Characterization of explosion signals from Tungurahua Volcano, Ecuador

David Fee and Milton GarcesInfrasound LaboratoryUniv. of Hawaii, Manoadfee@isla.hawaii.edu

Robin MatozaLaboratory 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 CourtesyInstituto Geofisico

ASHE Arrays - RIOE

• 4 Element Array, ~100 m aperture

• Chaparral 2 Microphones

• Flat between 0.1-200 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 0.5-4 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: 0 .018-24.4 Pa Mean = 0.64 Pa

• Durations: 0.1-16.5 sMean = 3.95 s

4/1/06 7/1/06 10/1/06 1/1/07 4/1/07 7/1/06 10/1/06

4/1/06 7/1/06 10/1/06 1/1/07 4/1/07 7/1/06 10/1/06

Acoustic Source Energy

• EAcoustic=2πr2/ρc ∫ΔP(t)2dt

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: 1.30x107 J

• Assume spherical spreading• More energy more eruptive material?

Energy Release

• Energy ratios: 7.5x10-5-502Mean = 0.81

Largest explosions follow 7/14/06 VEI 3 Eruption

• Group eruptive activity:– Background tremor– May 06– July 14-15th, 2006– August 16-17th, 2006– March 07

4/1/06 7/1/06 10/1/06 1/1/07 4/1/07 7/1/06 10/1/06

4/1/06 7/1/06 10/1/06 1/1/07 4/1/07 7/1/06 10/1/06

Effective Yield• Convert Explosion Energies to Effective Yield• 1 ton of TNT = 4.184 GJ• Largest explosion=1.56 ton, most around .001 ton (~1 kg of TNT)• Volcanic explosion in fluid, relationship may not hold

4/1/06 7/1/06 10/1/06 1/1/07 4/1/07 7/1/06 10/1/06

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 24th 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: 3/8/07 0745 UTC~10 Pa at 36.89 km368,900 Pa at 1 m 205 dB (re 20 μPa)!Effective Yield: 0.115 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 4th, 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: 2007-04-04 04:51:03, 2.91 Pa, 6.4 sec, 1.571 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 =Tacoustic-Tseismic)

• 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

0.1

0

-0.1

Cross-Correlation

• Pick “master” waveform for subset of events• Cross-correlation for each event• Look at evolution of correlation value?• Parameters: 0.1-5 Hz, Window: -3 s, +8 s from onset, amplitude >.5 Pa

Test Master Waveform

Cross-Correlation Results• Subset data between 5/11-5/16 2006• 385 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 0.02-.1 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 Family 5/14 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