SEISMOACOUSTIC SIGNALS OF EXPLOSION EVENTS AT TUNGURAHUA VOLCANO, ECUADOR

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SEISMO-ACOUSTIC SIGNALS OF EXPLOSION EVENTS AT
TUNGURAHUA VOLCANO, ECUADOR Mario C. Ruiz,
Jonathan Lees, and Jeffrey Johnson With the
collaboration of friends and colleagues of the
Instituto Geofisico EPN Gorki Ruiz, Patricio
Ramon, Diego Barba, Mercedes Taipe, Guillermo
Viracucha, among others. IRIS-UNAVCO, 2005

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WHY TUNGURAHUA Since 1999, Tungurahua volcano
shows a persistent volcanic activity with more
than 6,000 explosions. These explosions are
characterized by discrete explosions with
eruptive columns less than 7 km high, ballistic
ejection of blocks, generation of atmospheric
shock waves, emission of tephra, and deposits
that widely range in the percentage of juvenile
versus non-juvenile components. Tungurahua is a
young andesitic (last eruption57-58 SiO2 )
stratovolcano (5020m) on the Real Cordillera of
Ecuador, with a 500 m width crater on its upper
NW flank. Tungurahua offers the opportunity for
understanding the behavior of volcanic centers
with andesitic composition, that are present
especially along subduction zones.
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1999-2004 TUNGURAHUA ACTIVITY

• gt6,000 explosions since October 5th, 1999.
• November 1999 had largest number of explosions
  (1491).
• Since then, 4 smaller active periods were
  recorded.
• Last period started on May 2004.
• Seismic experiment coincided with climax and tail
  of May-August active period.

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DATA COLLECTION
Three Broad-Band Seismic-Infrasound
Stations distance to vent altitude MAS 3516
m 3310 m JUI 4168 m 2965 m RUN 5889 m
2700 m Operation Period June 29 to August
12 2004
RUN station
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Three main types of degassing events EX, RO, and
CH
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TEMPORAL DISTRIBUTION
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TUNGURAHUA DEGASSING MODES
Initial Time 0 s
Time 15 s
Plume moving to W
Degassing pulse
Time 60 s
Time 45 s
Constant flow degassing
Transition degassing phase
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CHARACTERISTICS OF SEISMIC SIGNALS OF EX
EVENTSNon-stationary (episodic) behaviorWide
range of amplitudes(three orders)Power law of
amplitude distribution and time intervals for the
complete set of explosions
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CHARACTERISTICS OF INFRASONIC SIGNALS
Non-stationary behavior Large amplitude range
reaching high sound level amplitudes Most of
events are related to small gas velocities at the
vent. Few signal get large exit velocities EX
events with clear arrivals times show a large
interval range between events
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Cluster 1 Sharp onsets, Quick compression, Slow
rarefaction
Cluster 2 Sharp onsets, Quick
compression, Quick rarefaction
Cluster 4 Variable onsets, Slow compression,
Slow rarefaction
Cluster 3 Sharp onsets, Quick compression, Slow
rarefaction
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CLUSTER ANALYSIS OF EX EVENTS
Group 4
Group 3
Group 4
Group 2
Group 3
Group 1
Group 1
Group 2
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Distribution of cluster events does not show
Spatial nor Temporal CorrelationClusters do not
depend on explosion location.Clusters are
related to different exit modes of mixtures of
gas and solids
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LOCATING EVENTS USING INVERSION OF ARRIVAL TIMES
OF SEISMIC EVENTS
Six EX events recorded on 6-9 seismic stations,
were located using Lquake software. Most of the
epicenters lay around 400 m south of active vent.
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LOCATING EXPLOSION SOURCES USING PARTICLE MOTION
Polarization angles of horizontal seismic
displacements are computed using SVD on 50 moving
windows of 0.75 s, after seismic onset. Median,
5th and 95th percentiles of back-azimuths are
plotted. Scaled vertical and horizontal
components are plotted. Epicenters are located
south of the vent.
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Constraining Vp with arrivals of secondary
seismic pulses and infrasonic waves
TP2
TP1
Gas expansion at crater floor can generate
simultaneously an infrasonic and a secondary
seismic pulse (Tp11.77 s) traveling across
parallel paths. Red line is expected curve for Vp
2.9 km/s. Dotted lines correspond to Vp
deviations of 1.2 km/s.
Tp1 first compressive seismic arrival from
expansion at the explosion source inside the
conduit Tp2 second compressive arrival observed
1.77 s later at MAS, which likely is related to
gas expansion at the vent.
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Constraining SOURCE DEPTH with arrival time
differences
Mapping residuals for different values of U
(ascending velocity in the conduit) and z (depth
below crater floor)
We found that events with ascending velocities
smaller than 100 m/s would have source depths
shallower than 120 m
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Constraining SOURCE DEPTH with arrival time
differences
1.- Find statistics of delay times between
acoustic and seismic arrivals 2.- Compute source
depths using a constant ascending velocity U 40
m/s, and a constant seismic velocity V2.9 km/s
(Molina, 2001) 3.- Define source area 5-100 m
below crater floor
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CONCLUSIONS

• Extensive degassing at Tungurahua shows a wide
  range of seismo-acoustic signals with three main
  types EXplosions, ROars, and CHugs). More than
  95 of all signals recorded at Tungurahua have an
  infrasonic component and, therefore, are related
  to different modes of degasssing.
• There are at least 4 families of EXplosions
  events based on cluster analysis of infrasonic
  waveforms. These clusters do not exhibit
  temporal, suggesting they are related to
  coexisting explosive processes.
• Epicenters of explosion events are located in two
  areas inside the crater and lt400 m south of
  active crater. Network geometry do not allow
  precise locations of explosion sources.
• Remarkable delay times between seismic and
  infrasonic signals are observed at all stations.
  Wind, temperature or humidity variations are not
  likely the main factor. Analysis of these delay
  times help to constrain the region source of EX
  events at depths ranging from 5 to 100 m inside
  the conduit.