Jökull - 01.01.2019, Side 59

Sgattoni et al.
for the interpretation of the 2011 unrest episode and
new seismicity. The seismic data of this catalogue
were recorded by the Icelandic national seismic net-
work (SIL) and the earthquake location obtained us-
ing a 1D SIL-velocity model (Stefánsson et al., 1993).
The permanent monitoring network around Mýrdals-
jökull has been densified through time (Figure 3). The
depth of the earthquakes is not well resolved, how-
ever, most events appear to be located at shallow depth
between 0 and 5 km (Vogfjörð and Slunga, 2008).
The analysis of the catalogue shows significant
changes through time in the seasonal patterns, size
and location distribution of the persistent seismic-
ity recorded at Katla since the 1970s (e.g. Einars-
son and Brandsdóttir, 2000). Because the network
configuration has improved through time, the magni-
tude of completeness of the catalogue has also im-
proved (lowered). Here we describe the seismicity
above M=1 (for the two main clusters, Caldera and
Goðabunga), which is presumably below the magni-
tude of completeness for the whole time span we con-
sider. However, the same features, albeit less pro-
nounced, would also be seen by increasing this thres-
hold to M=1.7, which is indicated as the magnitude of
completeness of the Katla seismicity between 1993–
2006 by Jónsdóttir et al. (2007). We also point out
that the changes we observe in the seismicity patterns
do not coincide in time with changes in the network.
The following are the main features of the seismicity
at Katla during 1998–2015, (Figures 4 and 5):
– The Goðabunga cluster was more active than the
caldera until the first half of 2011, with around 11600
events with magnitude ML>1 compared to 3300 in-
side the caldera. The most intense earthquake activity
occurred during the 2000–2004 seismic crisis, after
the 1999 unrest episode, when inflation was observed
at Katla (Sturkell et al., 2008).
– Increased seismicity at Goðabunga followed the
2010 Eyjafjallajökull eruption.
– Since July 2011, more earthquakes have occurred in
the caldera than at Goðabunga, with 1610 events with
ML>1 compared to 760 at Goðabunga.
– The magnitude of the largest events at Goðabunga
has been decreasing since 2009, with most events of
ML<2 as of 2010.
– The autumn seismicity peak of the Goðabunga clus-
Figure 3. Earthquake locations (in blue) at Katla during 1998–2015 from the IMO catalogue. All events with
magnitude ML>1 were selected for the caldera and Goðabunga seismic clusters, and events with MLML>ML0
for the southern flank seismicity. Black triangles: stations operating during the whole time period of each
panel. Grey triangles: stations that were deployed later. Station "snb" in (c) was substituted by "lod" in 2013. –
Skjálftastaðsetningar (bláir punktar) fyrir þrjú tímabil 1998–2015, gögn frá Veðurstofu Íslands. Fyrir skjálfta
innan öskjunnar og vestan Goðabungu eru eingöngu sýndir skjálftar með ML > 1. Fyrir þyrpinguna við Gvend-
arfell eru sýndir skjálftar með ML > 0. Svartir þríhyrningar: Skjálftamælistöðvar sem gengu allt tímabilið sem
sýnt er á kortinu. Gráir þríhyrningar: Stöðvar sem settar voru upp síðar. Stöðinni "snb" var skipt út fyrir "lod"
árið 2013.
58 JÖKULL No. 69, 2019