Characterization of explosion signals from Tungurahua Volcano, Ecuador David Fee and Milton Garces Infrasound Laboratory Univ. of Hawaii, Manoa Robin Matoza Laboratory for Atmospheric Acoustics (L2A) Scripps Institution of Oceanography

Overview Tungurahua Volcano Array(s) Explosion Algorithm and Events Examples –March 2007 Sequence –May 2006 Explosions Source Cross-Correlation Conclusions

Tungurahua Volcano 5023 m high, 3200 m of relief Frequent eruptions characterized by pyroclastic flows, lavas, lahars, as well as tephra falls Over 30,000 people live in close proximity, evacuated in 1999 Significant ash ejections resulting from nearly constant tremor and explosions Motivation: –Understand dynamics and evolution of explosions –Aid general understanding and monitoring Images Courtesy Instituto Geofisico

ASHE Arrays - RIOE 4 Element Array, ~100 m aperture Chaparral 2 Microphones Flat between Hz Collocated BB seismometer Porous hoses in open field Recorded signals from Tungurahua and Sangay Volcanoes 37 km 33° 43 km 132°

Explosion Detection Algorithm Time period: 2/15/06-11/1/2007 High-pass filter data >.5 Hz STA/LTA  event onset and end time –2/5 secs, 3/40 secs –Detection must be on all 4 channels Run PMCC between Hz –10 bands, 10 sec windows –Families with correct azimuth (±7°) during event time –Minimum RMS amplitude >0.02 Pa RMS –Minimum family size >15 pixels

Amplitude and Number Events 9331 Events detected >400 per day during peak Events clumped during periods of high activity Amplitudes: Pa Mean = 0.64 Pa Durations: s Mean = 3.95 s 4/1/067/1/0610/1/061/1/074/1/077/1/0610/1/06 4/1/067/1/0610/1/061/1/074/1/077/1/0610/1/06

Acoustic Source Energy E Acoustic =2πr 2 /ρc ∫ΔP(t) 2 dt r=source-receiver distance ρ=air density C=sound speed ΔP=change in pressure Energy normalized by reference event –Removes geometric spreading and topographical effects –Reference Event: 1.30x10 7 J Assume spherical spreading More energy  more eruptive material?

Energy Release Energy ratios: 7.5x Mean = 0.81 Largest explosions follow 7/14/06 VEI 3 Eruption Group eruptive activity: –Background tremor –May 06 –July 14-15th, 2006 –August th, 2006 –March 07 4/1/067/1/0610/1/061/1/074/1/077/1/0610/1/06 4/1/067/1/0610/1/061/1/074/1/077/1/0610/1/06

Effective Yield Convert Explosion Energies to Effective Yield 1 ton of TNT = GJ Largest explosion=1.56 ton, most around.001 ton (~1 kg of TNT) Volcanic explosion in fluid, relationship may not hold 4/1/067/1/0610/1/061/1/074/1/077/1/0610/1/06

March 2007 Sequence - Example Moderate-High Activity resumed between 2/15-4/15 Significant number of explosions and associated ash Seismic Tremor and LPs returned 2/23/07 Significant number of explosions starting 2/24

February 24 th Event 2/24/07 Impulsive Onset Signal lasts ~5 mins Sustained amplitude ~± 1 Pa Ash >40’000 ft Jetting? Similar spectrum

Acoustic Source Energy Example Explosion: 3/8/ UTC ~10 Pa at km 368,900 Pa at 1 m  205 dB (re 20 μPa)! Effective Yield: ton (105 kg) TNT

March 2007 Explosion Energy Most energetic explosions during middle of sequence Cloudy weather hampered visual monitoring for much of sequence Energy and number of explosions correlate well with heightened volcanic activity

Observation vs. Recording: April 4 th, 2007 Good recording and viewing conditions. Selected day for eyewitness, satellite, infrasound correlations Observation: 0450 UTC Explosion. Vibration of windows in Banos (7 km) and heard at observatory (13 km). Clear weather and constant emission reaching 8.5 km asl (~28,000 ft) Infrasound: :51:03, 2.91 Pa, 6.4 sec, energy ratio

Explosions  Infrasonic Harmonic Tremor Mid-May 06: Explosions trigger gliding harmonics lasting up to 30 mins Very little ash during these explosions/tremor New Injection of Magma?

Explosions  Seismic Harmonic Tremor Band-limited sustained seismic tremor Similar frequencies, but harmonics not very apparent (low SNR as well)

Explosion Source May 06 Explosions similar to acoustic recordings from Arenal Volcano, Costa Rica (Garces et al., 98) Explosion in low sound speed, low density magma-gas mixture would couple better into the atmosphere  acoustic impedance match Then decompression front propagates into conduit and create resonance Substantial pressure perturbation could destabilize the melt and initiate flow Explosion near surface of a gas-rich conduit creates a resonance that transmits into the atmosphere and couples into earth through the conduit walls Ruiz et al. 2005: analyzed travel times of seismic and acoustic first arrivals (ΔT =T acoustic -T seismic ) Large variations in ΔT  source location variability? Concluded explosions events originate <200m, followed by outflux of gas, ash, and solid material ~1 s later

Infrared Video Somewhat emergent onset, relatively low amplitude Long duration Liquid magma ejected ~300 m 2006/7/31 Explosion

Cross-Correlation Pick “master” waveform for subset of events Cross-correlation for each event Look at evolution of correlation value? Parameters: Hz, Window: -3 s, +8 s from onset, amplitude >.5 Pa Test Master Waveform

Cross-Correlation Results Subset data between 5/11-5/ Explosions Waveforms very similar on 5/14 Master

Cross-Correlation – Families 37 km away so atmosphere effects may affect waveform similarity Possible solution: compare waveforms with similar atmospheric conditions Use Hz as a proxy for wind speed (Fee and Garces, 2007) Sort explosions by waveform similarities (Green and Neuberg, 2006; Umakoshi et al., 2003) 5/11 Waveform Family5/14 Waveform Family

Conclusions Significant number of high s/n explosions recorded from Tungurahua Volcano Similarities and differences exist: amplitude, duration, energy, correlation, ash content, harmonics Other data sources necessary to understand effect of explosions –Some ash rich, some ash poor Understanding explosions key to hazard monitoring and dynamics at Tungurahua Future Work –Correlate explosions with observed activity: ash, pyroclastic flows, incandescent blocks –Waveform cross-correlation by families: group by correlation and similar atmospheric conditions –Model acoustic/seismic explosion source and determine how it relates to tremor