Infrasound and the 20042005 eruption of Mount St' Helens

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Infrasound and the 2004-2005 eruption of Mount
St. Helens

• Seth Moran, U.S. Geological Survey
• Cascades Volcano Observatory, Vancouver WA

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2004-2005 MSH eruption

• Groups involved in day-to-day MSH seismic
  monitoring
• Pacific Northwest Seismograph Network
  (PNSN)
• (Steve Malone, Tony Qamar, and many
  others)
• USGS CVO
• (Marv Couchman, Andy Lockhart, Pat
  McChesney,
• Seth Moran, and others)

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09/23/2004
09/24/2004
SEP webicorder (on 80-86 dome)
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PDT
09/24
09/25
09/26
VT events
09/27
transition
09/28
09/29
LF events
09/30
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Sep Dec 2004 MSH seismicity
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October 1, 2004, explosion

• Seismically quiet (!) explosion until very end

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09/22-10/09 seismicity RSAM perspective
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October 5, 2004, explosion

• Fourth and final explosion of 2004 (aseismic)

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Sep 04 May 05 seismicity
continuous dome-building
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DEMs courtesy Jim Messerich Steve Schilling
(USGS)
2003
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October 4
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October 13
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November 04
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November 20
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November 29
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December 11
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January 3
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February 1
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February 21
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March 10
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April 19
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March 11, 2005
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Reason for lack of explosivity?

• Very little gas (flat magma)

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Real-time infrasound monitoring during the
2004-2005 MSH eruption
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MSH near-cone stations, 10/03/04
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MSH near-cone stations, 10/13/04
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MSH near-cone stations, 11/05/04
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McMikes
McMike (18 element unit) in PVC pipe
soaker hose
Andy Lockhart, USGS
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SEP installation on 1980s lava dome
seismometer
Mike 1
Mike 2
Vent (500m)
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Infrasound recordings during the 2004-2005 MSH
eruption

• Dominated by wind noise and ground vibrations
• from passing seismic waves

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02/18/2005 2052 UTC rockfall event (east side
of dome, ash cloud over rim)
rockfall
Mic1
Mic2
Ground vibrations on mikes (?)
10 sec
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02/22/2005 1103 UTC rockfall event (west side
of dome, v. small ash deposit)
rockfall
Mic1
Mic2
whump
10 sec
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Infrasound recordings during the 2004-2005 MSH
eruption

• Dominated by wind noise and ground vibrations
• from passing seismic waves

but we also caught 4 explosions
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Explosion 1 10/04 1643z

• Small, 40-minute duration
• Cloud-top to 12,000

flank stations
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aseismic explosion
30 minutes
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McMike
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Explosion 2 10/05 1603z

• Largest of the October explosions, 60-minute
  duration
• Cloud-top to 12,000, ash dusted Rainier (70 km
  N)

flank stations
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aseismic explosion
40 minutes
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seismometer
mike
40 minutes
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Explosion 3 01/16 1118z

• Same size as 10/01 explosion, 20-minute
  duration?
• Cloud-top unknown

flank stations
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seismic explosion, barely detectable outside
crater
40 minutes
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seismic explosion, barely detectable outside
crater
seismometer
mike 1
mike 2
40 minutes
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BLIS
SEP mic1
wind whooshes
SEP mic2
SEP seismometer
Dropout
NED
YEL
350 seconds
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Explosion 6 03/09 0125z
1727 PDT (0127 UTC)

• Largest explosion to date, 25-minute duration
• Cloud-tops to 36,000, ash fall on communities
• 100 km east

(Pictures taken by my wife)
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1729 PDT
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1730 PDT
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Sugarbowl camera
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March 8, 2005 ash fall

• 50 x 105 m3
• Virtually no pumice
• (not magmatic)
• Cloud top 36K ASL

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Explosion 4 03/09 0125z explosion
flank stations
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03/09 explosion beginning of ramp-up
20 minutes
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and 2 hours later BOOM!
30 minutes
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and 2 hours later BOOM!
seismometer
mike 1
mike 2
30 minutes
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1726 PST
1727
1728
microphone
microphone
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172740
172750
172720
172730
SEP mic 1
ballistic impact
ash fall starts at SEP
SEP mic 2
172742
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Conclusions

• Small explosions _at_ MSH often generate little to
  no seismicity
• and/or infrasound. (half of all explosions
  have been aseismic)

• Reliable explosion detection (within 5 minutes)
  requires
• several sensors within 1-2 km of vent.
• (just 2 of 6 explosions have shown up on stas
  gt 2 km from vent)

• Explosions _at_ MSH have shown up first on seismic
  sensors.
• Infrasonic sensors are still critical,
  however.

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