Jökull - 01.01.2010, Blaðsíða 64

Martens et al.
Figure 13. Seismic velocity VP /VS ratios as a function of depth. Blue colour represents the shallow population, which
includes hypocentres that locate between 13.00 and 15.25 km below sea level. The deep population includes hypocentres
that locate between 15.25 and 18.40 km below sea level and is shown in red colour. The histogram depicts a statistically
significant bi-modal distribution of VP /VS ratios, representing the variation with depth.– Hlutfall bylgjuhraða VP /VS sem
fall af dýpi. Bláir punktar sýna grynnri þyrpingu með upptök á 13,00 til 15,25 km dýpi undir sjávarmáli. Upptakasvæði
dýpri þyrpingarinnar er á 15,25 til 18,40 km dýpi og eru þeir skjálftar sýndir með rauðu. Súluritið sýnir marktækan mun á
dreifingu VP /VS hlutfalls fyrir þessar tvær þyrpingar.
in this manuscript include both automatic and man-
ual quality checks that ensure precision phase-arrival
selection. Relative relocation techniques also provide
significant improvement in defining the spatial struc-
ture of the seismic swarms.
With respect to the Upptyppingar sequence, SIL
locations define the general morphology of the dyke;
however, high-precision locations reveal finer details
of the kinematics of its propagation and evolution.
The combination of a favourable network configura-
tion and precision phase picking has revealed that any
deformation off the plane of the dyke is not substan-
tially greater than the uncertainty in the relative lo-
cations of the hypocentres. We interpret the incred-
ible linear coherence of seismicity along the entire
length of the dyke as evidence for minimal brittle de-
formation adjacent to the active dyke. This implies
that melt-induced strain, at least for the 288 events
examined here, is primarily compensated for visco-
elastically, with only minor brittle fracturing occur-
ring near the leading edge of the dyke or within the
dyke itself.
We also note a relationship of VP /VS ratio with
depth that is consistent with the presence of melt. This
corroborates the interpretation that the spatially coher-
ent Upptyppingar seismicity indeed delineates a melt
intrusion at depth.
Acknowledgments
We thank G. B. Guðmundsson, B. S. Þorbjarnardóttir,
and the Icelandic Meteorological Office (IMO) for
providing data from six SIL stations, five of which are
funded by Landsvirkjun-Power, for use in our analysis
as well as the IMO-derived locations for all 2007 Upp-
typpingar events. We also appreciate the help of K.
Vogfjörð and R. Slunga in clarifying the IMO single-
and multi-event location techniques. Seismometers
64 JÖKULL No. 60