Results from the 2005 seismic experiment at the Torfajkull volcano, south Iceland a pretaster

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Results from the 2005 seismic experiment at the
Torfajökull volcano, south Iceland a pretaster

• Heidi Soosalu
• Bullard Laboratories, University of Cambridge

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Torfajökull, south Iceland

• In the eastern volcanic zone
• Largest rhyolitic centre in Iceland
• Caldera, 12 km in diameter
• High-temperature geothermal activity

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Torfajökull seismicity

• Persistent small-scale seismicity
• Ordinary high-frequency earthquakes in W part of
  the caldera (dots)
• Low-frequency earthquakes in S part of the
  caldera (stars)

May - October 2002
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High-frequency earthquakes

• Frequent, small in magnitude (typically lt 3)
• Cluster in W part of the caldera, under recent
  eruptive sites

• 1991-1995 data around a spherical aseismic
  volume with a
• centre at 8 km and 4 km diameter ? interpreted
  as
• expressions of a cooling magma chamber

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Low-frequency earthquakes

• Frequent, small in magnitude (lt 2)
• Can occur in swarms
• Cluster in south part of the caldera
• Narrow frequency band in 1-3 Hz
• P small, P and S emergent difficult to locate

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Low-frequency earthquakes in S Torfajökull
z 9 km, ML 0.8

• Filter 0.8-8 Hz

z 7 km, ML 0.7
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Spectra of a Torfajökull low-frequency event, ML
0.7
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The 2005 seismic experiment

• July September 2005
• 30 Güralp 6TD instruments
• Tighter net
• Several stations inside the caldera

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First preliminary locations
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Steam temperature defined with CO2
(Bjarnason and Ólafsson 2000)
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Station HAUH, 2 km distance
Bandpass 0.8 20 Hz
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Station HEIT, 4 km (same event)
Bandpass 0.8 20 Hz
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Station HAUH, 2-3 km distance
Bandpass 0.8 20 Hz
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Station HAUH, 2 km distance
Bandpass 0.8 20 Hz
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Station HAUH, 1 km distance
Bandpass 0.8 20 Hz
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Observations at close distance

• Before 2005 all observations show low-frequency
  appearance
• Closest stations HAUH and HEIT (within 4 km
  distance) have indications of higher frequencies
  and clearer phases
• Low-frequency appearance at least partly a path
  effect?
• Preliminary depth estimates in the uppermost 7
  km
• HEIT daily activity of small events not visible
  elsewhere

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Low-frequency events at Torfajökull

• This sort of continuous activity observed only at
  Torfajökull in Iceland
• Low-frequency events cluster in a small area in
  the southern part of the caldera, not observed
  elsewhere at Torfajökull
• Coincides with the area of highest temperature
  geothermal activity ( gt340 C)

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A working hypothesis

• Low-frequency seismicity expression of a
  cryptodome
• Supporting facts
• Torfajökull is an acidic volcanic centre, which
  are characterized by dome-building activity
• Domes are localised features, matches with a
  localised area of low-frequency seismicity
• Geochemical observations point to a source of
  extra heat at the same spot

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A toast to the field team Halldór Ólafsson,
Haukur Brynjólfsson, Fiona Campbell, Jósef
Hólmjárn, Anna Horleston, Auli Mikkola, Robert
White, Sarah White Icelandic Coast Guard