Jökull - 01.01.2010, Page 62

Martens et al.
0 1 2 3 4 5 6
19
18
17
16
15
14
13
Distance along azimuth 75 [km]
D
e
p
th
[k
m
]
06Jul07
08Jul07
10Jul07
12Jul07
14Jul07
16Jul07
18Jul07
20Jul07
22Jul07
24Jul07
Figure 12. ASN double-difference relative locations looking from the dip direction of the best-fit plane to the
hypocentres. Earthquakes are coloured by date allowing visualisation of the seismic progression along strike.
– Afstæðar ASN staðsetningar. Horft er þvert á strikstefnu besta flatar í gegnum þyrpinguna. Litaskalinn
endurspeglar þróun virkninnar með tíma.
Geophysical Interpretation
Melt injections at depth are not well documented ob-
servationally. As seismic swarms of the type observed
near Upptyppingar are impossible to predict, obser-
vations and accurate locations of such events are de-
pendent on serendipitous, a priori network placement.
Although the SIL network captured the Upptyppingar
events with an accuracy that allowed significant inter-
pretation, details of the intrusion became lost in loca-
tion ambiguities owing largely to a relatively sparse
network. The dense seismic network deployed by
Cambridge University around the nearby Askja vol-
cano during summer 2007 supplied the additional spa-
tial coverage needed to reveal fine morphological fea-
tures of the intrusion, thereby permitting an improved
investigation into its dynamics.
Brittle failure does not usually occur in the visco-
elastic region of the crust, except where there are high
strain rates (Rubin and Gillard, 1998). Hence, brit-
tle failure in the middle to lower region of the crust
is evidence for the presence of concentrated, high
strain rates that might be melt-induced. Rubin and
Gillard (1998) modelled the elastic stress field created
by propagating fluid-filled cracks and concluded that
most dyke-induced micro-earthquakes are likely to be
the result of slip on existing fractures near the tip cav-
ity of the dyke. Furthermore, they inferred the spatial
distribution of the seismicity to represent stresses that
are near to failure rather than delineating the extent of
the dyke.
For micro-earthquakes of the magnitude observed
at Upptyppingar, rupture diameters are on the order
62 JÖKULL No. 60