ACTIVE DEFORMATION DETERMINED FROM GPS AND SAR

Subproject 3.6 (A and B)

Freysteinn Sigmundsson
Nordic Volcanological Institute
University of Iceland
Grensasvegur 50
IS 108  Reykjavik
Iceland

Phone +354 525 4494
Fax   +354 562 9767
fs@norvol.hi.is

MILESTONES:

SAR interferometry study of the Reykjanes Peninsula has provided new results on plate spreading and volcano deflation in the area, and the study shows as well that the SAR technique is well suited to monitor crustal deformation in Iceland. A paper on the subject has been accepted in Science and will appear in few weeks time, and the results will be presented in two papers at the AGU meeting this December. Below is a summary:

We have used satellite radar interferometry to map the satellite-view component of a crustal velocity field, as well as volcano deformation, at the Reykjanes Peninsula in SW-Iceland. The area is the direct onland structural continuation of the submarine Mid-Atlantic Ridge. Oblique spreading between the North American and Eurasian plates of 1.9 cm/yr occurs there, causing both shearing and extension across the plate boundary. Using ERS-1 images from the 1992-1995 period we have formed interferograms, spanning up to 3.12 years. Coherence is preserved, and time-progressive fringes caused by crustal deformation are apparent. The most obvious deformation is time-progressive deflation of the Reykjanes central volcano, averaging to 15 mm/yr, probably caused by compaction of a geothermal reservoir in response to it's utilization by a power plant. The deflation we infer is in good agreement with leveling data. This gives confidence in the interpretation of more subtle deformation signal in the interferograms, fringes aligned in the direction of the plate boundary caused by plate boundary deformation. Relying partly on geologic evidence we assume the shape of the horizontal and vertical crustal velocity field. We estimate best-fit model parameters by maximizing the global coherence of the residual interferograms, the difference between observed and model interferograms. The data constrain the locking depth of the plate boundary to be about 5 km. Below that level the plate movements are accommodated by continuous ductile deformation, not fully balanced by inflow of magma from depth, causing about 6.5 mm/yr subsidence of the plate boundary. Previous regional geodetic data agrees with this interpretation.

The GPS geodesy project is still in its first phase, the planned installation of 3 GPS receivers in a semi-continuous mode in the South Iceland Seismic Zone has not been completed. Automatic GPS data collection with the 3 available Trimble 4000 SST instruments has been tested, and work has been done to automate the data analysis. Although continuous GPS has not been realized yet, an interpretation of older GPS measurments in the Hengill area has shead light on the 1994-1995 swarm of earthquakes there. A paper on the subject has been submitted to JGR, and these new results were presented at the EGS meeting in The Hague in May 1996. Below is a summary:

Since July 1994 an unusually persistent swarm of earthquakes (M < 4.0) has been in progress at the Hengill triple junction, SW Iceland. Activity is clustered around the center of the Hrómundartindur volcanic system. Geodetic measurements indicate a few cm uplift and expansion of the area, consistent with a pressure source at 6.5±3 km depth beneath the center of the volcanic system. The system is within the stress field of the south Iceland transform zone, and majority of the recorded earthquakes represent strike-slip faulting on subvertical planes. We show that the secondary effects of a pressure source, modelled as a point source in an elastic halfspace, include horizontal shear that perturbs the regional stress. Near the surface, shear stress is enhanced in quadrants around the direction of maximum regional horizontal stress, and diminished in quadrants around the direction of minimum regional stress. The recorded earthquakes show spatial correlation with areas of enhanced shear. The maximum amount of shear near the surface caused by the expanding pressure source exceeds 1 mstrain, sufficient to trigger earthquakes if the crust in the area was previously close to failure.

PUBLICATIONS:

H. Vadon, and F. Sigmundsson, 1992-1995 Crustal deformation at the Mid-Atlantic Ridge, SW Iceland, mapped by radar interferometry, Science, in press, 1996.

Sigmundsson, F., P. Einarsson, S. Th. Rognvaldsson, G.R. Foulger, K. M. Hodgkinson, G. Thorbergsson, 1994-1995 seismicity and deformation at the Hengill triple junction, Iceland: Triggering of earthquakes by a small magma injection in a zone of horizontal shear stress, J. Geophys. Res., in review, 1996.

Vadon, H., and F. Sigmundsson, Plate movements and volcano deformation from satellite radar interferometry: 1992-1995 crustal deformation at the Mid- Atlantic Ridge, SW Iceland, EOS Transactions American Geophysical Union, 77, AGU 1996 Fall Meeting abstracts.

Sigmundsson, F., and H. Vadon, Oblique spreading at the Mid-Atlantic Ridge, SW- Iceland: 3D crustal velocity field inferred from satellite radar interferometry and geologic evidence, EOS Transactions American Geophysical Union, 77, AGU 1996 Fall Meeting abstracts.

Sigmundsson, F., Inversion of geodetic data from Hengill triple junction, Iceland: Rift subsidence and volcano inflation, paper presented at the 21th General Assembly of EGS in the Hague, May 1996, Annales Geophysicae, Supplement I to vol. 14.

Sigmundsson, F., P. Einarsson, S. Rognvaldsson, G. Foulger, and G. Thorbergsson, 1994-1995 earthquake swarm at Hengill triple junction, Iceland: Triggering of earthquakes by an inflating magma chamber in a zone of horizontal shear stress?, paper presented at the 21th General Assembly of EGS in the Hague, May 1996, Annales Geophysicae, Supplement I to vol. 14.