Jökull - 01.01.2010, Blaðsíða 130

P. Einarsson
CONCLUSIONS
An overview is given of a project to map in de-
tail structures associated with Holocene faults in the
South Iceland Seismic Zone. Mapping has been ac-
complished with GPS-instruments with a resolution
of 1 m.
Holocene faults in South Iceland follow an E-W
zone that coincides with the SISZ as defined by epi-
centers of earthquakes. The zone is about 15 km wide.
The majority of fractures strike NNE-ENE and
form left-stepping arrays with northerly trend. This
implies right-lateral strike-slip faulting on northerly
striking faults. Push-up structures confirm strike-slip
faulting and the sense of slip.
Conjugate sets of ENE-striking, left-lateral fault
segments also exist, but they are less common by an
order of magnitude. Most of them are in a subordinate
role, bridging gaps between right-lateral segments.
Large earthquakes in the South Iceland Seismic
Zone thus seem to occur mostly by right-lateral fault-
ing on faults that are transverse to the zone. The earth-
quakes of 2000 and 2008 occurred by right-lateral
faulting on N-S faults and confirmed the above find-
ings.
The source faults of most of the largest historical
earthquakes since 1630 have been tentatively identi-
fied.
Kinematically, the model of "bookshelf faulting"
can explain how the over-all left-lateral movement
across this branch of the plate boundary is accommo-
dated by slip on numerous parallel transverse faults
and rotation of the blocks between them. This model
implies an unstable plate boundary configuration. The
rotation of the blocks will eventually lock the faults
and new faults are required to take up the motion.
The fissure swarm of the Grímsnes Volcanic Sys-
tem partly overlaps with the SISZ, but its fissure
swarm has characteristics different from those of the
SISZ faults. En echelon fracture arrays are not promi-
nent and push-up structures are absent.
Acknowledgments
Numerous persons have contributed to the map-
ping efforts reported here, to name a few: Jón
Eiríksson, Kristinn Albertsson, Páll Imsland, Amy
Clifton, Maryam Khodayar, Steingrímur Þorbjarnar-
son, Mathilde Böttger Sörensen, Ásta Rut Hjartar-
dóttir, Benedikt Ófeigsson, Bergur Einarsson, Kristín
Jónsdóttir, Vala Hjörleifsdóttir, Sigurjón Jónsson,
Daði Þorbjörnsson, Páll Bjarnason. Students of the
University of Iceland in courses on tectonics dur-
ing the period 1977–2003 also participated. Funding
for the work has come from many sources, includ-
ing the University of Iceland, National Power Com-
pany (Landsvirkjun), Township of Selfoss, and the
European Union (through the projects PRENLAB and
FORESIGHT). Ásta Rut Hjartardóttir helped with the
preparation of the figures. Constructive reviews by
Amy Clifton and Jeff Karson are gratefully acknow-
ledged.
ÁGRIP
Víða á skjálftabelti Suðurlands má finna merki um ný-
legar sprunguhreyfingar. Skjálftabeltið er um 15 km
breitt og 70 km langt (1. mynd) og er hluti af flekaskil-
um Atlantshafsins sem liggja gegnum Ísland. Beltið
markar suðurjaðar smáfleka, Hreppaflekans, sem ligg-
ur milli gliðnunar- og gosbeltanna tveggja á Suður-
landi. Á þessu belti eiga upptök margir mestu tjóna-
skjálftar Íslandssögunnar. Tvenns konar jarðmyndan-
ir koma einkum fyrir á skjálftasvæðinu. Annars veg-
ar er jökulmótað landslag, jökulöldur og jökulsorfin
hraun, mótað fyrir lok síðasta jökulskeiðs, og hins
vegar hraun, Þjórsárhraun, sem huldi stóra hluta lág-
lendisins fyrir um 8000 árum. Vísbendingar um ný-
legar sprunguhreyfingar, þ.e. frá síðustu 10000 árum,
varðveitast nokkuð vel í báðum tegundum landslags,
einkum þó í Þjórsárhrauninu. Gögnum um sprung-
ur og sprunguhreyfingar á svæðinu hefur verið safn-
að kerfisbundið í meira en þrjá áratugi og hafa nem-
endur í jarð- og jarðeðlisfræði við Háskóla Íslands
komið þar mjög við sögu. Sprungur og sprungutengd
fyrirbrigði hafa verið kortlögð með GPS-tækni. Hér
er gerð grein fyrir helstu niðurstöðum þeirra rann-
sókna. Langflestar sprungur stefna í NNA til ANA og
þær mynda skástígar sprunguraðir eða sprungufylki
(2. mynd) með norðlægri stefnu. Í hverju sprungu-
fylki hliðrast sprungurnar til vinstri miðað við aðliggj-
andi sprungur í fylkinu. Þessi röðun bendir til þess að
sprungufylkið sé tengt hægri handar sniðgengishreyf-
130 JÖKULL No. 60