Gravity changes in connection with the volcanic and earthquake activity in Northern Iceland 1975/76. (Þyngdarbreytingar tengdar eldsumbrotum og jarðskjálftum á Norðurlandi 1975/76)

In December 1975, a fissure eruption occurred at Leirhnjúkur, northern Iceland, whicli was followed by earthquake activity in the region north of Lake Mývatn.In orcler to detect associated gravity changes along the high precision gravity profile Akureyri-Mývatn-Grímsstadir, being established in 1938 and reobserved in 1965, 1970 and 1975 for the investigation of secular gravity variations, 48 stations of this profile have been reoccupied in 1976.This gravity survey, covering the region between Másvatn ancl Jökulsá á Fjöllum, and accompanied by a technical levelling, has been supplemented by the reobservation of about 50 gravity stations in the area between Mývatn-Grímsstadir and Axarfjördur, established between 1965 and 1970.As one result, a gravity increase ) of 0.04 mgal lias been found at Námaskard, 8 km soutli of Leirhnjúkur, being correlated with a ground subsidence of 0.3 m.Gravity variations increase to + a few tentlxs of a mgal, when approaching Krafla.Significant changes (-0.1 . . .+0.1 mgal) also occurred at Vestursandur near the coast line and in tlie southward adjoining fissure zone, while in the Gjástykki fissure system a relative gravity increase of 0.2 mgal has been observed.* Identification uncertain because of buiiding-site, point reconstructed.** Identification, uncertain, errors of the order >0.2 mgal cannot be excluded.