Task 2: Enhancing the basis for alerts, warnings and hazard assessments

This work has been carried out in relation to providing information and warnings about ongoing activity. It has been linked with increased probability of the occurrence of large earthquakes, on one hand in SW-Iceland and on the other hand near the Húsavík earthquake fault in N-Iceland.

Much work which concerns all aspects of Task 2 has been devoted to the Hengill-Ölfus area in SW-Iceland (Figure 1). An earthquake sequence has been ongoing in this region since 1994, related on one hand to E-W transversal motion across the plate boundary, and on the other to an expansion source at 8-10 km depth below the Hengill area. The largest earthquakes of this sequence took place on June 4, 1998, magnitude 5.1, and on November 13, 1998, magnitude 5. This sequence of events, as observed seismologically and geodetically, is of enormous significance for understanding the build-up of stress before earthquakes and for understanding the nucleating process or the short-term precursor activity before earthquakes (Figure 2) (Ágústsson 1998; Rögnvaldsson et al. 1998c; Tryggvason et al. 2000; Stefánsson et al. 2000d). Description of work developed on data from the Hengill-Ölfus area is also found in Subprojects 2, 3 and 4.

After the earthquake of June 4, 1998, and the following earthquake sequence and deformation, stress was modified up to 50 km distance to east and west from the epicenter, along the E-W plate boundary. This appeared in widespread seismic activity, but also in increases in shear-wave splitting delay time, which lead to an earthquake forecast (Crampin et al. 1999).

Work which is concerned with the possibility of an impending large earthquake, i.e. earthquake of magnitude 7, near the town Húsavík in N-Iceland (Figure 1), was discussed at a special PRENLAB-2 workshop in Húsavík on July 30, 1998. Work is going on under several subprojects with risk related research in this region. Subproject 1 has besides providing seismological data, taken initiative in planning new observations to be made in the area, on basis of the results of ongoing work. The objective is to provide observations which can create a better basis for modelling of the Húsavík earthquake, for an improved hazard assessment and for better real-time monitoring possibly involving short-term warnings (Stefánsson et al. 1998). IMOR.DG has been much involved in preparing and starting a new project, SMSITES, an EU project within "Support for Research Infrastructures", which is based on the results of the PRENLAB projects (see further Subproject 3). This involves seismic measurements and evaluations as well as monitoring of changes of water elevations in hot water boreholes in the area. Work is ongoing within Subproject 1 regarding the Tjörnes fracture zone in general (Rögnvaldsson et al. 1998b).

Increased activity in the volcanic complex of Katla (see position in Figure 4), Mýrdals-jökull and Eyjafjallajökull in S-Iceland from summer 1999 which has involved increased research and monitoring as well as multidisciplinary cooperation, was aimed at being able to provide useful warnings in case of a suddenly occurring of a possibly very dangerous eruption there. The website of IMOR.DG has gradually developed into a center of early warning and information activities in Iceland (http://www.vedur.is/ja/jar_inn).

The progress of this work on this task was put under test in the two large earthquakes in the South Iceland seismic zone, on June 17 and 21, 2000, as well as in the Hekla eruption which started February 26, 2000 (Figure 4). These events involved decision about warnings or predictions as further described and referred in Subsection 3.1.5.3.

  

Figure 4: The figure shows the southwestern part of Iceland. Iceland as a whole is shown in the upper right corner. Dotted yellow lines denote the western volcanic zone (WVZ) and the presently more active eastern volcanic zone (EVZ). South Iceland seismic zone (SISZ) is indicated as well as its prolongation in the Reykjanes peninsula (RP). The direction of the relative plate motion is shown by arrows. The faults of the earthquakes on June 17 and 21 are indicated by 17 and 21 respectively. Red dots, which are epicenters of small shocks following the large earthquakes, describe the area seismically activated.