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The Hengill triple junction sits between the Reykjanes Peninsula rift zone, the Western Volcanic rift zone and the South Iceland Seismic (transform) zone, in southwest Iceland. It consists of three volcanic systems, Hengill, Hrómundartindur, and Grensdalur, from west to east respectively. Mt. Hengill is the present center of volcanic production, Mt. Hromundartindur's last eruption occurred in early post-glacial times, and the Grensdalur system is considered extinct. A period of anomalous earthquake activity began near Mt. Hrómundartindur, centered beneath the Ölkelduháls geothermal field, in April, 1994. Between 1994 and 1998 over 80,000 earthquakes occurred in the area. This seismicity was associated with uplift of the area above a point source of pressure, interpretted as magma accumulating under Mt. Hrómundartindur volcano (Sigmundsson, et al., 1997). Activity culminated in a M = 5.1 earthquake on June 4, 1998 and an M = 5 earthquake on November 13, 1998. I have been using differential GPS to map surface effects from the earthquakes and uplift and have established a GIS database of the area to integrate geologic, geodetic and geophysical data. We are trying to understand why earthquake distribution during this time period was so asymmetric with respect to the center of volcanic uplift, and to determine whether surface effects of crustal deformation were focused along pre-existing weaknesses in the crust. |
