Jökull - 01.01.2005, Page 92

Heidi Soosalu and Páll Einarsson
25
20
15
10
5
0
20˚12’W 20˚00’W 19˚48’W 19˚36’W 19˚24’W 19˚12’W 19˚00’W
de
pt
h
(k
m
)
Hekla and Vatnafjöll Torfajökull
de
pt
h
(k
m
)
0 10 km
H S Lw Le
Figure 3. Vertical cross section of the study area from South Iceland seismic zone to east edge of the Torfajök-
ull caldera (between latitudes 63◦48.6’–64◦05’N), seen from south and without vertical exaggeration. Plotted
are all the well-located earthquakes at these latitudes. Black symbols are earthquakes that occurred before
the 1991 Hekla eruption, open symbols events that occurred between the 1991 and 2000 eruptions, and grey
symbols after the 2000 eruption. Dots denote ordinary high-frequency earthquakes and stars low-frequency
earthquakes. Inverted triangles show the surface locations of the South Iceland seismic zone faults: H-Hellar
fault, S-Skarðsfjall fault, Lw-western Leirubakki fault, Le-eastern Leirubakki fault. The locations of the central
volcanoes are shown with grey bars, the location of the Torfajökull caldera with darker grey shade. – Lóðrétt,
A-V snið í gegnum skjálftabeltið á Suðurlandi, Heklu og Torfajökulseldstöðina. Horft er úr suðri. Lóðréttur og
láréttur kvarði er hinn sami. Teiknuð eru öll skjálftaupptök milli 63◦48.6’og 64◦05’N, sama gagnasafn og sömu
tákn og á 2. mynd. Þríhyrningar á yfirborðinu sýna staðsetningar misgengja við Hella (H), Skarðsfjall (S) og
Leirubakka (Lw og Le). Gráar rendur við yfirborðið sýna staðsetningu megineldstöðvanna, Heklu, Vatnafjalla
og Torfajökuls. Torfajökulsaskjan er sýnd með dekkri rönd.
tudes of 0.6–1.7 during about ten hours, and the ana-
logue station HE at Hekla recorded over seventy ad-
ditional small events. The analogue stations HE and
LJ also recorded three volcanic-looking events within
the swarm, similar in appearance to hybrid events
(Chouet 1996), with a high-frequency onset followed
by a low-frequency coda. No volcanic tremor was
detected during this earthquake swarm. After June
1991, Hekla was seismically quiet for more than a
year. Later, sporadic small earthquakes started to oc-
cur, and their local magnitudeswere typically under 1.
A distinct feature of Hekla seismicity during non-
eruptive times is the low-frequency character of the
events (Figure 4a). The earthquakes occurring at
Hekla proper have a peculiar appearance: they con-
sist of only low frequencies (main frequency content
being below 5 Hz) but have clear S-waves, similar
to tectonic earthquakes. In contrast to this, the earth-
quakes related to the eruptions are ordinary-looking
high-frequency events (Figure 4b). High-frequency
earthquakes may continue to occur for a few months
after eruptive activity, but the few inter-eruptionHekla
earthquakes have a low-frequency appearance. This
phenomenon has been observed after both the 1991
92 JÖKULL No. 55