Tsunamis in Sweden: Occurrence and Characteristics

In the last 13,000 years, there are 17 tsunami events recorded in Sweden. This chapter highlights the characteristics of two high-magnitude events from the deglacial period and three events from Late Holocene age.


Introduction
In Sweden, we have documented 62 high-magnitude paleoseismic events in postglacial time [1,2].Most of those occurred in subaqueous environment and 17 events set up tsunami waves [3].All the 62 events are documented by multiple criteria and most of them are very precisely dated.
With recording by multiple criteria, I mean the recording of one and the same event in faults and fractures, in sedimentary deformations, in liquefaction characteristics and spatial distribution, in earth-and rockslides, in height and extension of tsunami waves, in distribution and age of turbidites, etc.
With very precise dating, I mean dating with a resolution as to a single year (sometimes even the season of a year) in the Swedish Varve Chronology [4].Therefore, studies in those uplifted former shelf areas may help us to understand the mode of offshore deformation and the special characteristics of structures created [5,6].
Sweden with its bordering Baltic Sea and Kattegat Sea were previously thought to be tsunamifree parts of the world.In 1995, we found the first evidence of a major tsunami wave that took place in the autumn of varve 10,430 BP [7].It was soon followed by the documentation of another big tsunami event in the varve-year 9663 BP [8,9].Today, we have a list of 17 events [1,3,6].In this chapter, I will discuss five of them in terms of case studies from this part of the world.

Paleoseismics and paleotsunamis
Table 1.lists the 17 tsunami events recorded and documented in Sweden up to now [3].The corresponding spectrum of inferred wave heights is provided in Figure 1 [6].In the present chapter, five of the events will be picked out and discussed as case studies from Sweden and its surrounding seas [1].  1. Tsunami events recorded in Sweden in association with paleoseismic events [1,2].

Figure 1.
Recorded tsunami heights in the Baltic (blue) and Kattegat (green) coasts of Sweden [6].Red figures (1-5) refer to the five events here discussed with event 4 being revised in time and wave height.

The 10,430 BP event
When ice receded over the Baltic basin, it dammed an ice lake in its front.The outlet was via the Great Belt area.The ice lake is known as The Baltic Ice Lake.Due to the successive land uplift with increasing amplitude to the north, the vertical damming above the corresponding Atlantic sea level successively increased to a final damming of 29 m.Due to ice recession, the ice lake grew in horizontal extension too.When the ice margin, shortly after the end of the Younger Dryas stadial, left the northern slope of Mt.Billingen in southern Sweden, the ice lake dropped to the level of the sea.This event occurred 10,740 varves BP [7].Pack ice and meltwater discharge blocked open excess to the sea, however [8].
At the varve-year 10,430 BP (i.e., about 300 years after the drainage), the entire Baltic basin suddenly turned marine and the Yoldia Sea stage (sensu strictu) commenced [8].The forces generating this change were a high-magnitude earthquake occurring in the autumn of varve 10,430 BP and a high-amplitude tsunami wave washing the Närke Straight free of blocking ice [1,3,4,8,9].The wave height must have been in the order of 15-20 m, and several lake basins were invaded by a high wave, recorded as tsunamites of graded bedding and including planktonic marine microfossils [1].
In 1995, we got excellent, extensive and multiple sections and trenches in connection with the construction of a new motor highway and a railway some 70 km west of Stockholm [10][11][12].There were remarkable liquefaction structures, ground-shaking structures and deformed annual varves (Figure 2).Section C shows heavy liquefaction with a venting pipe mushrooming in varve 10,430 BP.This is indicative of a major earthquake event in the autumn of varve 10,430 BP, and a major tsunami event that washed the Närke Straight free of blocking pack ice and icebergs and opened the straight for ingression of marine water, turning the Baltic into the Yoldia Sea stage [1,3,4,8].The dating to the autumn of varve 10,430 BP is obtained at three sited 85 km apart [4,9].
The earthquake in the autumn of varve 10,430 BP was exceptionally large with an estimated magnitude well above M 8 [1,2,4,9].It is recorded by multiple criteria (Figure 3).The tsunami event linked to this earthquake must have been both strong and high because it flushed the Närke Straight totally open allowing marine water into the Baltic in the varve-year 10,430 BP.At the same time, several lake basins were invaded [1].The total height of the tsunami wave is estimated at 15-20 m (Figures 1 and 3).The event is likely to have set up a sequence of six waves, judging from multiple graded bedding cycles as illustrated in Figure 4 in view of [4,9].A segment of an 11 m core taken on the island of Ornö, some 30 km south of Stockholm [13].After 321 years of deposition of normal annual varved clay, the sequence is broken by a sequence of six thick beds of graded bedding (1-6), interpreted to represent a sequence of six major tsunami waves of the 10,430 BP major earthquake and tsunami event [1][2][3][4].Paleomagnetic intensity shows a high peak representing "the Gålö Geomagnetic Intensity Peak," known to have occurred in varve 10,430 BP [14].The intensity curve records six cycles of stronger (s) and weaker (w) magnetization [15].The six beds of cyclic sedimentary (and magnetization) deposition represent a bed load, deposited at a water depth of about 130 m, as a hydride between tsunamites and turbidites [3,4].
Varve 10,430 BP is recorded in numerous cores and sections in southern Sweden.It is characterized by a simultaneous change from freshwater to marine environmental conditions, but also as a thick sandy-gravelly varve including rounded clay "pebbles" from the erosion of older clay beds, i.e., a layer transported under strong forces over the old seabed in the form of a turbidite.This turbidite varve has been recorded over an area of 200 × 320 km (Figure 3).In a few sites, it is seen as a sequence of cyclic deposits (Figure 4) representing a tsunamite deposited by a sequence of tsunami waves.This called for a new model where the tsunami wave forces and the seabed turbidite transport could be understood in terms of an integrated mode of transport, erosion and deposition [3,4].Figure 5 presents this integrated model (first presented in [3]).
Figure 5. Integrated model [3,4] for the tsunamite/seismite deposition in varve 10,430 BP.The tsunami wave has a large diameter and soon starts to trim the surface of the seabed setting up clay and silt in suspension and bed load of sand and gravel transported as a turbidite.At the time of the earthquake and tsunami event, sea level was about 150 m high than today in the Stockholm region.From [3].
Tsunamis in Sweden: Occurrence and Characteristics http://dx.doi.org/10.5772/63956 Varve 10,430 BP was originally interpreted in terms of a "drainage varve" [16], an old idea now substituted by the recording of the huge earthquake in varve 10,430 BP and its extensive tsunami event [1,3,4].The spatial distribution is given in Figures 6 and 7.  ).Red dot marks location of epicentre.Red double arrows refer to the Närke Straight opened by the tsunami wave so that marine water could enter the Baltic and turn it into the Yoldia Sea stage.Red cross marks the location of two sites recording an earthquake event with liquefaction and a tsunami event occurring 67 varves after deglaciation [17].

The 9663 BP event
In the Hudiksvall area of central Sweden, there occurred a very large earthquake in the varveyear 9663 BP [1,3,4,9,12,18,19].The paleogeography of this event is very well known (Figure 9).Sea level was in the order of +224-230 m, and only minor islands stack out of the sea in front of the ice margin.It is recorded by multiple criteria [1,4,9] as illustrated in  Figure 9 provides the paleogeography of the area in the varve-year 9663 BP and the location of the 14 sites where records of the tsunami event have been recorded in the near-field region [1].The area is traversed by six separate drainage basins marked by the corresponding esker systems from the subglacial drainage.The turbidite of varve 9663 BP traverses the entire area and expands 310 km to the south.
The tsunami was probably composed of two to three waves judging from the cycles recorded (Figure 10).Site 2 (Lake Svartsjön) is the key site of 14 closely spaced cores, five C14-dates and diatom analysis [1,3].The site was deglaciated 25 years before the tsunami event.The highest coastline (HK in  Figure 11 provides a good and representative record of the stratigraphy and sequence of events in Lake Svartsjön (Site 2).After the deposition of 25 varves, a tsunami wave invades the lake basin by overflowing the sill to the east (about +228 m) and depositing a thick tsunamite spread over the entire lake basin.This calls for a minimum height of the tsunami wave of 6 m (cf.15 m at Site 3).The tsunamite has a thickness 50-60 cm and is composed of two (occasionally 3) depositional cycles of graded bedding.The diatom flora of the tsunamite represents planktonic deep-water species from the open Baltic environment of the Ancylus Lake stage.This is a useful characteristic of sandy beds deposited by tsunami waves from the sea (contrary to shallow-water beach erosion).
The Lake Källsjön records are important [1].This lake was never a part of the Baltic.The sill in-between had an original level of about +236 m [3] (Figure 12).Five cores were obtained in the lake, all including a typical tsunamite of graded bedding, indicating a forceful overflow of the sill and ingression into the lake of the 9663 BP tsunami wave.The wave must be at least 12.5 m high and overflow the sill for 700 m.Therefore, we can set the tsunami height at 13 m or rather about 15 m.The full evidence of a major tsunami event comes from the content of diatoms in the tsunamite beds, viz., a typical planktonic diatom flora of the Baltic Lake Ancylus stage [1,3].Furthermore, in today's lake, a small fish, smelt, is living, which is considered to be a relict from the Lake Ancylus stage of the Baltic, washed into the lake by the 9663 BP tsunami wave.
Figure 12.Site 3, Lake Kjällsjön, was never a part of the Baltic because it was located above the highest Baltic limit (BL) and separated from it by a sill [1,3].At the 9663 BP event, the sill was overwashed and the lake was invaded by a wave from the Baltic.The tsunami height must have been at least 13 m, probably about 15 m high.From [3].
A final example of the 9663 BP tsunami comes from the harbour of Iggesund (site 14 in Figure 9), where we had five pits and a major trench cut [1,4].The area was free-melted in 9747 BP.Then, followed the deposition on 84 annual varves.In varve-year 9663 BP, the area was affected by a strong earthquake.The glacifluvial sand and silt was heavily liquefied, and the liquefied material vented to the surface where it spread laterally by mushrooming.Some lateral spreading also occurred along the surfaces of varve beddings.In pits 4 and 5, the intraclay sand layer had the character of a tsunami bed.In pit 4, we brushed off the sand covering the clay surface beneath.The surface exposed was traversed by furrows and wave patterns (Figure 13b) indicating that the tsunamites must have been deposited by a flow from the NE, which implied from the sea towards the coast.At that stage, we had not yet located the epicentre of the paleoseismic event.Later it was determined that the epicentre of the 9663 BP event must have been located 12 km to the NE (as marked in Figure 9).Consequently, our sedimentary records in the pits at Iggesund (Figure 13) are in full agreement with this location of the epicentre.Figure 14 provides the spatial distribution of the 9663 BP tsunami event in Sweden.The extension of this tsunami even will surely expand considerably with time to sites in Finland, Sweden and the Baltic coast to the SE, where there are numerous sites of proposed transgression deposits of the Ancylus Lake dated at the same age as the tsunami.Most probably, several of these sites must be reinterpreted in terms of tsunamites [1].

The Late Holocene events
The 17 tsunami events recorded in Sweden over the last 13,000 years (Figure 1, Table 1) are fairly regularly distributed over time.In the past 4000 years, there were as many as seven events observed [3,21,22], three of those are discussed below.
This includes a revision of the previously assigned age of the Hudiksvall event; now re-dated at 2900 BP, instead of 2000 BP.This makes the two events synchronous though recorded 160 km apart.

The 2900 BP event recorded at two sites 160 km apart
Site 1 refers to a well recorded at Skålbo, north of Hudiksvall [1,3,22,23].Here, violent methane-venting tectonics set up a tsunami wave recorded in five bogs at +8, +14, +18, +23 and +38 m [1,3,9,22].Lake Dellen, 25 km to the west and today at +37 m has a submerged peat dated at about 2000 BP [3].This age was used to determine the age of the event.At 2000 BP, sea level was at +18 m, implying a wave height of 20 m [1].Consequently, the run-up must have been at least 20 m in order to invade the +38 m bog.
Later, I come to question the age determination, however.First, the Lake Dellen pounding may have nothing to do with the tsunami event.Second, we find no records of the 2900 BP event in the Hudiksvall lakes and no records of the 2000 BP event in the Forsmark lakes.Third, subsequent C14-dates of the lake deposits provide ages older than 2000 BP.Now, it seems quite clear that the Hudiksvall event should be synchronized [23] with the Forsmark tsunami event closely dated at 2900 BP.
At 2900 BP, sea level in the Hudiksvall area would have been at +26 m [23].This implies a tsunami height of at least 12 m (to deposit a tsunamite in the +38 m bog).The tsunami wave had a trimming effect of the seabed (i.e., erosion) down to at least +8 or 18 m below sea level at that time.The Forsmark event (Figure 15) refers to a tsunami event very closely C14-dated at 2900 BP, which was recorded in seven lakebeds in northern Uppland, 160 km to the south of the Hudiksvall records [3,9,22].The lakes investigated and dated form a staircase in elevation, viz., +5.5, +10.8, +16.0, +16.0, +22.2, +22.5 and +24.7 m.At the time of the tsunami event, sea level was at +20.7 m, implying that four lakes were located below and three above sea level at 2900 BP (Figure 15).In all the lakebeds, we found nice tsunamites of sandy layers in graded bedding.A lakebed at +27.4 m [24] shows no sign of a tsunami ingression.Therefore, the upper limit of the tsunami event is set at about 26.5 m, indicating a tsunami height in the order of 6 m (and a submarine trimming depth of about 20 m).
Several lakes and bogs in the region had been investigated before [24][25][26], but none of those studies recorded the occurrence of tsunamites despite the fact that they are quite clearly present in the lakes at +5.5, +10.8, +16.0 and +22.5 m [26], all later cored and dated by the author, as shown in Figure 15.
Synchronizing the events recorded at Skålbo and Forsmark (Figure 16) implies a fall in tsunami height of 6 m over a distance of 160 km, which seems quite reasonable.
Figure 16.Synchronization of the tsunami events recorded at Skålbo (Hudiksvall) and at Forsmark (northern Uppland) at 2900 BP [23]; elevation of dated cores (red dots), sea level at 2900 BP (dark blue lines).The tsunami wave height of at least 12 m in Skålbo decreases to at least 6 m in Forsmark, i.e., a decrease of 6 m in 160 km.The basal trimming seems to go down at least 18 m below sea level.

The AD 1174 event
In SW Sweden, there are evidence of a young paleoseismic event with local faulting of the Viking shoreline.This implies a faulting post-dating the formation of this shoreline, known as PTM-10, and dated at 1000-950 BP [1,22,27].A fault offset of 1.1 m is recorded [1,22].
It seems highly likely that this earthquake also set up a tsunami wave, which buried two Viking ships in the ancient harbour of Galtabäck [27].A C14-date of the ship gave 1172 ±73 cal.years AD [28], which is quite close to a major historical earthquake event in 1174 as recorded in chronicles [29].The covering and burying of the two ships in silt are indicative of a very rapid (instantaneous) process that took the ship owners by surprise, fully in line with the process of a tsunami event.
Tsunami Therefore, it was proposed [27] that, indeed, the earthquake set up a tsunami and that the event refers to the earthquake in 1174 mentioned in the chronicles [29], which corresponds to 776 cal.years BP.
Having established this, it is possible to re-evaluate a turbidite record from the Gullmar Fjord [30] 230 km to the NW in terms of a tsunamite (Figure 17).
Figure 17.The time/depth relations of core 9004 [30] and the position of the "turbidite" here revised in terms of a tsunamite of the 1174 AD earthquake event.
An inferred age of the "turbidite" in the Gullmar Fjord core of 1174 AD fits perfectly well with the time/depth curve of [30].I therefore, quite confidently, re-evaluate this layer as a tsunamite of the 1174 paleoseismic event [27].

Discussion
With a total of 17 separate tsunami events documented in Sweden after the Last Ice Age [1][2][3][4]21], Sweden is a country of exceptionally high number of tsunami events.This is due to the fact that it represents an uplifted shelf environment [5] with sedimentary sequences-not least the varved clay-exposed on land where tsunami events can be documented in details and where their age can be determined with high precision [4].
It is most surprising that tsunamis have not yet been reported by other geologists in Sweden, Finland and Denmark, especially after the first report was brought out in 1995 [10] and 1996 [11].The evidence is there in the field, just to observe, interpret and document.
In Norway, there are excellent records of the tsunami from the gigantic Storegga submarine slide [31,32].

Conclusions
Tsunami events with the deposition of tsunamites are well recorded in Sweden [1,2,3,4].Up to today, 17 events have been documented and described (Table 1 and Figure 1).The triggering factor was high-magnitude paleoseismic events ranging from ∼13,000 BP to ∼800 BP.In two cases, however, the triggering factor was a violent methane-venting episode [1][2][3]23].The absence of recognition of tsunami events in Sweden by other scientists seems to be routed both in personal ignorance and in pressure from the nuclear power industry, which has chosen to neglect evidence of high seismic activity that might invalidate their claim of a safe deposition of the high-level nuclear waste in the bedrock.A safety analysis ignoring observational fact [33] can, of course, never be trustworthy as shown in a devastating manner by screening available field evidence in a long-term safety perspective [2].
A generally high paleoseismic activity during the deglacial phase of Sweden, a number of Late Holocene events reaching magnitudes of M∼ 7 and a sequence of 17 tsunami events are facts that must be assimilated in the history of Quaternary geology of Sweden [34] like in the longterm safety of a nuclear waste storage in the bedrock [2,35].

Figure 2 .
Figure 2. Three sections in the Turinge area exhibiting: (1) regressional sand and gravel, (2) varved clay deposited in marine-brackish environment, (3) varved clay deposited under fresh water conditions and (4) glacifluvial sand and gravel, strongly liquefied and with a subvertical venting pipe[1].Section A shows a concordant change from freshwater to marine conditions in varve 10,430 BP.Section B shows an erosional contact in the autumn of varve 10,430 BP.Section C shows heavy liquefaction with a venting pipe mushrooming in varve 10,430 BP.This is indicative of a major earthquake event in the autumn of varve 10,430 BP, and a major tsunami event that washed the Närke Straight free of blocking pack ice and icebergs and opened the straight for ingression of marine water, turning the Baltic into the Yoldia Sea stage[1,3,4,8].The dating to the autumn of varve 10,430 BP is obtained at three sited 85 km apart[4,9].

Figure 4 .
Figure 4.A segment of an 11 m core taken on the island of Ornö, some 30 km south of Stockholm[13].After 321 years of deposition of normal annual varved clay, the sequence is broken by a sequence of six thick beds of graded bedding (1-6), interpreted to represent a sequence of six major tsunami waves of the 10,430 BP major earthquake and tsunami event[1][2][3][4].Paleomagnetic intensity shows a high peak representing "the Gålö Geomagnetic Intensity Peak," known to have occurred in varve 10,430 BP[14].The intensity curve records six cycles of stronger (s) and weaker (w) magnetization[15].The six beds of cyclic sedimentary (and magnetization) deposition represent a bed load, deposited at a water depth of about 130 m, as a hydride between tsunamites and turbidites[3,4].

Figure 6 .
Figure 6.Red dots represent sites of recorded tsunamites.Green field represents the spatial distribution of firmly varve-dated tsunamites, seismites and liquefaction features with sites outside this field referring to sites dated by other means.

Figure 7 .
Figure 7. Paleogeography of the land-sea-ice distribution at the time of the big earthquake in varve-year 10,430 BP (modified from[3]).Red dot marks location of epicentre.Red double arrows refer to the Närke Straight opened by the tsunami wave so that marine water could enter the Baltic and turn it into the Yoldia Sea stage.Red cross marks the location of two sites recording an earthquake event with liquefaction and a tsunami event occurring 67 varves after deglaciation[17].

Figure 8 .
It seems to represent one of the best documented paleoseismic events in the world[1, 2].The intensity was estimated at XII and the magnitude as >8 [2, 9].Bedrock fracturing is recorded in some 100 sites over an area of 50 × 50 km.Liquefaction is recorded at 12 separate sites covering an area of 80 × 40 km.At two sites, it is recorded in five separate phases thought to represent the main event and a sequence of aftershocks.The liquefaction event is directly tied to varve 9663 BP.The water depth at the epicentre is likely to have been in the order of 250 m (allowing for a tsunami wave with the same diameter).A tsunami wave is recorded at 14 different sites, including nine lakes where a total of 44 cores were taken.It is dated both by varves (at 9663 varve-years BP) and by radiocarbon (at about 9150 C14-years BP).The tsunami wave must have a height of at least 15 m[1,3,4,9].A turbidite in varve 9663 BP extends for 310 km along the coast[1,19].

Figure 10 )
was closely determined at +31.3 m.At the time of the tsunami event, the Baltic level had fallen to +223.5 m, i.e., a drop of about 7.8 m in 25 years, providing a rate of relative uplift of about 312 mm/year.The five C14-dates all overlap at 9155-9135 C14-years BP.A C14-age of 9150 BP would correspond to an absolute age of 10,350 cal.years BP.The deviation between the absolute age and the varve age of 9663 BP records an error in the varve chronology of about 700 years[20].

Figure 11 .
Figure 11.Stratigraphy, dates, tsunami characteristics and diatom analyses of Core 3 in Site 2 [1, 3].The tsunamites spans 60 cm and is composed of a fragmentary first cycle and a full main cycle of graded bedding fining upward (sand-silt-clay).The diatom content of the tsunamite represents a planktonic deep-water flora of the open Baltic Lake Ancylus stage [3].

Figure 13 .
Figure 13.Stratigraphic records at site 14, Iggesund harbour [1, 4] recording liquefaction, venting and surface mushrooming of liquefied material, and the deposition of a tsunamite.The clay surface beneath the tsunamite in pit 4 is irregular (furrows and waves) in a manner indicative of a deposition from the NE (from right to left), i.e. from the sea outside.

Figure 14 .
Figure 14.Red dots represent sites of recorded tsunamites.Green field represents the spatial distribution of firmly varve-dated tsunamites (also seismites and liquefaction features) with sites outside this field referring to sites dated by other means.

Figure 15 .
Figure15.The 2900 C14-years BP tsunami event in northern Uppland with respect to the rate of land uplift and shore displacement over the last 4000 years (the oblique line of ∼7 m uplift per millennia passing through dated anchor points marked by blue dots).At 2900 C14-years BP, the shore was at +20.7 m with land above (yellow) and sea (blue) below as marked on the right side of the diagram.The stars mark tsunami beds recorded and dated in offshore sediments (all falling sharply at the 2900 BP level), in coastal deposits and in lakes and bogs on land where the tsunami beds have eroded down into the older sediments.The graph provides evidence of a tsunami event that deposited typical tsunami beds over a vertical range from −20 to +6 m.