Jökull - 01.01.2014, Side 67

Earthquake Sequence 1973–1996 in Bárðarbunga volcano
Table 1. Listings of Bárðarbunga main earthquakes 1973–1996. – Meginjarðskjálftar í Bárðarbungu.
Date time UT latitude longitude ML mb obsa MS obsa MW
ár dags. tími n.l. breidd v.l. lengd
1973 Apr 23 02:57:16 64.6 -17.3 3.9 4.5
1974 Jan 15 19:47:30.8 64.63 -17.34 4.7 4.7
1974 June 25 22:23:41.1 64.71 -17.53 4.8 5.1 27 5.2 2
1974 Dec 29 03:50:00.1 64.71 -17.42 4.8 5.2 31
1975 Oct 03 18:34:03.3 64.439 -17.285 4.8 5.4
1976 July 27 04:00:51.0 64.654 -17.406 4.9 5.2 4.9
1977 July 14 07:15:31.7 64.622 -17.403 4.2 4.7
1977 Dec 28 20:32:38.6 64.671 -17.372 4.6 5.0 5.2 5.4
1979 June 22 23:17:57.8 64.615 -17.396 4.7 5.4 4.8 5.2
1980 Aug 12 12:11:42.9 64.646 -17.352 4.5 5.2 5.3 5.5
1984 Sept 30 23:31:53.5 64.607 -17.403 4.9 5.3 45 4.6 5
1986 Nov 23 02:48:59.7 64.661 -17.333 4.9 5.2 49
1989 Feb 03 15:18:23.1 64.595 -17.406 4.9 5.3 58 4.9 3 5.1
1990 Sept 15 23:07:41.1 64.65 -17.39 4.8 5.5 58 5.2 13 5.6
1992 Apr 25 06:48:43.9 64.725 -17.482 4.4 4.8 58 4.4 6
1992 Sept 26 05:45:50.45 64.646 -17.537 5.5 99 5.4 45 5.7
1993 June 22 12:33:43.67 64.680 -17.180 4.1 5.1 76 4.8 26 5.3
1994 May 05 05:14:48.55 64.634 -17.430 4.2 5.7 91 5.2 45 5.4
1995 Dec 11 05:22:46.19 64.671 -17.504 3.9 4.9 72 4.5 4
1996 Sept 29 10:48:17.09 64.666 -17.444 4.4 5.3 78 5.4 37 5.6
Locations and ML are from IMO (2015), except for the Sept. 26th, 1992 (ISC, 2012) and the Sept. 29th, 1996 event (Einarsson et al., 1997). MS , mb and
anumber of observations from USGS (1999). MW are determined by Harvard University, Cambridge, USA.
ilar during the three decades, they are 4–5% larger in
the 1990s. The moment magnitude (MW ) that corre-
lates best with the energy release of earthquakes has
been determined for 9 of the earthquakes in the Bárð-
arbunga sequence. The three highest moment magni-
tude events all occurred in the 1990s, and the Sept.
1996 event was one of them. There is thus an indica-
tion of intensified seismic activity of the sequence in
the 1990s.
Prior to the Sept. 1996 earthquake, there had been
a highly active seismic period for about one year in
Bárðarbunga and the surrounding area (Figures 2 and
3). An earthquake swarm occurred within the Hamar-
inn volcano in February 1996 (Einarsson et al., 1997).
Seismic tremor accompanied two jökulhlaups from
the Skaftár cauldrons in July 1995 and August 1996.
These tremors had characteristics of eruption tremor
and have been interpreted as such, implying small
sub-glacial eruptions lasting 1/2–2 days on the vol-
canic Loki Ridge (Þorbjarnardóttir et al., 1997).
The combination of these events suggests an
increased pressure over large region of Northwest
Vatnajökull prior to the Gjálp eruption in Sept. – Oc-
tober of 1996. The subsequent eruption in Gjálp may
have caused pressure drop in Bárðarbunga and neigh-
bouring regions, although initially there was increased
seismic activity in the area of Loki Ridge and Ham-
arinn until middle of year 1997 (Figure 4 in Jakobs-
dóttir, 2008). There was a relative quiescence in seis-
micity of earthquakes of magnitude ≥3.0 from June
1997 to 2005 in the Northwest part of Vatnajökull, in-
cluding the most active volcanoes Bárðarbunga, Loki
Ridge and Hamarinn (ISC, 2012; IMO, 2015; Figure
4 in Jakobsdóttir, 2008). The Grímsvötn eruption of
1998 and 2004 may also have influenced the quies-
cence observed.
Earthquakes with hypocenters in the depth range
20–30 km are uncommon in Iceland, and can be de-
scribed as deep earthquakes relative to the common
seismicity. Interestingly in the years 2005 to 2009
large portion of the deep seismicity in Iceland oc-
curred in the Bárðarbunga region (64.5–64.8◦N, 17.0–
17.8◦W; IMO, 2015). The closest stations used for
the location of the deeper seismicity are within 40–
50 km and S phases are generally used in their lo-
cations (Martin Hensch, pers. comm., Feb. 2015).
Gomberg et al. (1990) showed that a good constrain
on focal depth is generally obtained given correctly
timed S phase recorded within ∼1.4 focal depth dis-
tance from the epicenter of an earthquake. The
IMO network is therefore on the border of fulfilling
this requirement for the deeper Bárðarbunga events.
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