Jökull - 01.01.2017, Page 19

Zeinab Jeddi et al.
tive rock strength. However, in the simplest Coulomb
failure models, a significantly increased pore pressure
would cause failure on a variety of fault planes, which
considering the similarity of waveforms we observe,
does not seem to be the case.
A localized, cooling magma body may be over-
saturated in gas and may, therefore, release gas which
can lead to similar rock failure as for the hydrother-
mal fluids discussed above. This mechanism was also
suggested for a localized seismic source above the
Upptyppingar intrusion in the Northern Volcanic Zone
(Martens and White, 2013).
In all cases, these speculations lead us to a small,
localized source of magma, heat or gas. But, we
cannot distinguish between these. However, such a
local source inevitably relates to heat, i.e. magma
(directly or indirectly) which highlights the potential
importance of the eastern events for flank volcanism
at Katla and associated volcanic hazard.
Acknowledgements
The authors would like to thank the Icelandic Me-
teorological Office for access to continuous wave-
form data from the permanent seismic network. The
temporary deployments producing data for this study
were supported by CNDS (Centre for Natural Dis-
aster Science) at Uppsala University. We also thank
Francesca Ferrarini and Zeno Lippi for help with data
processing. We are also grateful to the editor, Bryndís
Brandsdóttir, and the reviewers for their constructive
comments. The Generic Mapping Tool (Wessel et al.,
2013) was used to produce some of the graphics.
ÁGRIP
Óvenjuleg þyrping jarðskjálfta nálægt sporði Sand-
fellsjökuls austan i Kötlueldstöðinni hefur verið skoð-
uð í þaula út frá skráningum þétts, tímabundins nets
jarðskjálftamæla. Alls voru um 300 atburðir skráð-
ir frá júlí 2011 til ágúst 2013, flestir í hrinu á milli
4. og 12. desember, 2011. Dreifðara varanlegt net
Veðurstofu Íslands skráði og staðsetti fáeina þessara
skjálfta en einnig öflugri hrinu í nóvember, 2010. Það
er óljóst hvenær jarðskjálftavirkni hófst á þessum stað
vegna breytilegrar næmni varanlega netsins sem teng-
ist tímaþróun fjölda og þéttleika mælistöðva. Skjálft-
arnir eru litlir (-0.5 < ML < 0.5) og veldisvísir stærð-
ardreifingar þeirra (b gildi) er hár (1.6 ± 0.1). Þeir
eru dæmigerðir eldfjallabrotskjálftar (VT=volcano-
tectonic) þar sem tíðniinnihaldið er u.þ.b. 4–25 Hz og
bæði P og S fasar eru skýrir. Skjálftarnir skiptast í
tvo flokka, hvor um sig með snarlíkum bylgjuform-
um, sem líklega svara til mismunandi brotlausna. Þeir
eru staðsettir á um 3.5 km dýpi. Samkvæmt afstæð-
um staðsetningum þyrpast þeir þétt saman í rými sem
er um 400 m í þvermál. Nokkrir þeirra dreifast allt
að 800 m undir miðju þyrpingarinnar. Afstæðu stað-
setningarnar gefa óljóst í skyn línulega dreifingu med
stefnu í NNA/SSV og 60 gráða halla. Rök eru færð
fyrir því að þessir skjálftar geti hvorki tengst jökla-
hreyfingum né dreifðu spennusviði. Líklegri skýring
er að staðbundin uppspretta þrýstings, t.d. lítið rúm-
mál kviku ofarlega í jarðskorpunni sé bein eða óbein
orsök þeirra.
REFERENCES
Abercrombie, R. 1995. Earthquake source scaling relation-
ships from -1 to 5 ML using seismograms recorded at
2.5-km depth. J. Geophys. Res. 100, 24,015–24,036.
Aki, K. 1965. Maximum likelihood estimate of b in the
formula log (N) = a – bM and its confidence limits.
Bull. Earthq. Res. Inst. Tokyo Univ. 43, 237–239.
Árnadóttir, T., B. Lund, W. Jiang, H. Geirsson, H. Björns-
son, P. Einarsson and T. Sigurdsson 2009. Glacial
rebound and plate spreading: Results from the first
countrywide GPS observations in Iceland. Geophys.
J. Int. 177 (2), 691–716. doi:10.1111/j.1365-246X.-
2008.04059.x.
Björnsson, H., F. Pálsson and M. T. Guðmundsson 2000.
Surface and bedrock topography of the Mýrdalsjökull
ice cap, Iceland: The Katla caldera, eruption sites and
routes of jökulhlaups. Jökull 49, 29–46.
Böðvarsson, R., S. Th. Rögnvaldsson, R. Slunga and E.
Kjartansson 1999. The SIL data acquisition system –
at present and beyond year 2000. Phys. Earth Planet.
Int. 113(1-4), 89–101.
Chouet, B. A. 1996. Long-period volcano seismicity: its
source and use in eruption forecasting. Nature 380,
309–316. doi10.1038/380309a0.
Einarsson, P. 1991. Earthquakes and present-day tecton-
ism in Iceland. Tectonophysics 189(1-4), 261–279.
doi:10.1016/0040-1951(91)90501-I.
14 JÖKULL No. 67, 2017