Jökull - 01.12.1992, Blaðsíða 3

HISTORICAL ACCOUNTS OF EARTHQUAKES
ASSOCIATED WITH ERUPTIVE ACTIVITY
IN THE ASKJA VOLCANIC SYSTEM
Bryndís Brandsdóttir,
Science Institute, University oflceland
Dunhaga 5,107 Reykjavík
ABSTRACT
Scientific knowledge gained during the Krafla in-
trusive and eruptive activity in recent years facilitates
reinterpretation of earlier rifting events, such as the
the Askja rifting episode of 1874-1876. Most of the
intrusive activity during the Askja rifting episode took
place during the firstfew months. The biggest earth-
quakes attained intensity V or even VI on the Mercalli
scale at a distance around 35 km, and may have ex-
ceeded magnitude 5 on the Richter scale. Earthquake
swarms recorded on January 30 and June 12, 1962
which originated in the Askja region suggest that tM’o
brief intrusive events followed the 1961 eruption in
Askja. The total lava production in the Northern Vol-
canic Zone during the last three centuries was 0.25—
0.3 knv’ in each rifting event excluding the Askja 1961
eruption which had an extrusion volume of 0.1 krrv'.
The higher intrusion/extrusion ratio oftheAskja 1874-
1876 rifting episode compared to the recent Krafla
rifting episode is the result of temporal variations in
spreading capacity and magma availability along the
accretionary plate boundary.
INTRODU CTION
Volcanic activity in the Northem Volcanic Zone
during the last three centuries has been confined to
the Krafla and Askja volcanic systems. Three rift-
ing events took place in the Askja system and two in
the Krafla volcanic system. The recent rifting episode
in the Krafla volcanic system (from 1975) offered a
unique opportunity to study rifting and magmatism as-
sociated with plate divergence. Field observations dur-
ing the Krafla events along with various seismological,
geodetic, gravimetric, geochemical and petrological
studies created a data base which prompts reinterpre-
tation of earlier rifting events. This paper focuses on
earthquake and eruption reports from the Askja vol-
canic system in an effort to enhance our understanding
of older rifting events. The reinterpretation of these
events is based on current knowledge of the mechanics
of rifting.
A major rifting episode took place in the Askja
fissure swarm during 1874-1876, after more than 400
years of quiescence (Annertz et al., 1985). This rifting
episode activated most of the volcanic system, with fis-
sure eruptions up to 70 km north of the Askja central
volcano fed from a reservoir beneath the Askja caldera.
The subsequent pressure drop in the magma reservoir
is the most plausible explanation for the collapse and
formation of Öskjuvatn withinthe Askja caldera (Sig-
valdason, 1979; 1982; Sigurðsson and Sparks, 1978).
Further activity was observed during the years
1921 -1933 (Jónsson, 1945) when a minor rifting event
occurred that involved a fissure eruption south of the
central volcano and several eruptions along the rim
of Öskjuvatn (Sigvaldason et al., 1992). In 1926 a
small island was formed in the Öskjuvatn lake (Einars-
son, 1962). The present surface geothermal activity in
Askja is confined to these eruption sites on the north-
east, east and south rim of the Öskjuvatn caldera.
The latest eruption in Askja began on October 26,
JÖKULL, No. 42, 1992 1