Jökull - 01.01.2005, Page 97

Seismic characteristics of the Hekla volcano
20˚ 00'W 19˚ 50'W 19˚ 40'W 19˚ 30'W
63˚ 56'N
64˚ 00'N
64˚ 04'N
0 5 10
km
2
1
0
ML
Figure 7. Earthquakes during the onset phase of the Hekla 2000 eruption, open circles show the earthquakes
before the start of the eruption at 18:19 GMT and grey dots the events after. The black dot is a magnitude-2.1
earthquake at 3 km depth at 18:17. The magnitude scale is given in the inset. The fissure of the 2000 eruption
is shown with a thick line. Black bars at about 20◦W are the easternmost faults of the South Iceland seismic
zone. – Upptök jarðskjálfta undir Heklu sem urðu í tengslum við eldsuppkomuna 2000. Hringir tákna skjálfta
sem urðu áður en gosið kom upp klukkan 18:19, gráir deplar skjálfta eftir að gosið kom upp. Svarti depillinn
sýnir upptök stærsta skjálftans (ML = 2.1), en hann varð á 3 km dýpi klukkan 18:17, rétt áður en gosið sást.
Stærð táknanna sýnir stærð skjálftanna. Gossprungan sem var virk 2000 er sýnd með feitri línu. Einnig eru
sýndar sprungur sem voru virkar í jarðskjálftanum 1912, austast á skjálftabelti Suðurlands.
2000, showing a slight decline with time. In Fig-
ure 6 small events (ML < 1) are not seen after the
start of the eruptions, due to the masking effect of the
high-amplitude volcanic tremor. After some hours the
earthquakes become sporadic and soon stop occurring
altogether. Neither in 1991 nor in 2000 were earth-
quakes observed on the second day of the eruption,
and few events were observed during the later phases
in general.
The location accuracy of events on January 17,
1991 was not good because of insufficient station cov-
erage. The events of February 26, 2000were far better
observed and the depths of many earthquakes could
be constrained rather well (Soosalu et al. 2005). All
the first events, from 17:00 GMT on, were very small
and those that could be located were shallow, at 0–
4 km depth. After 17:36 the main activity jumped to
4–9 km depth. After the start of the eruption at 18:19
earthquakes were observed at all depths from the sur-
face down to 14 km, but mainly at 2–12 km depth.
All the Hekla earthquakes forming the initial
swarms, as far as was possible to discern, can be
classified as normal, high-frequency earthquakes with
clear S-phases (see Figure 4b) and are indistinguish-
able from earthquakes caused by brittle failure. Ac-
tually, the higher-frequency content was used for vi-
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