabitants from mid October to late December. November saw a shift to vulcanian explosions, which showered the cone with ballistics and fine ash over the region, but no large eruptions or pyroclastic flows transpired. During these early months SO2 outputs rose to 8-10 KT/day, while magma discharge remained low—probably less than 5 x 106 m3. Low level eruptive activity (VEI =1) continued from 1999 to mid 2006, punctuated by periods in Aug. 2001, Sept. 2002, Oct. 2003 and in mid 2006 of high energy release, persistent Strombolian fountaining, large explosions, and light regional ash fallout. Quiet periods were also registered, notably from Feb. to Dec. 2005. From Jan. to Mar. 2006 another rash of VT´s was registered followed by a reactivation in May to July, when the number of explosions/day increased (N= 900), SO2 levels rose, deformation accelerated and there was a temporary evolution of LP seismicity with hypocenters ascending from 7 to 3 km beneath the summit. Due to these changes, on May 12, the IG issued a report to authorities stating that activity at Tungurahua could lead to several scenarios—the generation of ash falls and pyroclastic flow were main concerns. Subsequently, the 14 July (VEI= 2) and the 16-17 August (VEI= 4) eruptions were preceded by heightened activity in all monitored parameters roughly 10 days before each eruption, followed by several days of calm, then by 3 to 12 hours, respectively, of dramatic ramping up before pyroclastic flows were produced. The IG gave early warnings to local and regional authorities, who evacuated vulnerable populations and closed highways. On the SW foot of the cone incandescent flows claimed the lives of six people on 17 August, when they did not heed official orders to evacuate. Since 1999, the IG has maintained a staffed observatory near the volcano, operating 24 hr/day, has emitted daily, weekly and special reports, and has been in continual contact with local volcano observers, authorities and the populace. The observatory also monitors rainfall and lahars on the cone and emits early warnings of secondary lahars en route to critical areas. The lessons learned at Tungurahua include: 1. Magma injections identified by VT swarms advance slowly to the surface, often taking months before superficial manifestations are apparent. 2. The quantity of precursory VT and LP events has been decreasing over time and therefore is an uncertain "clue" in determining the occurrence of magmatic intrusions. 3. From 1999-2006, the small magma injections benignly degassed before arriving to the surface, not having sufficient volume or gas content to produce gravitational flows. 4. Only during the two eruptive episodes of mid 2006 were pyroclastic flows generated, probably because of greater magma supply and its rapid ascent. 5. Drawn-out andesitic eruptions, although carefully monitored over long periods, may lead to waning concerns in the population and boredom in observatory staff. 6. Maintaining frequent and direct contact with the local population is important for fostering credibility with them.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8419 Volcano monitoring (7280)
DE: 8428 Explosive volcanism
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly