Task 3.1: Measuring the present-day crustal displacements in the Tjörnes fracture zone and adjacent areas

The Tjörnes GPS network

The network consists of about 45 sites distributed in the northern Iceland seismic zone (Figure 37). It completes the geodetic networks already installed over the whole Iceland at a smaller scale. The TGN has been designed to measure the surface displacement field on each side of the Húsavík-Flatey fault (HFF). It has been done in order to estimate if there are locked fault segments in the area and how these segments can contribute to increase the seismic risk.

 

Figure 37: The Tjörnes GPS network. 32 points have been measured in 1995, 1997 and 1999. The whole network has been measuring both in 1997 and 1999.

1995-1997 velocity field

The velocities (Figure 38) have been computed by reference to a point located in the southern part of the network. Two tendencies can be distinguished on the Tjörnes peninsula: eastward velocities reaching 13 mm/year for the most northern points of the peninsula and NNE velocities up to 15 mm/year for the points located on both sides of the HFF. Displacements to the east have been computed for the points located in the fissure swarms. Points on Flateyjardalur move to the north. The point on the Flatey island reveals a large displacement to the NE that could be due to a local instability.

 

Figure 38: 1995-1997 horizontal displacements on the TGN.

The assumption of an interseismic strain has been tested by using a simple dislocation model. This model assumes that a set of buried planar fault surfaces are locked above a given depth and are affected by uniform aseismic creep below this depth. In order to determine this brittle/ductile transition we used the microseismicity recorded by the SIL network from 1995 to 1997. We assume that most earthquakes are localized in the brittle crust. Thus gives us a limit at a 10 km depth in average. We founded a solution that minimizes the differences between simulated and observed velocities.

The model (Figure 39) assumes: (1) a dyke opening of 20 mm/year affecting all the brittle crust along the Kolbeinsey ridge; (2) two dyke openings of 30 mm/year and 20 mm/year respectively along the Krafla and Þeistareykir fissure swarms; (3) a dextral strike-slip fault striking N$100^\circ$E between the two previous rift segments with a velocity of 50 mm/year below a depth of 10 km and completely locked above the brittle/ductile transition; (4) a 15 mm/year opening zone striking N$140^\circ$E south of the HFF; (5) a fault along the Grímsey lineament with both a 15 mm/year opening and 20 mm/year dextral strike-slip movements. In addition small wavelength tendencies has been adjusted by superficial faults.

This model based on our 1995-1997 TGN comparison revealed extension and strike-slip movements 3 to 4 times larger than the average velocity. The transform motion is locked on a large (150x10 km) fault surface and this represents the main risk for destructive earthquakes in the near future. From a mechanical point of view, the lockage could be due to the increase of normal stress on this surface following the double opening north and south of the fault zone.

 

Figure 39: Dislocations used in the simulation of the 1995-1997 displacement field (after Jouanne et al. 1999). Two main kinds of dislocation can be distinguished: superficial dislocations that affected the upper crust with mainly aseismic opening or strike-slip (Kolbeinsey ridge) and a major dislocation simulating a fault locked at 10 kilometers depth and affected by a constant slip below this limit simulating ductile slip at depth.

1997-1999 velocity field
Comparison with the 1995-1997 velocity field

32 common points have been measured during the 3 campaigns (1995, 1997 and 1999). In comparison with to the first period and using the same reference point located in the southern part of the network, the 1997-1999 velocity field has the following main characteristics:

The eastward tendencies north of Tjörnes are always present but the average velocity has been divided by 2. We observed less than 1 cm/year of dextral displacement north of Tjörnes. The NNE displacements observed in 1995-1997 on both side of the Húsavík fault are always observed but their component are in the opposite direction of those observed during the first period. These points are now moving to the south also with smaller velocities. Similar conclusions are drawn with the 2 points on Flateyjardalur. The large displacement observed in 1995-1997 for Flatey is no more observed.

From this comparison we can argue that the displacements on Tjörnes have varied significantly in sense and size in less than 2 years. A model compatible with both period is presently being elaborated.

Velocities for the points added to the 1995 network

13 new points have been added to the 1995 network both to the east and to the west to the first study area. All eastern points shows very similar ESE azimuth of displacements. A difference of 1 cm/year has been observed between the points respectively inside and outside the Krafla fissure swarm. This demonstrates the activity of the eastern margin of this swarm. The points added to the west of the network show similar displacements to those located on Flateyjardalur, which mean a general sinistral displacement of the SW area, relatively to a point located in the southern middle part of the network.