Pyroclastic density currents (PDC) of the 16-17 August 2006 eruptions of Tungurahua volcano, Ecuador: Geophysical registry and characteristics

摘要: Abstract Tungurahua, located in the Eastern Cordillera of Ecuadorian Andes, is a 5023 m-high active volcano, notable for its extreme relief (3200 m), steep sides, and frequent eruptive cycles. From 1999 until 2006 Tungurahua experienced short periods low to moderate strombolian activity, characterized by fire fountaining, explosions, ash falls debris flows, no PDC events. Without warning, initiated activity on 15–16 July 2006, which became more intense night 16–17 August focus this study. Continuous monitoring has employed seismic (both period broadband (BB) instruments), SO 2 gas emission (COSPEC DOAS), geodetic methods (EDM, tilt meters, GPS), addition thermal imagery (airborne ground-based). Acoustic flow monitors (AFM) installed monitor lahar were important detecting signals monitored at Riobamba, 40 km SW, as well infrasound sensors Tungurahua's BB stations. Based geophysical parameters, visual observations, deposit characteristics, four phases distinct are recognized during episode. Phase I (08H37 21H13 16 Aug.) (local time) with occasional high energy impulsive bursts small PDC. II (21H13-16 Aug. 00H12-17 was number discrete events amplitude seismo-acoustic signals, followed generation larger that overran stations had velocities 30–33 m/s. After midnight, III (00H12 01H14) saw an unrelenting corresponding episode's greatest release. It subplinian accompanied series outbursts constant frequency jetting together formed continuous plume. phase largest produced, reaching surrounding river valleys. IV (after cessation paroxysmal eruption, but witnessed many granular generated degassed lava spill outs from crater developed lobe channel morphology cone's lower flanks. Hours later blocky issued crater. During these episodes, than 30 detected, majority being flows remained cone. The two occurred after probably fountain collapse. Their descent down upper flanks (~ 28°) 2.4 km length favored air entrainment, resulting greater fluidity. These volumes 9 17 × 10 6  m 3 produced widespread, relatively thin (1–2 m thick) normally-graded deposits their distal ends. character evolution apparently reflect decreasing volatile contents magma diminishing supply.