characterization of explosion signals from tungurahua volcano, ecuador
DESCRIPTION
Characterization of explosion signals from Tungurahua Volcano, Ecuador. David Fee and Milton Garces Infrasound Laboratory Univ. of Hawaii, Manoa [email protected] Robin Matoza Laboratory for Atmospheric Acoustics (L2A) Scripps Institution of Oceanography. Overview. Tungurahua Volcano - PowerPoint PPT PresentationTRANSCRIPT
Characterization of explosion signals from Tungurahua Volcano, Ecuador
David Fee and Milton GarcesInfrasound LaboratoryUniv. of Hawaii, [email protected]
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