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The Reykjanes Peninsula in southwest Iceland is a highly oblique spreading segment of the Mid-Atlantic Ridge oriented about 30° from the direction of absolute plate motion. We present a complete and spatially accurate map of fractures for the Reykjanes Peninsula with a level of detail previously unattained. Our map reveals a variability in the pattern of normal, oblique- and strike-slip faults and open fractures which reflects both temporal and spatial strain partitioning within the plate boundary zone. Fracture density varies across the length and width of the peninsula, with density maxima at the ends and at the northern margin of the zone of volcanic activity. Fractures with similar strike cluster into distinct structural domains which can be related to patterns of faulting predicted for oblique extension and to their spatial distribution with respect to volcanic fissure swarms. Additional structural complexity on the Reykjanes Peninsula can be reconciled with magmatic periodicity and associated temporal strain partitioning implied by GPS data, as well as locally perturbed stress fields. Individual faults show variable slip histories, indicating that they may be active during both magmatic and amagmatic periods associated with different strain fields.

Two large Ms=6.6 earthquakes occurred on June 17 and 21, 2000 in the South Iceland Seismic Zone. The roughly E–W trending zone is undergoing left-lateral shear. However, most known surface rupture has been along north-striking, right-lateral strike-slip faults. Rupture associated with the June 2000 events follows a similar pattern. Although the two earthquakes had similar magnitude, fault plane solutions, and overall rupture lengths of 15 to 20 km, the pattern of rupture from each was notably different in character. The June 17 event ruptured along a series of NNE-trending, left-stepping segments, giving the fault as a whole an almost due north trend. At the largest scale rupture is relatively straight and continuous. At the smallest scale, rupture style seems to vary with small-scale topography and ground texture. Rupture from the June 21 event is more complex and can be divided into five discrete segments. To the north, deformation is distributed across two zones of left-stepping fractures, along which widening and subsidence have occurred. The central segment consists of right-stepping fractures defining a 2 km long, ENE trending zone. Sense of shear is clearly left-lateral strike slip. The two southernmost segments define an NNE trend. In several places along the rupture zones of both earthquakes it can be verified that the ruptures occurred along preexisting faults. The observed faulting structures are similar to those of earlier earthquakes in South Iceland, both with regard to style and spatial arrangement. However, our observations suggest that some of the historical earthquakes may have been larger

Three large earthquakes (Mw>4.5) were triggered within 5 min, 85 km west of a Mw 6.5 earthquake in the South Iceland Seismic Zone (SISZ). We report on surface effects of these triggered earthquakes, which include fresh rupture, widespread rockfall, disrupted rockslides and block slides. Field data confirm that the earthquakes occurred along N-striking right-lateral strike-slip faults. Field data also support the conclusion from modeling of InSAR data that deformation from the second triggered event was more significant than for the other two. A major hydrological effect was the draining of water through an open fissure on a lake bed, lowering the lake level by greater than 4 m. Field relationships suggest that a component of aseismic slip could have been facilitated by water draining into the fault zone. D 2003 Elsevier Science B.V. All rights reserved.

The Hengill triple junction, SW Iceland, is subjected to both tectonic extension and shear, causing seismicity related to strike-slip and normal faulting. Between 1994 and 1998, the area experienced episodic swarms of enhanced seismicity culminating in a ML = 5.1 earthquake on June 4, 1998 and a ML = 5 earthquake on November 13, 1998. Geodetic measurements, using Global Positioning System (GPS), leveling and Synthetic Aperture Radar Interferometry (InSAR) detected maximum uplift of 2 cm/yr and expansion between the Hromundartindur and Grensdalur volcanic systems. A number of faults in the area generated meter-scale surface breaks. Geographic Information System (GIS) software has been used to integrate structural, field and geophysical data to determine how the crust failed, and to evaluate how much of the recent activity focused on zones of pre-existing weaknesses in the crust. Field data show that most surface effects can be attributed to the June 4, 1998 earthquake and have occurred along or adjacent to old faults. Surface effects consist of open gashes in soil, shattering of lava flows, rockfall along scarps and within old fractures, loosened push-up structures and landslides. Seismicity in 1994^1998 was distributed asymmetrically about the center of uplift, with larger events migrating toward the main fault of the June 4, 1998 earthquake. Surface effects are most extensive in the area of greatest structural complexity, where N- and E-trending structures related to the transform boundary intersect NE-trending structures related to the rift zone. InSAR, GPS, and field observations have been used in an attempt to constrain slip along the trace of the fault that failed on June 4, 1998. Geophysical and field data are consistent with an interpretation of distributed slip along a segmented right- lateral strike-slip fault, with slip decreasing southward along the fault plane. We suggest a right step or right bend between fault segments to explain local deformation near the fault. B 2002 Elsevier Science B.V. All rights reserved.

We use clay models to simulate how fault population systematics vary as a function of rift obliquity. Rift obliquity is related to the acute angle, α , between the rift trend and the displacement direction, so that the value of α is inverse to the degree of obliquity. The range of azimuths in a fault population increases as rift obliquity increases (i.e. as α decreases). The length of the longest faults, the sum of fault lengths, and the width of the deformed zone all increase as rift obliquity decreases (i.e. as α increases). The majority of faults in our models are segmented and have highly tortuous traces. Tortuosity is maximum when α =30° as segments of widely varying azimuth link during fault growth. Significant changes occur in the fault patterns between α =30° and α =45°. At α =30° two fault populations of equal importance develop in the center of the rift zone, one approximately rift-parallel and the other displacement-normal. Between α =30° and 45°, the number of faults more than doubles, and between α =45° and 60°, summed fault length more than doubles. Fault patterns for all models are fractal, with fractal dimensions that increase with increasing α and that are comparable to those found in the field. Fault populations are not multi-fractal because the cumulative frequency distributions of fault lengths do not generally follow a power-law relationship. An exponential distribution best describes the data for whole faults, with the characteristic length increasing with increasing α . Segment lengths also follow an exponential distribution with characteristic length varying very little with α . The fault patterns in our models resemble the spatial pattern of brittle deformation observed at oblique mid-ocean ridge segments and are similar in geometry to those in oblique continental rift basins. At the rift margins, tensional stresses are modulated and reoriented by a secondary stress field related to a change in boundary conditions, resulting in the formation of two distinct sub-populations of faults during oblique rifting.

[1] We have used high-resolution scanned air photos and field measurements to analyze fracture population systematics for the Reykjanes fissure swarm in order to determine the effect of rift obliquity on the evolution of fracture populations on the Reykjanes Peninsula, SW Iceland. The peninsula is oriented approximately 30° oblique to the direction of plate motion. Data show significant differences between the strike, length, and degree of development of fractures along the margin of the fissure swarm and those in the center of the zone of active volcanism. Along the margin, fractures are long, highly segmented extension fractures, and normal faults with offset on the order of several meters and a predominant strike 20° oblique to the trend of the plate boundary (rift axis). In the center of the zone of active volcanism, fractures are generally shorter, straighter, fewer in number, and strike approximately perpendicular to the direction of maximum horizontal extension and parallel to the trend of eruptive fissures. Right-lateral oblique-slip faults striking approximately perpendicular to the spreading direction are present in the center of the rift zone. Scaled experimental models of oblique extension predict a significant change in fracture strike at the rift margins due to the presence of a secondary stress field; on the Reykjanes Peninsula, this corresponds to the boundary between thicker and thinner brittle crust. The models confirm that right-lateral oblique- or strike-slip faults accommodate significant left-lateral shear where deformation is distributed in a rift zone.