Ragnar Stefánsson, Kristján Ágústsson.
Continuous measurements of volumetric strain in boreholes has been carried out since 1979 at 7 sites in and around the SISZ. The most significant results are the observations of slow strain changes related to 2 eruptions in the volcano Hekla and to an earthquake of magnitude 5.8 on the prolongation of the SISZ south of the volcano Hekla. In both cases strain signals observed to a distance of up to a few tens of kilometers could be used for modelling volcano and earthquake related processes with duration of hours to a few days. with a few tens of hours duration.
More long lasting changes are seen in the strainmeter monitoring records after careful cleaning of athmospheric and tidal effects. However it has so far not been possible to verify these changes by other measurements. Increased compression is observed in the SISZ after 1990, lasting for one or two years. It has benn suggested that a sequence of seismo/volcanic epizodes between Reykjanes peninsula and Hekla in this time period is related to this wave of increased compression. There are also indications for increased compressive strain rate during early 1996 to the north of SISZ and simultaneous decreased strain to the south of it, while no change is seen in the SISZ itself. At the same time increaed compression was inferred from observations of shear wave splitting near the SISZ, which has been related to dyke injection in in the Vatnajökull area in the preparation stage of the Vatnajökull eruption of October 1996.
A program for monitoring radon in borehole wells in the SISZ was initiated in 1977. Up to 9 stations were monitored and the length of the time series varies from 3-16 years. Samples were taken every 1-3 weeks. A significant correlation was found between radon signals, especially increase, and small earthquakes, i.e. the earthquakes following the signal within 6 weeks. It was observed that some measuring stations are more sensitive than others and some anomalies are observed at great distance from the earthquake which they are considered to be related to. This can possibly be explained by that both the radon anomaly and the earthquakes are related to a common source.
The radon measurements were discontinued in 1993 because of lack of funding. It is intended to start them again partly with a support from and as a part of the PRENLAB project.
A new project is now being planned in Iceland named: A search for earthquake precursors in South-Iceland geothermal reservoirs. The aim of the project is to find and to monitor possible earthquake precursors in South-Iceland geothermal systems. It is planned to install simple pressure, temperature or vibration devices near periodic eruptive geysirs or in deep geothermal wells or to collect similar signals from already operating monitoring systems of a few district heating systems in S-Iceland. It is considered that the deep roots of geothermal systems are likely to be sensitive to changes in stress conditions in the crust. There is historical evidence for such a precursors in Iceland, and P.G. Silver and N.J. Wakita, 1996 and P. G. Silver and N.J.Valette-Silver, 1992 have reported in Science changes shortly before large earthquakes in groundwater- and hydrothermal-systems.
This project is planned as a cooperation between the National Energy Agency of Iceland, which does thermal energy prospecting and research and the Icelandic Meteorological Office, which would provide monitoring facilities, which is the SIL system. The project is linked to the PRENLAB project although there is no direct input from PRENLAB funds to the project. The project is a low budget project based on good-will of the hydrothermal power companies and utilizing in some cases monitoring facilities that are already in use.