Jökull


Jökull - 01.01.2019, Page 51

Jökull - 01.01.2019, Page 51
Einarsson quake appears to be almost simultaneous with the be- ginning of the eruption. In one instance (1947), the first felt earthquake occurred even a few minutes after the initial explosion. Eruptions from the Hekla vol- canic system outside the main edifice behave differ- ently. They are effusive and are accompanied by in- tense seismicity. The precursor times of the eruptions of 1554, 1725, 1878 and 1913 were more than three hours, in one case possibly more than two weeks. The short precursor times of the edifice eruptions are in accord with the instrumental observations of the last four decades (Einarsson, 2018) that show exception- ally short precursor times of Hekla, if compared to other volcanoes in Iceland. Both historical eruptions of Öræfajökull, in 1362 and 1727, appear to have been preceded by felt earth- quakes. Documentation on the very large explosive eruption of 1362 is in the form of legends, but archae- ological data suggest that the population may have been prepared for the cataclysmic phase of the erup- tion. The moderately large eruption of 1727 was clearly preceded by earthquakes by at least one day. Major rifting episodes in th Icelandic rift zones, like the Mývatn Fires episode of the Krafla system in 1724–1746, the Laki episode of the Grímsvötn sys- tem in 1783–1784, and the Sveinagjá episode of the Askja system in 1874–1876, were accompanied by earthquakes felt in the neighboring areas but their temporal relationship to beginning eruptions or indi- vidual rifting events within the episodes is unclear, mostly due to remoteness. The known cases are, how- ever, entirely compatible with experience of recent episodes that have been instrumentally observed, like the Krafla episode of 1974–1989 and Bárðarbunga rifting in 2014–2015. Leading a modern society in a geologically active country like Iceland, it is of paramount importance to be able to issue warnings of potentially catastrophic events like volcanic eruptions. A monitoring system of geophysical instruments has been successfully ap- plied for this purpose in the last few decades (Einars- son, 2018). But historical documents also contain im- portant information on events in the past, before sen- sitive instruments were installed. This information is particularly important for Katla and Öræfajökull, two of Iceland’s most feared volcanoes, for which no in- strumental data of pre-eruption behaviour exist. Acknowledgements This work made extensive use of the historical studies of Þorvaldur Thoroddsen, Sigurður Þórarinsson and many others, and in particular the recent book of Már Jónsson. Ásta Rut Hjartardóttir made the maps. Con- structive reviews of Jurgen Neuberg and Martin Hen- sch improved the paper considerably. ÁGRIP Í eldfjallalandi skiptir miklu að hægt sé að gefa út viðvaranir til íbúa um yfirvofandi eldgos. Til þess að gefa út skammtímaviðvaranir þarf að gera mæl- ingar og bera kennsl á óvenjulega virkni í aðdraganda goss. Eldstöðvar á Íslandi eru fjölbreytilegar að gerð og gjósa auk þess í fjölbreytilegu umhverfi, á landi, sjávarbotni eða undir jökli. Þessi fjölbreytni býður upp á óvenjugóð tækifæri til rannsókna á aðdraganda og byrjunarfasa eldgosa. Nýleg rannsókn og saman- tekt á mælingum á gosum á Íslandi síðan næm mæli- tæki komu til sögunnar, sýnir að öll gos sem urðu á síðustu fjórum áratugum áttu sér mælanlegan aðdrag- anda. Við hagstæðar aðstæður var hægt að bera kennsl á þessa forboðavirkni og gefa út viðvörun til fólks í grenndinni. Það var gert í 14 tilfellum af 21. Tals- verðar upplýsingar eru einnig til hér á landi í rituðum heimildum frá fyrri öldum, áður en mælingar komu til sögunnar. Stór hluti upplýsinganna varðar virkustu eldstöðvarnar, einkum þær sem næstar eru byggð, það er, Kötlu og Heklu. Jarðskjálftar fundust á undan öll- um staðfestum gosum Kötlu síðan 1625. Skjálftarnir fundust einum til níu klukkustundum áður en sást til goss, og frá tveimur til meira en tólf klukkustundum áður en hlaup náðu til byggðra bóla. Hegðun Heklu er með talsvert öðru móti. Gosum í Heklu sjálfri, sem orðið hafa síðan 1510 fylgdi yfirleitt fremur væg skjálftavirkni. Skjálftar fundust oftast fáum mínút- um áður en sást til goss, í gosinu 1947 jafnvel nokkr- um mínútum eftir að fyrsta sprengingin sást. Annað er uppi á teningnum þegar gos verða í eldstöðvakerfi Heklu utan Heklu sjálfrar. Slíkum gosum, t.d. 1878 og 1913, fylgdi talsverð skjálftavirkni sem byrjaði meira en þremur tímum áður en gos hófst. Gosin tvö í Ör- æfajökli á sögulegum tíma, 1362 og 1727, áttu sér lík- 50 JÖKULL No. 69, 2019
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