Introduction

Among achievements of great significance for the project and for its outcome following can be mentioned:

• While Iceland as a whole is the test area for the project, The Hengill-Ölfus area has come to play a very significant for the project. The reason is basically the very high seismic activity in this area since 1994. The seismic activity is a result of one hand the strain caused by transversal motion along the EW plate boundary in SW Iceland and on the other hand fluid expansion source near the center of the Hengill volcanic complex. It has been possible to carry out deformation measurements of various kinds to keep track of the deformation in addition to very detailed observations of frequent seismic swarms and individual earthquakes up to 5.1 in magnitude. Frequent observations by the SAR technology since 1993 show a steady uplift of 1.5 cm/year above a postulated pressure source at 7 km depth. The horizontal deformation is observed by GPS measurements, which have shown well constrained displacements related to individual earthquake sequences in two cases. The stress modifications related to both of these earthquakes have been observed. Thus an earthquake cycle has been observed from the start time of build-up of stress on June 4, 1998, in a large area towards concentration of stress in a focal region and foreshocks of an earthquake that occurred on November 13, 1998. After the earthquake of November 13, it was then observed how an E-W fault zone served as a stress guide, and how a sequence of earthquakes was observed related to that guide. This has consequences for modelling large South Iceland seismic zone earthquakes which occur in sequences of earthquakes reaching magnitude 7.

• There has been significant progress in utilizing microearthquakes to study faulting processes. An automatic process has been developed to relate small earthquakes with individual fractures within an earthquake fault system on basis of very accurate relative location procedure and on basis of fault plane solutions. This opens the possibility to use microearthquakes to observe stable motion within a complicated fault system, changes of stresses and stress directions and earthquake nucleation. In seismology microearthquakes are mostly considered or treated as chaotic feature. By work in this field in the PRENLAB projects we are gradually discovering causality which in a physically understandable way relates the microearthquakes to each other and to larger events. Among significant indications of this mostly methodological study is that small earthquakes, also at depths near the base of the seismogenic crust, show similarity with hydraulic fracturing and changes in pore pressure that may have strong influence of the periodicity of and triggering of seismic activity.

• Very significant progress in observing relation between shear-wave splitting delay and deviatoric stress build-up has resulted in a successful stress forecast. On basis of experience in studying shear-wave splitting time patterns in the very active Hengill-Ölfus area in SW Iceland a successful stress forecast was issued. After a general information about increase in stress in the Hengill area in SW Iceland and on the basis of observations and modeling of nearby premonitory activity at end-October 1998 a definite stress forecast was issued by Stuart Crampin of the PRENLAB-2 project, on November 10, 1998. This forecast said that an earthquake of magnitude 5-6 could occur anytime between the issuing of the forecast (M=5) and the end of February 1999 (M=6) if stress kept increasing. An earthquake of magnitude 5 occurred near the center of the region included in the forecast on November 13. Although this kind of forecast is far from being a complete earthquake prediction this is a step forward for short-term warnings. It does not in itself specify the epicenter of the earthquake. In this case the most likely epicenter could be guessed based on former activity, i.e. to complete an ongoing seismic cycle. The earthquake itself had foreshock activity, which in fact defined the most likely epicenter for the earthquake, and also indicated that it was impending within short. Of course it is always a question if a sequence of small earthquakes is a foreshock activity or not. However, the pattern of foreshock activity in this case and methods for automatic evaluations of observations which are ongoing on basis of the collected data, give hopes that procedures can be developed to complete such a stress forecast by observations which aim at finding the place and the time of the earthquake nucleation before it ruptures.

• The current stress fields near the earthquake zones in North Iceland and in SW Iceland have been calculated by inverting a large number of fault plane solutions of the SIL system for stress. The average direction of extension was observed to be N66$^\circ$E for the Tjörnes fracture zone in North Iceland and N143$^\circ$E for the area around the South Iceland seismic zone. This is in good agreement with the postulated pattern of the general divergent plate motion in Iceland. This result is also a significant base for studying the spatial and temporal variations in stress directions, which are related to uneven transversal plate motion and fluid intrusions, and thus to earthquake occurrence.

• Significant results have been obtained in geodetical and geological studies near the Húsavík-Flatey transform fault zone in northern Iceland. A model has been created mainly on basis of repeated GPS measurements which describes the fault system as a locked system down to a depth of 10 km but with a right-lateral transversal motion of 5 mm/year below that depth along the fault, i.e. the same sense of motion expected in a large earthquake on the Húsavík fault. Thus stress seems to be fast built up by time increasing the probability of a large earthquake in this area. Studies of aspect ratio of fluid filled veins studied in exposed parts of the Húsavík-Flatey fault zone indicate that fluid overpressure above the minimum compressive principal stress is 20MPa.

• Modelling work has been ongoing within some of the subprojects to explain observations of various kinds. A model has been developed to explain the historical earthquake sequence of the South Iceland seismic zone. This is a simple model assuming that the earth is a homogeneous halfspace and the plate divergency is constant, and that all the strain energy or stress build-up comes from the plate motion. The stress build-up in elastic lithosphere caused by magma upwelling from a medium with different rheological parameters has been modelled and studied with respect to earthrealistic conditions. A model has been proposed, based on modelling results and observations, which assumes that a significant part of the stress build-up before earthquakes comes from heat energy from the mantle. It is probable that basaltic fluids extracted from the Iceland mantle plume at depth of less than 100 km play a significant role, not only in triggering earthquakes in Iceland but also a significant role in the stress build-up. Work has started to develop such a model.

• Most significant extension of the observational network is that continuous monitoring of deformation has been initiated in Iceland by the installation of continuous GPS at 4 sites in an area of high seismic activity in SW Iceland, the Hengill-Ölfus area (Figure 2). These four stations are linked to observations of two former continuous GPS stations, which create a reference base for the local deformation monitoring. Observations and research within the PRENLAB-2 project of this activity, and the need to understand what is going on in this area, made it possible to obtain funds to start this innovating work. On the other hand the continuous GPS measurements provide new constraints in using the activity in this area as a basis for modelling earthquake processes.