Jökull - 01.01.2014, Side 66

Ingi Þ. Bjarnason
2
3
4
5
6
2
3
4
5
6
1973 1974 1975 1976 1977 1978 1979
magnitude
4.5
4.7 5.1 5.2 5.4 5.2 4.7 5.0
2
3
4
5
6
2
3
4
5
6
1979 1980 1981 1982 1983 1984 1985
magnitude
5.4 5.2 5.3
2
3
4
5
6
2
3
4
5
6
1985 1986 1987 1988 1989 1990 1991
magnitude
5.2 5.3 5.5
2
3
4
5
6
2
3
4
5
6
1991 1992 1993 1994 1995 1996 1997
magnitude
4.8
5.5
5.1 5.7 4.9
5.3
Figure 3. Magnitude time graph of earthquakes (ML ≥ 3.0) in the Bárðarbunga region (∼64.5–64.8◦ N, 17.0–17.8◦ W)
during 1973–1996. Magnitudes are mostly local (ML), but listed (PDE-USGS) mb magnitudes of main events are also
labeled (see Table 1). Note, that a period of foreshock activity is not uncommon, but aftershocks are rare, suggesting an
efficient stress release of the main events, possibly because of low stress environment. Earthquake data are from Björns-
son and Einarsson (1990), USGS (1999) and ISC (2012). – Tímaruna jarðskjálfta (ML ≥ 3.0) á Bárðarbungusvæðinu
(∼64.5–64.8◦ N, 17.0–17.8◦ V) frá árunum 1973–1996. Lengd lína gefur til kynna staðbundnar stærðir jarðskjálftanna,
en mb stærðir meginskjálfta eru auk þess sýndar með tölustöfum (sjá Töflu 1). Ekki er óalgengt, að forskjálftar fylgi meg-
inskjálftum, en eftirskjálftar eru óalgengir (undantekning er 1996 skjálftinn). Skortur á eftirskjálftum bendir til skilvirkrar
spennulosunar meginskjálftans, enda spenna tiltölulega lág við upphaf hans. Jarðskjálftagögn frá H. Björnssyni og P.
Einarssyni (1990), USGS (1999) og ISC (2012).
The Bárðarbunga 1973–1996 earthquake sequence
and other seismic activity in the Northwest Vatna-
jökull region
The sequence of earthquakes that started in Bárðar-
bunga in 1973 can be described as a series of main
events with magnitudes in the range 4.5–5.7 mb and
associated seismicity, occurring ∼yearly (Figure 3
and Table 1). The majority (2/3) of the main events
are in the magnitude range of 5.0–5.7 mb, but some
as small as 4.5 mb are defined as main events on
the basis of the associated seismicity. By defini-
tion, a main event (mainshock) is the largest earth-
quake in a sequence of earthquakes close in space and
time, the so-called pattern of fore-, main-, and after-
shocks. Foreshocks have been recorded with many of
the Bárðarbunga main events, but the sequence is un-
usual in that it lacks significant aftershocks (Einars-
son et al., 1997). An exception to this pattern was
the 1996 main event (MW =5.6). There active fore-
and aftershock sequences happened. The interval be-
tween main events has varied. The seismic activity
was significantly less in the 1980s (interval between
main events ∼2.5 years) compared with the 1970s and
1990s (interval ∼1.0 year). Body-wave magnitudes
(mb) have been determined for all 20 main events of
the sequence (Table 1). A comparison of the mb mag-
nitudes shows that although they are on average sim-
66 JÖKULL No. 64, 2014