Mechanical behaviour of the Árnes and Hestfjall Faults of the June 2000 earthquakes in Southern Iceland: inferences from surface traces and tectonic model

Abstract

Two major earthquakes (M S=6.6) occurred on June 17th and 21st, 2000 in southern Iceland. This paper presents mapping and measurements of representative segments along their surface fault traces: the Árnes Fault (June 17th) and the Hestfjall Fault (June 21st). The rupture trace of the Árnes Fault at Mykjunes shows a conjugate strike-slip pattern with a N30°E-trending right-lateral fault and a N60°E-trending left-lateral fault. Each of these faults is formed by en-échelon arrays of fractures and push-ups. The compression observed in the push-ups is inferred to represent the lateral displacement across the fault, which can be quantified by treating the push-ups as a shortening problem. Thus the mapped push-ups indicate an average strike-slip offset of 0.22 m for both these faults. The surface trace of the Hestfjall fault near Bitra reveals a N50°–60°E trending left-lateral strike-slip fault including large open fractures, N25°–35°E-trending en-échelon fractures and some push-ups indicating a left-lateral offset of 0.50–0.60 m along the Bitra fault segment. Two other fault segments on the Hestfjall Fault, near Eyvik and near Torfagil, show association of open fractures and normal faults. The most characteristic feature of the June 2000 fault traces is the en-échelon arrangement of fractures and push-ups. The 2000 rupture traces are compared with some historical seismic fault traces of the South Iceland Seismic Zone in terms of scale hierarchy of fracture traces. According to our observations at Bitra and Mykjunes, the general fault map of the Orkustofnun (National Energy Authority in Iceland) is interpreted as a conjugate system of strike-slip faults. A deformation model is proposed, aiming at reconciling the simple velocity boundary conditions imposed by plate kinematics along the left-lateral, E–W-trending transform zone, and the brittle deformation inside the South Iceland Seismic Zone. The observations made along the Árnes and Hestfjall Faults show that the actual earthquake-related fault pattern is not restricted to the N–S-trending dextral strike-slip revealed by focal mechanisms and N–S alignment of aftershocks, but includes conjugate systems consistent with the structural pattern of the South Iceland Seismic Zone.