Gravity and elevation changes at Askja, Iceland

Ground tilt measurements demonstrate that Askja is in a state of unrest, and that in the period 1988 - 1991 a maximum 48 +/- 3 µrad tilt occurred down towards the centre of the caldera. This is consistent with 126 mm of deflation at the centre of the caldera with a 2.5 - 3.0 km depth to the source of deformation. The volume of the subsidence bowl is 6.2 x 106 m3. When combined with high precision microgravity measurements, the overall change in sub-surface mass may be quantified. After correction for the observed elevation change using the free air gradient of gravity measured for each station, the total change in mass is estimated to be less than 109 kg. A small residual ground inflation and net gravity increase in the eastern part of the caldera may be caused by dyke intrusion in this region. The minimum dimensions of such an intrusion or complex of intrusions are 1m width, up to 100m deep and up to several hundred metres thick

Subsidence events in the Krafla area: Preliminary report based on tilt and distance measurements

In the inflation-deflation sequence of the Krafla magma chamber since its beginning in 1975, 10 deflation or subsidence events have been identified until July 1978. The tilt and distance measurements relating to these subsidence events are discussed in some detail. All of these subsidence events are associated with horizontal magma flow along the N-S trending fissure zone, which goes through the central part of the suggested Krafla caldera to form a dike 3 to 5 meter wide, 80 km long and 1.0 to 2.5 km high from the bottom to the top. The total volume of magma, which flowed out of the Krafla magma chamber during these 10 events, is estimated as 392 x 10{sup 6} m{sup 3}, whereof 318 x 10{sup 6} m{sup 3} are estimated to have flowed northwards, 72 x 10{sup 6} m{sup 3} southwards and about 2 x 10{sup 6} m{sup 3} came to the surface as basaltic lava

Earthquakes in Iceland during the years 1956, 1957 and 1958

During this period, three seismograph stations were operated in Iceland, at Reykjavík, Akureyri and Vík. A new seismograph station was erected at Kirkjubæjarklaustur (Síða) in 1958, where one vertical Willmore seismograph is operated. The seismic activity in South Iceland was rather low during these years, but higher activity was observed in North Iceland. Earthquakes were felt in Iceland on 6 days during 1956, II days during ]957 and 9 days during 1958. The most pronounced felt earthquakes occurred as follows: 1956. June lst. in Southwest Iceland, maximum intensity VI, radius of perceptibility about 70 km. Ociober 29th in North Iceland, maximum intensity V, radius of perceptibility about 200 km. Epicenter off the coast. 1958. September 27th in North Iceland, maximum intensity VI, radius of perceptibility about 80 km. Desember 6th in North Iceland, maximum intensity IV, radius of perceptibility about 100 km. Epicenter off the coast. After the foundation of the seismograph stations at Akureyri, Vík, and Kirkjubæjarklaustur, the epicentre of Icelandic earthquakes can be determined with some accuracy. The epicentres seem to lie in severai smali regions, where some are known by frequent felt earthquakes, but other were unknown until recently. Two of these previously unknown epicentral regions have been very active in recent years, one in northwestern Vatnajökull (64.4°—64.8°N, 16.5°—18°W), and another around Torfajökull (63.7°-64.0°N, 19.0°-19.5°W). Some 25-30% of all Icelandic earthquakes during the period 1954 to 1958 have occurred in the first of these regions, and about 10% in the second region

Observed ground deformation during the Krafla eruption of March 16, 1980

International Symposium on the Activity of Oceanic Volcanoes. Ponta Delgada, 4-9 August 1980.The Krafla volcano has been monitored with continuously recording tiltmeters and seismometers and frequent geodetic measurements since the beginning of the present episode of activity in 1975. The inflation-deflation sequence of the volcano showed striking regularity in 1977 and 1978 but it became more erratic in late 1979 with slow inflation interrupted by small deflations. The eruption of March 16, 1980 was preceded by rapid deflation which started at 15:15 (GMT) and intense volcanic tremor started simultaneously. The subsidence became very rapid at about 16:00, about three times more rapid than had been seen in any previous subsidence event. The eruption was first seen at about 16:20, but it may have started 10 to 20 minutes earlier. It lasted until about 22h that same night. The deflation of the volcano ceased at about 03h next morning, March 17, and a new inflation started within a few hours. Tilt observations indicate that roughly 30 x 106m3 of magma left the Krafla magma chamber, but only some 10 % of this carne to the surface as very fluid basaltic lava. New fissures extended through the Krafla volcano over a distance of a:bout 21 km and the widening od' the fissure zone was about 1.5 m.info:eu-repo/semantics/publishedVersio

Earthquakes in Iceland 1953

The seismic activity was relatively low in Iceland during this year. No large shock occurred, and no damage was done by earthquakes. The largest shock came on February 10th, with epicenter off the north coast (about 66°40'N, 17°0'W), origin time l ^ e ^ S l 8 GMT, and instrumental magnitude about 4%. This shock was felt over a wide area, up to a distance of about 150 km from the epicenter, but the intensity was low, maximum IV degrees (Modified Mercalli Intensity Scale of 1931). On August 20t h to 22nd came a swarm of small shocks originating at Mt Hengill (about 64°05'N,21°16'W). The largest shock of this swarm, at 13Mlm on 20th was felt at Reykjavík (30—35 km distance), and in Hveragerði and Hveradalir many shocks were felt with the maximum intensity of about V degrees. In Reykjavík, the seismometers recorded more than 120 shocks during this swarm. Earthquakes were felt on 14 days during the year, and sometimes more than one shock were felt on the same day. On seven days, earthquakes were felt in Southwest-Iceland, six days in North- or Northeast-Iceland, and one small shock (somewhat doubtful) was felt in Northwest-Iceland

Earthquakes in Iceland during 1954 and 1955

The seismograph station at Reykjavik was operated with three short period Sprengnether seismographs during these years. New stations were erected at Akureyri (65°40.3' N, 18°06.0' W) and at Vík (63°25.3' N, 19°01.0' W). At both these stations, one Mainka seismograph is operated, static magnification about 70. The seismic activity in Iceland was rather high during these years, especially during the period September 1954 to May 1955. At Reykjavík about 500 tremors were recorded during 1954 and about 365 during 1955. Of these, about 350 in 1954 and 300 in 1955 were of local origin, epicentral distance less than 500 km. At Akureyri about 90 earthquakes were recorded during 1954 and about 70 during 1955. Earthquakes were felt on at least 21 days during 1954, three days in Northlceland and 18 days in Southwest-Iceland. During 1955 earthquakes were felt on at least 37 days, 24 days in North-Iceland and 15 days in Southwest- Iceland. The most pronounced earthquakes occurred as follows: 1954. September 15th. About 10 shocks were felt at Grindavík. October 29th. Numerous shocks felt at Hveragerði and Hveradalir. The largest shocks were felt over area of about 7000 km-. October 30th. One shock felt over the same area as on October 29th. November 21st. Several earthquakes felt at Hveragerði and Hveradalir. Largest shocks felt over the same area as on October 29th. 1955. January 15th. Considerable earthquakes at Grindavík. Macroseismic area of the largest shocks about 30000 km2. January 18th. Earthquakes felt over 8000 km- area in South-Iceland. February 27th. Strong earthquakes in Axarfjörður, North-Iceland, where tremors were felt on almost all days untill March 20th. Macroseismic area of the largest shocks about 30000 km2. In the epicentral area about 200 shocks were felt during this swarm, the maximum intensity beeing VII on modified Mercalli scale. Minor damage was done in the epicentral region. March 13lh. Strong earthquake in South-Iceland, felt over 10000 km2. April lsl. Strong earthquakes at Hveragerði, where minor damage was done. The intensity of the largest shock was VII—VIII in the epicentral region, and macroseismic area about 60000 km2. This was the largest shock occurring in Iceland during the years covered by this report, instrumental magnitude about 5i/£. May 19th. Earthquake felt over large area in North-Iceland, maximum intensity about V and macroseimic area about 60000 km2. The epicenter was situated off the north coast, east of Grímsey