At about 780-750 BC, a major earthquake struck southeast Sweden. At Brantetrask, the bedrock of quartzite was heavily fractured into big, flat blocks. Local people turned the site into a quarry for flat blocks to be placed around the Late Bronze Age graves at Brantevik, the big flat blocks of the sarcophagus, and two 5 tons monoliths transported 30 km to the SSW and erected as the bow and stern stones in the huge ship monument of Ales Stones. Rock carvings from the Bronze Age at Jarrestad became traversed by numerous fractures. Similar rock carving fracturing was observed at six other sites within a radius of 5 km from Brantetrask. In the shore cliff at Ales Stones a seismite was recorded and dated at 780-750 BC. At Glimme hallar, 4 km WSW of Brantevik, the bedrock shows signs of young tectonization. At Lillehem, 40 km to the NNW of Brantetrask, seismically disturbed beds were recorded and dated at the Late Holocene. The seismic event is concluded to have occurred around 780-750 cal.yrs BC and to have had a magnitude in the order of 6.3 to 6.8 and an intensity of about IX on the IES scale.
During the time of deglaciation, when the glacial isostatic uplift peaked at rates in the order of tens of cm per year, Sweden was a high-seismic region [
Therefore, it may not be so strange that we are now able to present evidence of a major earthquake in SE Sweden [
Furthermore, there was a quite strong earthquake on December 16, 2008, in SE Sweden with a magnitude of 4.3, felt over an area of a radius of at least 200 km and with occasional fracturing of churches and other buildings in the vicinity of about 30 km.
The present seismotectonic study refers to SE Sweden, an area often named “Österlän”.
The geological setting will be discussed below in connection with the tectonics studies. The old fault patter is dominated by NW-SE and SW-NE trending faults.
The sea level changes in the Brantevik area are presented in
The main area is in the Brantevik-Järrstad-Simrishamn area with additional sites at Kåseberga 30 km to the SSW and at Lillehem 40 km to the NNW (
Branteträsk is the key site [
The fractured quartzite bedrock surface was cleaned over a large area. The quartzite surface is ice scoured and strongly weathered from postglacial exposure. The fracture surfaces on the other hand are fresh and have very
Geographical location of the main sites discussed
Sea level changes at Brantevik during the last 5000 years [12]
Branteträsk is a swampy, partly water filled, depression. To the northwest it is bounded by a heavily fractured bedrock of quartzite, partly broken up in big blocks by multi-directional extensional forces indicative of seismic de- formation. Later, the blocks and the depression were utilized as quarry of monoliths for Ales Stones and flat blocks for the Late Bronze Ages graves at Brantevik. Yellow dot marks place of block laid up in inclined position. Red line and dots mark main coring section and sites
sharp fracture edges.
A total of 70 fractures were measured: 33 in 355˚ ± 14˚, 19 in 283˚ ± 13˚, 12 in 57˚ ± 15˚, 4 in 324˚ ± 12˚ and 2 in 21˚ ± 3˚, which is quite different directions as those of the main old tectonic pattern.
The lithology of the quartzite is almost identical to that of the bow and stern stones in the Ales Stones megalithic monument some 30 km to the SW, suggesting that they were indeed collected in the Branteträsk quarry about 750 BC and transported by raft to the shore below the place of erection of the monument [
At Brantevik, there previously was a huge grave from the Bronze Age known as “Brantarör” [
Only one core (7 in
The fractured bedrock of Cambrian quartzite. The fracture surfaces are fresh and the fracture edges are knife-sharp. The bedrock surface, on the other hand, is strongly weathered with marks from glacial scouring. The main deformational force is extensional, indicating a seismic origin. A secondary deformation is recorded in the removal of blocks (top image) and lying-up of a big block in inclined position (in the back-ground of the top image), which can only have been achieved by man, transforming the site into a quarry
Core 7 penetrating the entire postglacial period after the deglaciation some 15,000 years ago and exhibiting a “spall horizon” of sharp quartzite fragments interpreted as a record of the quarrying in Late Bronze Age time. In all the other 20 cores, the quartzite surface is hit at a depth of about 0.5 m (equaling the depth of quarrying)
The present coast around Brantevik is rocky consisting of lower Cambrian quartzite, known as “Hardeberga sandstone”. The bedrock is traversed by major open fractures in 72˚ - 76˚ and secondary fractures in 28˚ successively moving over into 326˚.
At the resting place just off the site of rock carving “Simris 19”, 2 km north of Brantevik, the fractures can be observed at a level not reached by the waves today, but well reached by the waves at the +2.1 m high-stand at about 1000-800 BC (
This was also observed just south of Brantevik: the fractures are affected by the abrasion of sea waves up to the level of the waves of the high-stand at +2.1 m (
Just north of Järrestad, a flat quartzite bedrock surfaces is exposed. It is covered by over 100 rock-carvings from the Bronze Age [
This post-rock-carving fracturing was documented at 6 additional sites of rock-carvings (petroglyphs) from the Bronze Age in the Simrishamn-Brantevik-Järrestad region (
The surface of the quartzite bedrock is very smoothly polished by the glacial flow, and it contains excellent glacial striae and crescent-marks indicative of a main ice flow from the NE (measured value: 51˚ ± 3˚). The surface is traversed by a few large and open fractures in 34˚ and 316˚, and numerous minor fractures; viz. 63 in 290˚ ± 4˚ and 19 in 15˚ ± 8˚ (
Both glacial striae and crescent-marks are often cut by fractures indicating at least one phase of postglacial fracturing ([
Consequently, there are good observational reasons documented in Järrestad and six additional sites for an
The abraded fractures just above the +2.1 m sea level high- stand at 1000-800 BC (Figure 2) exposed at the resting place opposite the rock carving site “Simris 19” (compare with the very sharp-edged fractures in Figure 4)
Fractured rock-carvings (yellow dots) and sites of bedrock fracturing analyses (blue dots [8] -[10] )
episode of general bedrock fracturing within an area of 5 × 5 km (
Glimminge hallar area constitutes the eastern part of a local granite horst (
The northwestern side of the granite horst is full of structures indicating deformations in postglacial time. Two examples are given in
The smoothly glacial polished quartzite surface of the Järrestad rock- carvings, traversed by a few open fractures in 34˚ (central horizontal) and 316˚ (slightly winding from left top to bottom), and numerous parallel minor fractures in 290˚, traversed by a nearly perpendicular system in 15˚, which often cut across the pictures
Postglacial fracturing of crescent-marks at site Simris 19 (A) and a post-carving fracturing of a foot picture from the Bronze Age at Järrestad (B)
Signs of postglacial deformation are present also on the eastern side of the horst. At site 2, there is a heavily fractured scarp (
With the addition of the Glimminge hallar sites, the size and mode of bedrock deformation (
The shore section was found in 2009 and described in [
Right above the beach cobble of a sea level high-stand dated at around 1000-800 BC, there is a black layer of humus and charcoal, which was C14-dated at 785 ± 20 cal.yrs BC [
At the 2011 excavation [
After additional work in the section in 2013, it is now possible to identify the layer right above the organic bed as a layer of liquefied, tectonized and down-slid material; i.e. it is classified as a “seismite” (
The stratigraphy of
7: base of a Viking Age midden [
6: eolian sand 2 with a strong humus soil of the Late Iron Age.
5: hearth with charcoal dated 385 ± 35 cal.yrs BC.
4: eolian sand 1 representing the general sand drift phase at 500-600 BC.
3: tectonized layer (i.e. a seismite) including down-slid boulders and a reworked bone dated at 1195 ± 85 cal.yrs BC (cf.
2: organic layer rich in charcoal dated 785 ± 20 and 775 ± 35 cal.yrs BC, further inland this land horizon is dated at 1285 ± 65 cal.yrs BC (
1: beach shingle representing the high-stand 1000-800 BC (cf.
Close to the west (site 2 in
In a road junction at Lillehem, there is a minor gravel pit. After cleaning, an interesting stratigraphy was exposed including an old ice wedge cast filled with organic material, humus and at the base a bed of precipitated carbonate (
The development of a strong humus soil in the ice wedge cast and the precipitation below of carbonate need a lot of time. This is consistent with the age obtained of the horizontal carbonate layer C14-dated at 4503 ± 43 cal.yrs BC.
The left side of the section is heavily tectonized with progressive down faulting (lower image). The tectonized beds include fragments of the carbonate bed dated at 4503 BC. Consequently, the tectonization must post-date this age.
The general down faulting in the order of 0.5 m is indicative of a significant liquefaction in the subsurface. All this suggests the effects from a major earthquake in Late Holocene time. It seems reasonable that this earthquake event is the same as the one identified in Branteträsk and Kåseberga.
Multiple indications of a Late Holocene earthquake event have been presented above, and will now be considered together as to location, magnitude and age. In the first preliminary report, a magnitude of “M > 6” and an
age “around 1000 BC” or “in the period 1000-800 BC” were proposed [
The Glimminga hallar site (Section 2.4) documents extensional fracturing to the NE and the E, i.e. in directions off the granite horst.
The bedrock fracturing at Branteträsk follows a clear 3-dimentional extension. The main direction is perpendicular to the main fracture direction 355˚ - 175˚. This implies that the long axis of the strain ellipsoid must be in 265˚ - 85˚. The short axis should be perpendicular to this. The vector of the two additional fracture directions is 350˚ - 170˚, which is only 5˚ off the perpendicular direction of the long axis. If the long axis is extended westwards, it passes the granite horst (
At Järrestad, 63 fractures were measured in 290˚ and 19 in 14˚. It is this fracture system, which has fractured the petroglyphs (
The combined record (
The tectonic map of the area [
The estimate of intensity and magnitude depends on the character of deformational structures and the spatial distribution of observations e.g. [
Bedrock fractures in the order of centimeters to a decimeter suggest an intensity of about IX (on the IES scale; [
The accumulated extension of the 33 fractures at Branteträsk is 123 cm in 265˚ - 85˚ direction, and the opening along the two other directions is 48 and 47 cm. This seems rather to suggest an intensity of about X (or at least IX-X). At Järrestad 63 fractures were measures. Their width ranges form millimeters up to a centimeter at the most. This gives a total accumulated extension in the order of a decimeter. The spatial distribution (5 km from Järrestad and 4 km from Branteträsk to the granite horst) suggests an intensity of VIII-IX.
The spatial distribution of liquefactions provides a better mean of estimating intensity and magnitude. Lillehem lies 32 km north of the granite horst at Glimminge, and Kåseberga lies 20 km south of it. Applying the magnitude/spatial distance distribution relation (e.g. [
The fracturing of the petroglyphs at Järrestad and six additional sites (
must be younger than the Mid Bronze Age; i.e. younger than 1000 BC. The peat cover of the fractured (and quarried) quartzite bedrock at Branteträsk indicates quite some time back in the Late Holocene. The Brantarör grave seems to incorporate numerous blocks quarried at Branteträsk. Because the grave included bronze tools and an urn dated at 800-500 BC, the quarrying post-dating the earthquake fracturing is likely to have occurred in the Late Bronze Age at around 700 BC. Finally, if the quartzite megaliths were shipped out by boat or raft from Brantevik to Kåseberga, the natural harbor only existed at the sea level high stand 1000-800 BC up to about 750 BC (
At Kåseberga, charcoals just below the tectonized horizon are dated at 780 BC. Taking into consideration that the wood burned into charcoal must have some age, we may set the actual burning at abound 800 BC. The subsequent earthquake must have happened shortly after or at about 780-750 BC.
The earthquake deformation at Lillehem is dated as younger than 4500 BC. An age of 750 BC seems quite reasonable from the stratigraphic evolution. Besides, we have no reasons to infer a second seismic event for this deformation.
In conclusion, the age equation seems to provide a quite sharp age determination at around 780-750 BC.
At the time of deglaciation, Sweden was a high-seismic area as a function of the very high rates of isostatic uplift [
The Late Holocene records from NW Skåne may be of particular interest in comparison with the present record. A number of events of magnitudes well above M6 and maybe even reaching M7 were reported [
Because the instrumental seismic records do not exceed M 4.3 (for example the Dec. 16, 2008 event with epicenter 60 km west of the horst area here discussed), the seismological community has great difficulties in accepting any earthquake in the Late Holocene reaching M6 or above e.g. [
The present paleoseismic event is recorded by different geological and archaeological means (bedrock fracturing at 13 sites, fracturing of rock-carvings at 7 sites, liquefaction at two sites 43 km apart, and dating by radiocarbon, archaeology, sea level history, etc).
Analyses of measured fracture pattern suggest that the epicentre of earthquake is to be searched for in vertical movements of the local granite horst at Glimminge hallar.
The seismic intensity (on the IES scale) is likely to have been in the order of VIII-IX. The magnitude is estimated at M 6.3 - 6.8.
The age can be fairly well established at 780-750 BC. The following quarrying episode at Branteträsk must be older than 700 BC, and probably around 750-730 BC.
This investigation documents a new Late Holocene earthquake with a magnitude significantly higher than those with the maximum magnitudes recorded by seismological instruments. Hence it should affect our hazard assessment [