Rapid Changes in 1 - 4 Hz Continuous Tremor, Grímsfjall
- Subject: Rapid Changes in 1 - 4 Hz Continuous Tremor, Grímsfjall
- From: "Matthew J. Roberts" <matthew@vedur.is>
- Date: Mon, 08 Nov 2004 12:09:13 +0000
- Organization: Physics Department, Icelandic Meteorological Office
- Reply-to: matthew@vedur.is
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Sæl öll,
In the last two days, we have observed distinct periods of reduced seismic
tremor in the 1 - 4 Hz frequency range at Grímsfjall. (See enclosed GIF
images.) For instance, at ~0220 hours, ~0415 hours, and ~0740 hours on 06
November, tremor intensity decreased abruptly and then rose steadily to
pre-existing levels. The rapidity of the tremor changes are suggestive of
changes in hydraulic resonance rather than meteorological conditions.
Recent photographs on Magnús' Web-site show water draining from Grímsvötn
into Skeiðarárjökull; see:
http://www.raunvis.hi.is/~mtg/04gv/041105-129SG.JPG
http://www.raunvis.hi.is/~mtg/04gv/041105-115SG.JPG
Perhaps the sudden variations in 1 - 4 Hz tremor are due to the development of
water-flow constrictions within the (rapidly healing) ice dam?
Additionally, see http://hraun.vedur.is/ja/oroi/grf.gif
for an overview of recent tremor data from Grímsvötn. Note that the higher
frequency measurements remain elevated above pre-jökulhlaup conditions. More to
follow in a separate message from Kristín on spectrograms derived from recent
Grímsfjall data.
Kveðja,
Matthew.
Eftirlitsmaður 45. viku á Eðlisfr.sviði Veðurstofu Íslands
P.S. The following URL is still being up-dated, so please stay tuned!
http://hraun.vedur.is/ja/eldgos_grimsvotnum.html
