Jökull - 01.06.2000, Page 67

Páll Einarsson and Bryndís Brandsdóttir
an earthquake sequence in one is accompanied by a
few earthquakes in the others. It is noteworthy that
the two historic eruptions of Eyjafjallajökull were al-
most coincident with Katla eruptions. The 1823 erup-
tion closely followed the 1821 Katla eruptions, and
both volcanoes appear to have erupted in 1612 (Thor-
oddsen, 1925; Þorkelsson, 1939). Tephra layers with
chemical characteristics of both volcanoes have been
identified from that year (Larsen, pers. comm.).
As noted above, the vertical error of the locations
is usually quite large. All reliable solutions turned
out to be shallow and no compelling evidence was
found for deep earthquakes. Thus, most of the data
are consistent with shallow seismicity, 0-5 km. This
contrasts with a swarm of "deep" earthquakes in July
1985 that occurred off shore south of Mýrdalsjökull
(Einarsson, 1989; Einarsson and Sæmundsson, 1987).
This swarm occurred near the insular shelf edge at
about 30 km depth and is not directly related to any
of the volcanic centers in the region.
MAGNITUDES OF MÝRDALSJÖKULL
EARTHQUAKES
Magnitudes used in this paper are duration magni-
tudes based on signal duration
at the analog seismic
stations, i.e. from the onset of the first P-wave until the
signal in the coda falls below a prescribed level for the
last time (Einarsson and Björnsson, 1987). The dura-
tion magnitude is derived from formulas of the form:


 
The constants for the different stations are given
by Einarsson and Björnsson (1987). They are found
by correlation between log
and M . Local mag-
nitudes are determined from amplitudes at calibrated
stations, for a set of earthquakes distributed within the
seismically active areas of S-Iceland. The magnitude
of the earthquakes in this study are within the range
2.0-4.0 (Figure 6), and the list appears to be nearly
complete above magnitude 2.5. Earthquakes in the
western cluster have a narrower magnitude range than
earthquakes in the eastern cluster. Events in the west-
ern cluster reach magnitude 3.4 whereas the largest
event in the eastern cluster has a magnitude of 3.9.
0

6

12 18 24 30

Z
N-S

E-W

P

??
??
s
880816 2117 SKO
0

6

12 18 24

30

P
S

s
880823 1136 SKO
Z

N-S

E-W

A

B

Figure 5. Digital, three-component seismograms from
the temporary station SKO (see Figure 1) of typical
events in the eastern cluster (A) and the western
cluster (B). Note the emergent P-wave and irregular
wave train in (B). – Stafræn, þríása skjálftarit úr
færanlegum mæli á Skógaheiði (SKO á 1. mynd).
Skjálftarit (A) sýnir skjálfta úr eystri þyrpingunni,
en skjálftarit (B) skjálfta úr vestari þyrpingunni.
Takið eftir ógreinilegri byrjun P-bylgjunnar og
ruglingslegri S-bylgju í (B).
66 JÖKULL No. 49