Jökull


Jökull - 01.12.1984, Side 149

Jökull - 01.12.1984, Side 149
epicenters were north or northwest of Gríms- vötn. The first two earthquakes nray be regarded as foreshocks of the M = 4.3 earthquake in Bárðarbunga on February 21st, suggesting that the January earthquakes were unrelated to Grímsvötn and the jökulhlaup. 1965 Jökulhlaup began in the end of August and culminated on September the 7th ( Rist 1973). Four Vatnajökull earthquakes (M > 3.0) were recorded in Reykjavík that year, in May and November. (Fig. 2). Apart from these quakes many smaller ones were recorded on the AKU and SID seismometers. Four smaller earthquakes with epicenters possibly in Grímsvötn coincide with the culmination of the jökulhlaup on the 8th of September i.e. at 1856 (M=2.4), 1857 (M= 2.4), 1857 (M=2.2) and 1858 (M=2.5). Accord- ing to the Seismological Bulletin (1965) they all originate in the Grímsvötn area and the bulletin comments, „some more tremor during this time”. This tremor was recorded on the SID seismo- meter and does not resemble any known volcanic tremor but the wind tremor often recorded at SID. 1972 A jökulhlaup culminated on March 23th ( Rist 1973). During February and March three Vatna- jökull earthquakes were recorded but none of them seems to originate at Grímsvötn (Fig. 2). An unusual tremor was recorded at Skammadals- hóll (SK) on March 13 1900-March 14 0500 and on March 20th at 1800-2400. The seismogram from SID on March 13 is not readable but on the 20th of March wind disturbance was recorded during the evening, especially from 1900—2000 hours, and culminating around 1200 hours on the 21 st. Helgi Björnsson and Magnús Hallgrímsson were stationed in Grímsvötn from March 28th until April the 4th. They felt earthquakes twice during the night between March 28 and 29 {Björnsson and Hallgrímsson 1976), but nothing was recorded on the seismometers at SID and SK. Helgi and Magnús operated a seismometer during their expedition in Grímsvötn. The seismometer recorded many small ice shocks (Björnsson pers. comm. 1984). According to analysis of the sediment discharge in Skeidará during the jökulhlaups in 1965, 1972 and 1976, the ratio of dark glass/light glass in the suspended sediment was exceptionally high in 1972. The coarse silt fraction became almost 90% glass with the same refractive index as the Grímsvötn volca- nic ash (Tómasson 1974, Tómasson et al. 1980). Tómasson (1974) explained this exceptional increase in the glass ratio as a result of a small subglacial eruption that lasted less than a day and produced big quantities of sand and silt that was carried away with the jökulhlaup water. Stein- thórsson and Oskarsson (1983) concluded that their geochemical studies of jökulhlaup water in 1972 supported the idea of a volcanic event. Taking into account all available seismic data collected before and after 1972 nothing supports the idea of a small volcanic event in 1972 or in later years until May 1983 when a small eruption was discovered from seismic analyses ( Einarsson and Brandsdóttir 1984). 1976 Jökulhlaup culminating on Sept. 22nd ( Rist 1976). Helgi Björnsson and Magnús Hallgríms- son were stationed in Grímsvötn from Sept. 22nd until October the 6th. They did not sense earth- quakes when the jökulhlaup was at its peak (Björnsson and Hallgrímsson 1976). No Vatna- jökull earthquakes were recorded during the four months following a big Bárðarbunga earthquake in July 1976 (Fig. 2). 1982 An unusually small jökulhlaup occurred in February (Björnsson 1983). No earthquakes (Fig. 2). CONCLUDING REMARKS Available data from the jökulhlaups and erup- tions described in this chapter is summarized in Table 2, along with all available data on the earthquake activity. Note that no reliable reports are of earthquakes felt during these events and only twice have they been accompanied by earth- quakes (M > 3.5). EARTHQUAKESAND SKAFTÁRHLAUPS According to farmers in Skaftártunga and Meðalland jökulhlaups in Skaftá and outlet rivers were frequently observed in the first half of this century ( H. Björnsson 1977). During the Skaft- árhlaup in 1955 an ice cauldron was formed about 15 km NW of Grímsfjall ( Thórarinsson and Rist JÖKULL 34. ÁR 147
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