**2.1. The 10,430 BP event**

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

**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

*Age in BP Area affected Earthquake magnitude Observed tsunami record* 12,400 Kattegatt At least 8 Very high & strong wave

10,430 Mälardalen Well above 8 Very high & strong wave 9663 Hälsingland Well above 8 At least 15 m wave height 9428 Umeå area At least 7 (height unknown) 9221 Umeå area 7–8 At least some meters 8600 Södermanland 6–7 Probably some 5–10 m 7800 Stockholm region At least above 6 Maybe 13 m run-up 6100 Hälsingland Well above 8 At least 10-15 m height

11,600 Kattegatt At least 7 High wave 11,250 Kattegatt About 7 High wave

4000 Umeå area 6–7 Tsunamite

**Table 1.** Tsunami events recorded in Sweden in association with paleoseismic events [1, 2].

refer to the five events here discussed with event 4 being revised in time and wave height.

**Figure 1.** Recorded tsunami heights in the Baltic (blue) and Kattegat (green) coasts of Sweden [6]. Red figures (1–5)

3–4000 Södermanland Explosive gas venting At least 11 m run-up 3200 Lake Marviken Around 7 Local lake tsunami 2900 Hudiksvall area Explosive gas venting At least 12 m wave height 2900 Forsmark area Same as above At least 6 m wave height 1600 Kattegatt Unknown Some meters run-up 776 South Kattegatt Around 7 Destructive, tsunamites

world.

116 Tsunami

its surrounding seas [1].

**2. Paleoseismics and paleotsunamis**

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 [10–12]. There were remarkable liquefaction structures, ground-shaking structures and deformed annual varves (**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].

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**).

**Figure 3.** The 10,430 BP paleoseismic event is documented by multiple criteria [1, 2, 4, 9].

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].

**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 [1–4]. 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 3.** The 10,430 BP paleoseismic event is documented by multiple criteria [1, 2, 4, 9].

of [4, 9].

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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

**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].

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**.

**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.** 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].
