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Maryam Khodayar, Iceland GeoSurvey (Iceland)
Sveinbjörn Björnsson, National Energy Authority (Iceland)
Páll Einarsson, Institute of Earth Sciences, University of Iceland (Iceland)
Hjalti Franzson, Iceland GeoSurvey (Iceland)
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The South Iceland Seismic Zone (SISZ) is an active transform zone connecting two rift segments. It is a shear zone about 80 km long and 25 km wide, striking E-W, and acting sinistral conformably with relative spreading direction N104°E across Iceland. Six fracture sets exist within the SISZ. However, the main faults responsible for large earthquakes are northerly dextral strike-slip, but the conjugate ENE sinistral strike-slip also contributes. Historical records from the last seven centuries suggest that sequences of strong earthquakes (M > 6) occur with a recurrence interval of 45-112 years resulting in surface ruptures such as en échelon open segments, sink holes, and push-ups. The latest earthquake sequences occurred in 1896 and June 2000. The two largest events in 2000 were caused by strike-slip on two parallel northerly faults with 12 km long and up to 2.5 m offset. Locally, extension and dip-slip up to 0.5 m were observed along fault segments. We carried out structural analysis of a number of Holocene surface ruptures in the SISZ including the Skardsfjall Mountain. Most of the basaltic lavas, sediments, and hyaloclastites in Skardsfjall originated in an early rift zone since lower Matuyama. They are covered in places by Bruhnes interglacial lavas, 8000-year old post-glacial lavas, and eolian sediments. We used the geometry of hundreds of dykes and faults, and fault displacements in time to suggest the activity of a transform fault regime and reactivation of the same crustal weaknesses from early history of Skardsfjall up to the present. (a) Fracture geometry: Matuyama dykes and basement faults strike dominantly northerly and ENE, then NNE and WNW. A major N-S fault dissects Skardsfjall and plays also the contact between the lower Matuyama lavas and the hyaloclastites. Dykes are numerous and bend from WNW to NNW around this fault, reflecting an eruptive centre located at the intersection of the N-S and an ENE faults. The dykes fed the hyaloclastites from this centre and magma injected also into older conjugate northerly and ENE faults; (b) Fault displacements: Most basement faults and some dyke edges were reactivated during Holocene earthquakes. There, the main N-S fault shows the highest number of ruptures. Horizontal offset could not be estimated along fractures of any age. However, apparent dip-slips are 3 to 35 m along the older faults (the largest being along northerly faults), and 0.5 to 2.5 m along Holocene ruptures including the main N-S fault. These differences indicate that in general slips accumulate with earthquakes; (c) Present-day seismicity indicates activity along at least two major northerly basement faults, one of which is the main N-S fault. The initial shock of the 1896 earthquake sequence most likely originated on this major N-S fault, which has been an active crustal weakness since Matuyama.
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