In-situ strain relief measurements in Iceland

Abstract

In-situ measurements of rock stress in Iceland have been made by Hast in 1967–1968 at five sites and two sites by Haimson and Voight in 1976. All of the received data revealed compressive stress, although three sites (Keflavik, Burfell and Akureyri) investigated by Hast and the two (Reykjavik) measured by Haimson and Voight are located close to the axial rift zone. During the summer of 1976 in-situ strain relief was measured at 20 new sites. The localities were concentrated in the axial rift zone in Kelduhverfi/ North Iceland. Strain relief was recorded during an episode of intensive seismic activity and of major rifting. Pre-existent fissures and faults were subjected to considerable vertical and horizontal movements a few months earlier. Additional in-situ strain measurements were carried out between the northern axial rift zone and the east coast, and along the south coast from Höfn to Reykjavik. Most of the measurements have been made at the surfaces of Interglacial and Tertiary flood basalts polished by glacial abrasion. These surfaces were most suitable for in-situ measurements since bedrock outcrops were unaltered. There were no major geological criteria that influenced the stress field after the recession of the glaciers other than stress caused by the driving mechanism. At seven sites within the axial rift zone and at thirteen sites outside, in-situ strain was relieved by overcoring and horizontal strain components were measured. The orientations and magnitudes of maximum and minimum horizontal stress were calculated after determination of Young's modulus and Poisson's ratio. Outside the axial rift zone the orientations of maximum horizontal stress ( σ H max ) are between N30°W and N70°W, and the magnitudes of stress vary from 650 to 10 bar (most values: 50–100 bar). The magnitudes of minimum horizontal stress ( σ H min ) vary from 150 to −160 bar (most values: 0–50 bar). Within the axial rift zone there are two trends of orientation of σ H max , one of N35°E to N40°E in the central part of the axial rift zone in the south as well in the north, and another of E-W in the marginal areas of the rift zone. The σ H max values vary from 120 to 10 bar (most values: 50–100 bar) and σ H min was calculated to vary from 100 to −100 bar (most values: 50 to −50 bar). Absolute tensile stress could be found only in a narrow zone within the northern rift area. In July 1976, shortly after the episode of intensive recent crustal movements and during increased seismic activity in the northern rift zone only a N-S-striking corridor of about 2.5-km width was subjected to tensile stress, while to the east and west of that zone σ H max was measured to be perpendicular to the gaping fissures.