Tsunami Hazards in Iceland

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1 Tsunami Hazards in Iceland Byggingafræðifélag Íslands 23 January 2012 Matthew J. Roberts Veðurstofa Íslands With contributions from: Kristín S. Vogfjörð, Esther Hlíðar Jensen, Jón Kristinn Helgason, and Einar Kjartansson Contact:

2 Background: TRANSFER research project European funding under the Sixth Framework Programme (FP6). TRANSFER: Tsunami Risk and Strategies for the European Region. Project led by Prof Stefano Tinti, University of Bologna. Twenty nine research partners involved between 2006 and Project web-site: IMO involved in research on seismicity and mass movements. 2

3 Background: Tsunami physics A tsunami is a series of waves (wave-train) generated in a body of water by an abrupt vertical displacement of the water column. Earthquakes, landslides, volcanic eruptions, and submarine mass-movements all have the potential to generate tsunamis. A tsunami can have a wavelength in excess of 100 km and a period on the order of one hour. Tsunamis behave as shallow-water waves, moving at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m s 1 ) and water depth. The rate at which a tsunami loses its energy is inversely related to its wave length. High speed > great distances > limited energy loss. 3

4 Known tsunamis in Iceland Generation mechanism Tsunami extent Example Offshore earthquake Local to regional Flatey, 1872 Inland earthquake Local (confined to lakes and rivers) Ölfus, 1896 Coastal rockfall Local No historic example Inland rockfall Local (confined to lakes and rivers) Steinholtshlaup, 1967 Snow avalanche Local Suðureyri, 1995 Jökulhlaup Local to regional Kötluhlaup,

5 Sedimentary evidence of the Storegga tsunami One of the largest mass movements to have occurred during the Holocene. The slide originated from a 20,000 km 2 area at the edge of the Norwegian continental shelf. Estimates of the volume of displaced material range from 2,400 to 3,200 km 3. The displaced sediments are thought to have been mobilised about 7,250 ±250 C 14 yr BP. 5

6 Possible seismic sources 6

7 Seismicity in the Tjörnes Fracture Zone (TFZ) The Tjörnes fracture zone (TFZ) links the Northern volcanic zone to the Kolbeinsey Ridge. 7

8 Tsunamis in the Tjörnes Fracture Zone Large earthquakes in the TFZ are known to have generated minor tsunamis. For instance, in June 1934 an M s 6.2 earthquake occurred in Eyjafjörður, near to the village of Dalvík in Northern Iceland (Thorarinsson, 1937). Sailors in the fjord at the time of the earthquake reported seeing a large series of waves, which lifted ships high above the surrounding sea. There are also indications that a tsunami was observed on Hrísey a small island about 6 km north-east of Dalvík. 8

9 TFZ: Hazardous offshore seismicity in the 18 th century Date Longitude Latitude Magnitude Tsunami? Likely Unknown Unknown Yes Unknown (M s ) Unknown (M s ) Yes (M s ) No (M s ) No 9

10 Rockfall-triggered tsunamis Since the beginning of the Holocene, 30 coastal rockfalls, ranging in volume from m 3, have occurred (Hjartarson, 2006). From the dimensions of the greatest deposits, some rockfalls would have impacted large expanses of water. 10

11 Rockfall-triggered tsunamis To date, the Víkurhólar rockfall is one of the largest Holocene massmovements on Iceland's coast (Hjartason, 2006). The rockslide took place ~7,000 C14 yr BP, and it undoubtedly gave rise to a tsunami (Davíðsdóttir, 2008). The total volume of the rockslide is estimated at ~ m3, with ~ m3 (23%) of fragmented rock entering the sea (Davíðsdóttir, 2008). 11

12 Estimated parameters of the early Holocene Víkurhólar rockslide Rockslide parameters Tsunami variables Slide width 1,600 m Wave formation time 33 s Slide volume 12,000,000 m3 Wavelength 350 m Slide velocity 30 m s 1 Amplitude 4.3 m Stillwater depth 45 m Subaqueous run-out 700 m Source: Davíðsdóttir (2008) 12

13 Historical coastal floods Wave height (m) Date Location Eyjafjöll Heimaey, Grindavík, Þorlákshöfn Húsavík, Flatey Flatey, Skjálfanda ~1 Localised flooding 3 Generated by a M 6.5 earthquake Ölfusá ~5 Localised flooding 4 River seiche generated by a M 6.9 earthquake Heimaey ~1 Boat damaged 5 Volcanogenic jökulhlaup from Skeiðarárjökull Stokkseyri, Eyrarbakki Damage to sea defence 1 Flooding during calm conditions Bolungarvík Unknown 1 Flooding during calm conditions Eyjafjörður Unknown 6 Generated by a Ms 6.2 earthquake ~1 Damage? Ref. Remarks Boats damaged 1 NA Barrels washed into the sea at Heimaey 2 Volcanogenic jökulhlaup from Mýrdalsjökull Generated by a M 7 earthquake Unknown ~1 13

14 Avalanche-triggered tsunamis Since 1839 one debris-flow and at least 14 snow avalanches have generated tsunamis in Iceland. These mass flows are source from only three fjords: Súgandafjörður in the West fjords (11), Siglufjörður in the North (2), and Mjóifjörður in the East (1). At Súgandafjörður, all but one tsunami-generating avalanche originated from the same hillside, known as Norðureyrarhlíð. Tsunami waves induced by these avalanches have repeatedly damaged the coastal village of Suðureyri, located less than a kilometre from the base of Norðureyrarhlíð on the opposite site of the fjord. 14

15 Avalanche-triggered tsunamis Súgandafjörður 26 October 1995 The avalanche was sourced from an 800-m-wide channel at ~500 m a.s.l., where compacted snow ranged 2 3 m in thickness. It is estimated that the avalanche extended 200 m into the fjord, from which a 10-m-high wave spread to nearby Suðureyri (Ágústsson and Sigurðsson, 2004). The destructive power of the wave was such that it destroyed a swimming pool, damaged sea defence structures, sank a six-ton ship moored in the harbour, and killed 30 sheep. 15

16 Volcanic sources Elevation profiles of Iceland's active coastal volcanoes. Snæfellsjökull has the steepest profile, averaging a slope of 28%. Four active, ice-capped volcanoes lie within 10 km of the Icelandic coast. 16

17 Volcanic sources 1721 eruption of the Katla: jökulhlaup entered the sea with sufficient hydraulic force to create a tsunami. Coastal flooding was reported from the villages of Þorlákshöfn and Grindavík. 17

18 Volcanic sources Submarine topography south of Mýrdalsjökull, as revealed by multi-beam sonar data. Submarine canyons represent the erosive effects of repeated, jökulhlaup-induced turbidity currents from Mýrdalsjökull. 18

19 Volcanic sources Date Location Mode of generation Remarks Reference Mýrdalssandur Flood entered sea Jökulhlaup due to subglacial volcanism Þórarinsson (1975) Flood entered sea Jökulhlaup due to subglacial volcanism Þórarinsson (1974) Rockfall into lagoon Displacements confined to a lagoon and a river Kjartansson (1967) Skeiðarársandur Eyjafjallajökull Historical jökulhlaup thought to have caused a locally damaging tsunami. 19

20 Inundation simulations As a first step, outlines of coastal towns in south-west Iceland were overlaid on a digital elevation model (DEM). Contours at 5 m intervals were then applied to these regions to illustrate zones of potential flooding. Such an approach often referred to as a 'bathtub' inundation model allows simple assessments of lowlying areas without the need to consider dynamic coastal influences. 20

21 Inundation simulations Static simulation of coastal flooding due to a hypothetical tsunami. Two wave heights are used: 5 and 9 m. The symbols denote dwellings sited at or below 9 m a.s.l. The nationwide accuracy of the DEM varies according to location, with spatial errors ranging 2 10 m (ÍSOR, 2008). 21

22 Inundation simulations A 4-m-high wave superimposed on a high spring tide could easily ingress parts of Reykjavík. In the Ölfus region, a 5-m-high wave would inundate several coastal towns and inland villages. It is noteworthy that 15% of Iceland's population (~49,000 inhabitants) live within a 5-m-elevation range of MSL. A far-field tsunami due to volcanic activity at Snæfellsjökull is a possible but highly improbable hazard; nevertheless, assessments of such underrated hazards should be made. 22

23 Mass movements either reputed or known to have generated tsunamis Note that the sites of possible submarine slides are approximated from SIL seismic data between 1994 and It is unknown whether submarine slides from the Icelandic Shelf have caused tsunamis; however, offshore sediment accumulation from glaciofluvial sources, combined with the bathymetry of the shelf, make such failures probable. 23

24 Conclusions Tsunamis represent hazards of low probability but extreme consequence, particularly for catastrophic processes such as the collapse of a volcanic flank. The inundation maps presented here emphasise that many settlements in Iceland are susceptible to wave action. Icelandic tsunamis can be generated via one of three principle mechanisms: (i) water disturbances due to seismicity; (ii) impulsive impacts at the coastline or into a lake due to rockfalls and (iii) ingress of mass flows into the sea or an enclosed body of water. 24

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