Crustal deformation near Hengill volcano, Iceland 1993-1998: Coupling between magmatic activity and faulting inferred from elastic modeling of satellite radar interferograms

Tectonic activity in the Hengill volcanic area in southwestern Iceland acce lerated in July 1994, when an unusually persistent swarm of moderate-sized earthquakes began. Although the largest events were magnitude 5, the patter n of upward crustal deformation at 2 cm/yr indicates that most of the activ ity is related to inflation of a magma chamber at depth. To monitor this ac tivity, we analyze synthetic aperture radar (SAR) images acquired by the ER S-1 and ERS-2 satellites between July 1993 and September 1998 using interfe rometry. Interferograms composed of images acquired during the snow-free su mmer months remain coherent on Holocene lava hows, even after 4 years. Some of the interferograms show a discontinuity in the fringe pattern, which we interpret as 8 mm of (aseismic) dip slip on a 3-km-long segment of a N5 de greesE striking normal fault, part of which had been mapped previously. Thi s slip must have occurred between July 31 and September 3, 1995 (inclusive) , and has been confirmed by observations in the field. The predominant sign ature in all the interferograms spanning at least I year, however, is a con centric fringe pattern centered just south of the Hromundartindur volcanic center. This we interpret as mostly vertical uplift caused by increasing pr essure in an underlying magma source. The volume source that best fits the observed interferograms lies at 7 +/- 1 km depth and remains in the same ho rizontal position to within 2 km. It produces 19 +/- 2 mm/yr of uplift. Thi s deformation accumulates as elastic strain energy at a rate 2.8 times the rate of seismic moment release. Accumulated over 5 years, it increases the Coulomb failure stress by > 0.6 bar in an area that includes some 84% of th e earthquakes recorded between 1993 and 1998. Under our interpretation, mag ma is injected at 7 km depth, just below the seismogenic zone formed by col der, brittle rock. There the inflation induces stresses that exceed the Cou lomb failure criterion, triggering earthquakes, possibly in a cyclical fash ion.