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Thesis University of Iceland > Verkfræði- og náttúruvísindasvið > Meistaraprófsritgerðir - Verkfræði- og náttúruvísindasvið >

Please use this identifier to cite or link to this item: http://hdl.handle.net/1946/3990

Title: 
  • is Use of relatively located microearthquakes to map fault patterns and estimate the thickness of the brittle crust in Southwest Iceland. Sub-surface fault mapping in Southwest Iceland
Other Titles: 
  • is Endurstaðsettir smáskjálftar frá 2000 notaðir til þess að kortleggja sprungufleti og meta þykkt brotgjörnu skorpunnar á Suðvesturlandi
Abstract: 
  • is

    The two rift zones in southern Iceland, the western and the eastern volcanic zones, are
    connected by an approximately 70 km long and 15 km wide left lateral shear zone, the
    South Iceland seismic zone (SISZ). Approximately every 100 years a sequence of large
    earthquakes of magnitudes M 6-7 occurs in the zone and traces of many Holocene faults
    have been mapped on the surface. Although the trend of the zone is east-west, major
    earthquakes take place on north-south striking faults, which leads to “bookshelf” type
    faulting. In June 2000, two large earthquakes of magnitudes ML6.4 and ML6.5 struck in the
    SISZ, three and a half days and approximately 17 km apart. Seismicity greatly increased in
    all Southwest Iceland and during 2000, roughly nineteen thousand microearthquakes were
    recorded there. The aim of this research is to relocate the earthquakes using a double-
    difference relative location method and to map the faults that they occur on. Due to the
    high clock-accuracy of the Icelandic SIL seismic network, the method can, through cross-
    correlation, reduce the uncertainties in relative arrival times and thus increase relative
    location accuracy to as far as tens of meters. This enables fault mapping by grouping
    together relocated events that form apparent lineaments. A suite of possible mechanisms
    are calculated for each event based on polarities and spectral amplitudes. Through the joint
    interpretation of fault mechanisms with the event distribution defining the fault, the slip
    direction on the fault plane can be inferred. Using this kind of mapping, about 240 faults,
    fault segments and small clusters, which were active in year 2000, have been mapped
    during this research. A detailed image of the two large fault planes of the 17 June event
    (J17) and the 21 June event (J21) has been revealed. The aftershock activity on the 12.5
    km long, 10 km wide, north-south striking, near vertical J17 fault is mainly confined to its
    margins and centre, and the fault is composed of three patches, each striking a few degrees
    east of the overall fault strike, which is N197°E. The J21 fault, on the other hand, is more
    linear but with varying dip. South of the epicentre the fault is vertical but north of it, dip
    changes to 77°. It is 15.5 km long, strikes N179°E and deepens to the south from 6 km to 9
    km. Many other smaller faults in Southwest Iceland that were illuminated by the 2000
    activity have also been mapped, including the fault planes of three M~5 events which were
    triggered within minutes of the J17 event. The depth distribution of the relocated
    seismicity shows shallower focal depths in geothermal areas. Generally the results indicate
    a 6-8 km thick brittle crust and abrupt thickening is observed between 20.8°W and 20.6°W
    in the central SISZ. The thickness of the brittle crust increases eastwards to 10 km near the
    1912 Selsund fault, and southwards to 13 km south of the transform zone.

ISBN: 
  • 978-9979-9914-6-5
Description: 
  • is Verkefnið var unnið á Veðurstofu Íslands og var að miklu leyti hluti af Evrópusamstarfsverkefninu PREPARED, yfirverkefnisstjóri var Ragnar Stefánsson.
Accepted: 
  • Oct 9, 2009
URI: 
  • http://hdl.handle.net/1946/3990


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