Geodynamic signals detected by geodetic methods in Iceland

The geodynamics laboratory provided by Iceland’s position on an active mid-ocean ridge has been recognized for several decades. Geodetic experiments have been designed and carried out in Iceland since 1938 to verify various global geodynamic theories, such as Wegener’s theory of continental drift, the sea floor spreading hypothesis, plate tectonics, mantle plumes etc. State-of-the-art techniques have been used to obtain data on crustal displacements with ever increasing accuracy to constrain the theories. Triangulation and optical levelling were used in the beginning, later EDM-trilateration. Network GPS surveying began in 1986 and has been used extensively since then to study crustal movements. With the addition of InSAR and continuous GPS in the last decade we have made a significant stride towards the goal of giving a continuous representation of the displacement field in time and space. The largest and most persistent signal is that of the plate movements. Geodetic points in East and West Iceland move with the Eurasia and North America Plates, respectively, and the vectors are consistent with global models of plate movements. The plate boundary zones are a few tens of kilometers wide, within which strain accumulates. This strain is released in rifting events or earthquakes that have a characteristic displacement field associated with them. In the Krafla rifting episode in 1975-1984 a 100 km long section of the plate boundary in North Iceland was affected and divergent movement as large as 8-9 m was measured. The June 2000 earthquakes in the South Iceland Seismic Zone were the most significant seismic events in the last decades. Two magnitude 6.5 earthquakes and several magnitude 5 events were associated with strike-slip faulting on several parallel faults along the transform-type plate boundary. Slow post-rifting and post-seismic displacements were detected in the months and years following these events, caused by coupling of the elastic part of the crust with the visco-elastic substratum. Viscosities in the range 0.3-30 x 1018 Pa s have been estimated from the time-decay of these fields. Similar values are obtained from crustal uplift measured around the Vatnajökull glacier due to the reduced load of the glacier in the last century. Magma movements in the roots of volcanoes are reflected by deformation fields measureable around them. The volcanoes inflate or deflate in response to pressure increase or decrease in magma chambers, and intrusive bodies are revealed by bulging of the crust above them. The most active volcanoes in Iceland, Katla, Hekla, and Grímsvötn, appear to be inflating at the present time, whereas Krafla and Askja are slowly deflating. An intrusion episode was documented near the Hengill volcano in 1994-1998 and two intrusion events occurred in the Eyjafjallajökull volcano in 1994 and 1999, all of which were accompanied by characteristic deformation fields.