SEISMOLOGI

Neotektonikk er studiet av bevegelser og deformasjon av jordskorpen, som er nåværende eller nyere i geologisk tid. Kartet viser jordskjelv, fokalmekanismer, spenningsmålinger, postglasiale forkastninger, landhevning, submarine skred, vulkaner, oppløft/erosjon og sedimentavleiring i det norske område. (Neotectonics is the study of motion and deformation of Earth's crust that are current or recent in geological time. The map displays earthquakes, focal mechanisms, stress observations, postglacial faults, glacial isostatic adjustment, submarine slides, volcanoes, uplift/erosion and sediment deposition in Norway and adjacent areas.) Kartet også vedlagt i NGU-rapport 2018.010

Litteraturen om neotektonikk i Fennoskandia og østlige Canada er undersøkt. Sen- eller post-glaciale forkastninger i Fennoskandia opptrer som veldefinerte, ofte lineære trinn i et ellers jevnt morenedekke eller berggrunnsoverflate. Den maksimale vertikale spranghøyde er 25 m. Den vestlige blokka er vanligvis senket. I Canada opptrer de fleste påviste forkastningene på glatteroderte berggrunnsblotninger. Disse forkastningene er vanligvis mindre enn tilsvarende i Fennoskandia. I begge områder er de reversforkastninger. De er i stor grad styrt av eldre svakhetssoner. Det er en geografisk sammenheng mellom de neotektoniske strukturene og jordskjelvaktivitet i Fennoskandia. Dette antyder at de kreftene som forårsaket forkastningen fremdeles er aktive. Retningen på forkastningene er hovedsakelig parallell de Midt-Atlantiske og Arktiske spredningsryggene. In situ spenningsmålinger og forkastningsplanberegninger fra jordskjelvregistreringer viser i de fleste tilfeller at den maksimale horisontale spenningen er vinkelrett disse spredningsryggene og at Fennoskandia og det østlige Canada er under kompresjon. Det er derfor sannsynlig at spenningen som oppstår ved havbunnspredning, er et viktig bidrag til dannelsesmekanismen for forkastningene. Utløsningen av disse spenningene kan imidlertid skyldes den postglaciale landhevingen.

The Ranafjorden area in northern Norway is a region of increased seismicity and
anomalous land uplift. There is evidence for recent movements in the bedrock;
1) A total of 0.89 m uplift of a bladder wrack mark from 1894 to 1990 in
Hemnesberget; 2) Anomalous low uplift of the islands of Hugla and Tomma in the
outer Ranafjorden area (0,0 0.06 and 0.07 m from 1894 to 1990 compared to 0.25-
0.30 m in the area to the north and the south); 3) Associated with the 1819
magnitude 5.8-6.2 earthquake in the Ranafjorden area, an uplift of a shallow
sea floor above sea level during an aftershock was reported in the bay Ut-
skarpen. During the main earthquake a major land slide occurred at the same
location. This earthquake is the largest North European near-shore earthquake
recorded in historical time; 4) An uplift of approximately 1 metre of a farm-
house in the 1870's at Båsmoen. The observation has been made relative to the
two neighbouring mountains Snøfjellet and Høgtuva. 5) Approximately 0.5 m
uplift of a boathouse in the bay Straumbotn during the last 50 years.
There are, consequently, indications of recent faulting in the Ranafjorden area
It is, however, difficult to find conclusive evidence for postglacial and
present-day movements along specific faults. We have therefore established
a Global Positioning System (GPS) network designed to measure the active
geological strain in the area.

Bleikvassli Gruber ble den 25.september 1997 utsatt for setninger og vanninn- trenging. Dette resulterte i at gruva ble fylt med vann i løpet av en uke og at en 10 meter høy vannkraftsdam tilhørende Statkraft ble ødelagt. NTNU, NORSAR og NGU har fått i oppgave av Gjensidige Forsikring å vurdere årsakene til setningen i gruva, og spesielt muligheten for at et jordskjelv som er registrert i området kan ha utløst setningen. Denne rapporten utgjør NGU's bidrag til vurderingene. Det vil bli utarbeidet en felles konklusjon fra de tre institusjonene.

Neotectonic crustal deformations have been reported at a large number of locations in Norway (both on local and regional scales). The NEONOR Project represents a national effort by several national research and mapping institutions to investigate these phenomena through a multidiciplinary approach. Both the industry and the Norwegian Research Council contribute with major financial support. The main objectives of the NEONOR. Project are to systematically collect data, and to provide answers to the questions: 1) How can recent crustal deformation be characterised in time and space? 2) What processes cause the neotectonic crustal deformations? 3) What are the implications for migration and occurrence of fluids (especi- ally hydrocarbons and groundwater) in bedrock? 4) What are the implications for geohazard related to constructing sensitive installations like pipelines, gas-terminals and hydropower-plants. The initial phase of the project includes a systematic collection and charac- terisation of known deformations. New geological, geodetic and seismological data are, however, also acquired.

This report constitutes the itinerary for a two-day field excursion to postglacial faults and rock avalanches in northern Troms and western Finnmark. Day 1 has three stops along the route from Tromsø to Nordmannvikdalen in Kåfjord, Troms. Day 2 has two localities along the Stuoragurra fault in Masi. The Nordmannvikdalen and Stuoragurra faults are part of the Lapland province of postglacial faults which consists of nine reverse faults and two normal faults in northern Fennoscandia. The faulting was most likely associated with major earthquakes with magnitudes of 7-8 on Richters scale.
The reverse Stuoragurra fault has a length of 80 km and a scarp height of maximum 7 metres. The fault is located within the regional Mierujavri Sværholt Fault Zone which is of Proterozoic age. The fault cross-cuts glaciofluvial deposits and is consequently younger than 9600 C14 yars. During 1998, two trenches were made across the Stuoragurra Fault, between Kautokeino and Masi. For the first time, the fault was directly observed in the bedrock. The fault did not penetrate the overlying glacial materials, but rather folded them, forming a blind thrust. Large liquefaction and other deformation structures were found in the glaciofluvial sediments in both trenches. Veins of angular and subangular pebbles from the local bedrock (Masi Quartzite) penetrate more than 10 metres laterally from the thrust plane and into the sediments in the footwall. It is thought that these veins were possibly injected during the fault activity. Deformational structures seen in the trench can be explained as a result of one major fault event.
The normal Nordmannvikdalen fault has a length of 1.5-2 km and a scarp height of one metre. The fault is most likely due to a deep-seated deformation, but a gravitational origin can not be ruled out. Two rock avalanches in Nordnesfjellan and Manndalen in Lyngenfjorden, northern Troms are described. The avalanches in the area were most likely triggered by the large earthquakes which were formed during the postglacial faulting.

The NEONOR project represents a national effort by several national research
and mapping institutions to study neotectonic phenomena through a
multidisciplinary approach. Information on present land uplift, seismiticiy,
rock stress and postglacial faults is compiled in a 1:3 million map of Norway
and adjacent areas. There is a good evidence for conjugate set of normal
faults perpendicular to the extensive system of NE-SW trending reverse faults
in northern Fennoscandia. (Forkortet)

Geodynamic modelling of the present crustal uplift indicates that the uplift of western Norway and northern Norway is partly due to other mechanisms that the glacioisostatic rebound. We have also deduced a new model based on the 'seismic pumping' mechanism to explain the observed correlation between land uplift and groundwater yield in Norway. Rock avalanches and landslides represent the most hazardous effects of earthquakes in Norway with its mountainous terrain, deep fjords and relatively large areas with unstable quick-clay. New seismic mini-arrays in the Ranafjorden area and the northern North Sea have sharply defined zones of increased seismicity. A total of 350 earthquakes have been detected in the outer Ranafjord area during the project period (2 1/2 years) with magnitudes up to 2.8. This is very high onshore Baltic Shield areas. The return perios of magnitude 6 and 5 earthquakes have been estimated to 1500 and 130 years respectively. The M6 earthquake in 1819 in Mo i Rana triggered several rockfalls and a landslide.

The NEONOR project represents a national effort by several national research and mapping institutions to study neotectonic phenomena through a multi-diciplinary approach. Information on present land uplift, seismicity, rock stress and postglacial faults is compiled in a 1:3 million map of Norway and adjacent areas.