Earthquakes and present-day tectonism in Iceland

Einarsson, Páll

doi:10.1016/0040-1951(91)90501-i

Cited by 439 publications

(441 citation statements)

References 31 publications

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“…Pre-main shock earthquakes in the N-cluster are located at shallow depths of around 5 km. This fits observations on the adjacent Reykjanes peninsula, where seismic activity occurs in comparable local normal and strike-slip faulting regimes (Einarsson 1991;Keiding et al 2009). In contrast, earthquakes in the S-and EW-clusters mainly occur between 4 and 8 km depth with almost pure strike-slip faults.…”

Section: Discussionsupporting

confidence: 88%

“…Hengill marks a triple junction between the Eurasian and Northern American plates, as well as the border to the Hreppar microplate in central Iceland (Einarsson 1991). Hengill is located at the southern end of the Western Volcanic Zone, with similar NE-SW striking normal faults outlining the fissure swarm as observed on the Reykjanes peninsula.…”

Section: Discussionmentioning

confidence: 79%

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Temporal stress changes associated with the 2008 May 29M_W 6 earthquake doublet in the western South Iceland Seismic Zone

et al. 2015

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two magnitude M W ∼ 6 earthquakes occurred on two adjacent N-S faults in the western South Iceland Seismic Zone. The first main shock was followed approximately 3 s later by the rupture on a parallel fault, about 5 km to the west. An intense aftershock sequence was mostly confined to the western fault and an E-W aligned zone, extending west of the main shock region into the Reykjanes oblique rift. In this study, a total of 325 well-constrained focal mechanisms were obtained using data from the permanent Icelandic SIL seismic network and a temporary network promptly installed in the source region following the main shocks, which allowed a high-resolution stress inversion in short time intervals during the aftershock period. More than 800 additional focal mechanisms for the time period 2001-2009, obtained from the permanent SIL network, were analysed to study stress changes associated with the main shocks. Results reveal a coseismic counter-clockwise rotation of the maximum horizontal stress of 11 ± 10 • (95 per cent confidence level) in the main rupture region. From previous fault models obtained by inversion of geodetic data, we estimate a stress drop of about half of the background shear stress on the western fault. With a stress drop of 8-10 MPa, the pre-event shear stress is estimated to 16-20 MPa. The apparent weakness of the western fault may be caused by fault properties, pore fluid pressure and the vicinity of the fault to the western rift zone, but may also be due to the dynamic stress increase on the western fault by the rupture on the eastern fault. Further, a coseismic change of the stress regime-from normal faulting to strike-slip faulting-was observed at the northern end of the western fault. This change could be caused by stress heterogeneities, but may also be due to a southward shift in the location of the aftershocks as compared to prior events.

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Section: Discussionsupporting

confidence: 88%

Section: Discussionmentioning

confidence: 79%

Temporal stress changes associated with the 2008 May 29M_W 6 earthquake doublet in the western South Iceland Seismic Zone

et al. 2015

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“…At north Cleft, geologic mapping and side-scan sonar data indicate that a young sheet flow was erupted several years before, and just south of, the 1983-1987 pillow mounds Previous studies on land have also noted that when multiple intrusions occur in the same area, later events are more likely to lead to eruptions at the surface, presumably because the earlier events had increased the horizontal compressive stress perpendicular to the rift axis, which forced later dikes to be intruded with a higher magmatic pressure [Ando, 1979;Bj6rnsson, 1985;Einarsson, 1991 ]. This was probably also a factor affecting the recent JdFR eruptions.…”

Section: -15 M (Table 2) This Comparison Suggests That Dikeinducedmentioning

confidence: 99%

Graben formation associated with recent dike intrusions and volcanic eruptions on the mid‐ocean ridge

Chadwick¹,

Embley²

1998

J. Geophys. Res.

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Abstract. Grabens have been mapped immediately adjacent to recently erupted submarine lava flows on the Cleft and CoAxial segments of the Juan de Fuca Ridge (JdFR). The grabens are 10-100 m wide and 5-15 m deep and are located uprift and/or downrift of the eruptive vents that fed the flows. We interpret that these structures formed (or were reactivated) directly over the dike that fed the eruptions as it was intruding toward the surface. These graben structures are primary conduits for diffuse hydrothermal venting on the seafloor during the cooling of newly intruded dikes. The axial summit "graben" or "caldera" on the East Pacific Rise (EPR) in some locations has similar dimensions to the grabens observed on the JdFR, and we interpret that it too may be a dike-induced graben structure (although often buried by subsequent eruptions where it is narcowest). These grabens on the JdFR and the EPR are distinctly narrower and deeper than grabens that formed during well-documented dike intrusions on slow spreading rifts on land (Iceland and Afar). Mechanical modeling suggests that narrow grabens would form when a dike is at shallow depth where it imposes a high stress perturbation on the ambient stress state to cause faulting. Therefore the narrow grabens on the JdFR and EPR imply that a relatively high level of horizontal compressive stress typically exists perpendicular to the ridges, and this must be overcome by high dikeinduced perturbations to cause faulting. This is probably because gradual plate spreading rarely gets enough time to lower the compressive stress significantly due to the high frequency of dike intrusion events relative to the plate spreading rate. The dikes that do intrude in this environment must have relatively high internal magma pressure. Therefore the size and character of dike-induced grabens that form at the surface on intermediate to fast spreading ridges reflect the fact that volcanism dominates over tectonism in regulating the local stress state where a robust magma supply is available.

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“…This explosive eruption was accompanied by heavy tephra fall to the northeast of Askja, creating 0.21 km 3 dense rock equivalent (DRE) of silicic tephra [Sparks et al, 1981]. It has been suggested [Sigurdsson and Sparks, 1978] [Einarsson, 1991]. A third cluster is northwest of Askja.…”

Section: Introductionmentioning

confidence: 99%

Continuous deflation of the Askja caldera, Iceland, during the 1983–1998 noneruptive period

Sturkell¹,

Sigmundsson²

2000

J. Geophys. Res.

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Abstract. The Askja volcano at the spreading plate boundary in north Iceland consists of nested calderas, the latest formed in an eruption in 1875. Several eruptions have occurred since in Askja, the most recent in 1961. Precise leveling has been conducted yearly at Askja since 1983. In 1993, a dense GPS network was measured in and around the Askja caldera consisting of more than 20 points, and we remeasured this complete network for the first time in 1998. Askja subsided during the period from 1983 to 1998. Observed deformation fits broadly with a "Mogi" point source model with best fitting location near the center of the main Askja caldera (65.0448øN, 16.7805øW) at a depth of 2.8 km. From 1983 to 1991 the yearly subsidence rate between the end points of a leveling profile decayed gradually from ---10 mm/yr to an average of---7 mm/yr in the 1991-1998 period. Total subsidence at the Askja center in the 1983-1998 period is at least 75 cm, and the integrated volume of surface subsidence is -0.037 km 3. This period has been a "quiet" period at Askja with no eruptions, large earthquakes or known dike injections. Such a high rate of "background" deformation has not been observed at other volcanoes in Iceland. The eruption of a neighboring volcano in 1996 has not resulted in a modified deformation pattern at Askja, indicating no pressure connection at depth between the two systems. Solidification may account for part of the observed contraction and subsidence in Askja.

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Earthquakes and present-day tectonism in Iceland

Cited by 439 publications

References 31 publications

Temporal stress changes associated with the 2008 May 29M_W 6 earthquake doublet in the western South Iceland Seismic Zone

Temporal stress changes associated with the 2008 May 29M_W 6 earthquake doublet in the western South Iceland Seismic Zone

Graben formation associated with recent dike intrusions and volcanic eruptions on the mid‐ocean ridge

Continuous deflation of the Askja caldera, Iceland, during the 1983–1998 noneruptive period

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Earthquakes and present-day tectonism in Iceland

Cited by 439 publications

References 31 publications

Temporal stress changes associated with the 2008 May 29MW 6 earthquake doublet in the western South Iceland Seismic Zone

Temporal stress changes associated with the 2008 May 29MW 6 earthquake doublet in the western South Iceland Seismic Zone

Graben formation associated with recent dike intrusions and volcanic eruptions on the mid‐ocean ridge

Continuous deflation of the Askja caldera, Iceland, during the 1983–1998 noneruptive period

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Temporal stress changes associated with the 2008 May 29M_W 6 earthquake doublet in the western South Iceland Seismic Zone

Temporal stress changes associated with the 2008 May 29M_W 6 earthquake doublet in the western South Iceland Seismic Zone