Subproject 6.
Agust Gudmundsson
Nordic Volcanological Institute
University of Iceland
Grensasvegur 50
IS-108 Reykjavik
Iceland
Phone: 354-525-4488
e-mail: agust@norvol.hi.is (office)
agustgud@mmedia.is (home)
1. The South Iceland Seismic Zone
The work has focused on aerial inspection of the main fault systems. Some of the faults have been selected for a detailed areal study for comparison with on-land field studies. The principal questions addressed are: Why are the fault systems in the Holocene lava flows so complex? How do these systems compare with the nearby faults in the Pleistocene rocks.
With the view of answering these questions partly, a detailed aerial inspection and mapping of the Leirubakki-Svinhagi seimogenic fault has been made. This fault is just over 7 km long strikes N12°. The fault consists of many segments within a zone that is 100-250 m wide. In the northern part of the zone, individual fractures strike around N12°, around N15° in the middle part, and around N25° in the southernmost part. The fractures thus become more easterly striking towards the south. The fractures consist of en echelong segments, with small push-ups or hillocks as well as pull-apart structures between the nearby ends of the segments. These structures are associated with areas of transpression and transtension, respectively, along the main strike-slip fault.
The Leirubakki-Svinhagi fracture is location in the 9000-year-old Thorsarhraun lava flow. The age of the fracture itself is, however, not known. It is obviously less than 9000 year old, but may be only several hundred years old. At the southern end of the fracture, near the farm Svinhagi, hot water (15-20°) emits from the fracture. The presence of warm water in the fracture indicates that it is not very old as otherwise it would have been sealed up with silica.
There are at least three main trends of fractures associated with the Leirubakki-Svinhagi fault: NNE, ENE and WNW. Most of these cracks are presumably strike-slip faults. In addition, there are NE trending tension fractures. All these trends are also found in the fault populations in the nearby Pleistocene rocks. Nevertheless, the WNW trend of segments associated with the Leirubakki-Svinhagi fault is unusually clear and conspicuous and must be explained by any model that attemts to account for the fracture pattern associated with the South Iceland Seismic Zone.
Work on the SISZ is in close collaboration with Jacques Angelier, Francoise Bergerat, Sigurdur Rögnvaldsson and Helgi Torfason. A manuscript on the Origin and tectonic evolution of the South Iceland Seimic Zone is near to being completed.
2. The Tjörnes Fracture Zone
Work is in progress on combining field and numerical studies with seismic studies from the extension of the SIL network in this area. A manuscript is in preparation, entitled: Seismotectonic analysis of the Tjörnes Fracture Zone, an active transform fault in North Iceland. The purpose of this paper is to combine a detailed seismic analysis by Sigurdur Rögnvaldsson and Ragnar Slunga with on-land field studies of the faults in the Tjörnes Fracture Zone. Our conclusion is that many of the strike-slip faults in the Grimsey fault, as well as those associated with the Dalvik "lineament" are northerly trending sinistral faults rather than westerly trending dextral faults. We are working on numerical (boundary element) models with a view of explaining the current stress field in this area.
Part of the work on the Tjörnes Fracture Zone is made in collaboration with Thierry Villemin, particularly on the on-land parts of the Husavik-Flatey Fault. A detailed GPS network, set up by the NVI in 1994, was expanded in collaboration with Thierry Villemin in 1995. One objective of this work is to correlate the seismicity on the Husavik-Flatey fault with the on-land slip of the fault, as measured by the GPS network. An abstract has already been submitted to the EUG 9 meeting in Strasbourg on some of the results of this work.
3. Effects of fluid pressure on fault development
Pore-fluid pressure greatly affects the probability of failure and reactivation of the faults, both in the South Iceland Seismic Zone and in the Tjörnes Fracture Zone. A detailed study has been made of the mineral veins (old channels of geothermal water) on the fault planes in the Pleistocene rocks of the South Iceland Seismic Zone and in the Pleistocene-Tertiary rocks in the Tjörnes Fracture Zone. One of the principal questions addressed in this study is: How rapidly do seismogenic faults in these zone heal and how do changes in fluid pressure in one region (e.g. in association with major earthquakes or volcanic eruptions) affect slip on faults in other regions. It is likely that changes in fluid pressure can be transmitted over considerable distances and thus trigger earthquakes in areas relatively far away from the source of the initial pressure change. Fluid pressure also affects friction on fault planes, hence the probability of fault slip.
This research is particularly important in view of the major geothermal activity associated with earthquake fractures in South Iceland and elsewhere. The work on earthquake fracture healing is partly in collaboration with Phil Meredith, University College, London.
4. Analytical and numerical studies on fault populations
Fault populations develop in space and time. Both analytical and numerical studies are important in order to be able to predict the evolution of fault populations, in particular those in the South Iceland Seismic Zone and in the Tjörnes Fracture Zone. Part of this work uses data from other fault areas in Iceland, such as the Borgarfjördur area where strike-slip faults are common. This work is made in collaboration with Maurizio Bonafede and Maria Elina Belardinelly (Italy).
PUBLICATIONS
1. Gudmundsson, A., 1996. Geometry, displacements and driving stresses of seismogenic faults in Iceland. In Seismology in Europe, Bardi Thorkelsson (ed), Reykjavik, pp. 193-198. (Also an Abstract)
2. Gudmundsson, A., 1996. Stress fields controlling strike-slip faulting in Iceland. Physics and Chemistry of the Earth (in press).
3. Gudmundsson, A., 1996. Stress field controlling strike-slip faulting in Iceland. General Assembly of EGS in the Hague, May 1996, Annales Geophysicae, Supplement I to vol. 14.
4. Gudmundsson, A., 1996. Formation and development of tension fractures in a lava pile. Tensile Fracturing in the Earth's Crust. Programme with Abstracts. University College London, Department of Geological Sciences, September 1996.
5. Gudmundsson, A., Bergerat, F. and Angelier, J., 1996. Strike-slip faults in the central part of the South Iceland Seismic Zone. Abstract submitted to EUG 9, Strasbourg 1997.
6. Gudmundsson, A., 1996. Origin and tectonic evolution of the South Iceland Seismic Zone (MS in perparation).
7. Gudmundsson, A., 1996. Mid-ocean ridge processes in Iceland. General Assembly of EGS in the Hague, May 1996, Annales Geophysicae, Supplement to vol. 14.
8. Rögnvaldsson, S., Gudmundsson, A., and Slunga, R., 1996. Seismotectonic analysis of the Tjörnes Fracture Zone, an active transform fault in North Iceland (MS in perparation).
9. Gudmundsson, A., 1996. Formation and development of fractures, with application to fluid flow in solid rocks. Lectures at the University of Bergen, October, 1996.