Seismotectonics of the Swedish West Coast IASPEI Excursion Guide

Abstract

The Kattegatt Sea is traversed by a major fault trending in NE–SW direction (Mörner, 1969, 2003, 2004). Mörner has recorded 13 paleoseismic events during the last 13,000 C14-years (Mörner 2003, 2009, 2011). We visit the spectacular liquefaction site at Hunnestad (12.400 BP) and 6 Late Holocene seismotectonic sites in the Båstad-Torekov region.

Full text

Seismotectonics
of the Swedish West Coast
IASPEI
Excursion Guide
July 27, 2013
Nils-Axel Mörner
P&G-print, 2013

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The Kattegatt Sea is traversed by a major fault trending in NE–SW direction
(Mörner, 1969, 2003, 2004); Fig. 1. Mörner has recorded 13 paleoseismic
events during the last 13,000 C14-years (Mörner 2003, 2009, 2011); Fig. 2.
Fig. 1. A major active fault (red line) from the northeast side of the Hallands-
åsen Horst across the Kattegatt in NW-direction (from Mörner, 2004).
Fig. 2. 13 paleoseismic events have been documented in the Kattegatt–West
Coast region (from Mörner, 2003, 2011, 2013). Blue dots = tsunami events.

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HUNNESTAD
A Late Glacial liquefaction site
This site was first described in Mörner (2003). It was shown at Excursion 11B
of IGC 2008 (Mörner, 2008; Stop 8-8) and described as follows:
“In these gravel pits, I recorded very clear liquefaction structures (Mörner, 2003, p. 282;
cf. Påsse, 1990). The liquefied beds are covered by a littoral bed including pebbles and
stones of flint and chalk coming from the Öresund region by drifting icebergs. The site
is interpreted as giving evidence of a paleoseismic event at about 12,400 cBP and a
related tsunami event (flooding the surface with littoral material and drifting icebergs
(Mörner, 2003).”
At the excursion very excellent new liquefaction structures were discovered
(Fig. 3) including liquefied sand and silt with a large variety of structures and
big blocks “swimming” in the liquefied sand.
At the reconnaissance for this excursion, new structures were found, especially
in the still active pit close-by; viz. mega-structures, internal faulting, sliding-in
of liquefied wedge and stones sunken (vibrated) down into liquefied sand as
shown in Figs. 5-8.

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Fig. 3. Heavy liquefaction including big boulders
“
swimming
”
in the fluidized sand.
Foto taken at the IGC Excursion B (Mörner, 2008; Stop 8-8; p. 93).

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Fig. 4. Exposure 2013 in the old gravel pit with two main units of strong liquefaction.
Fig. 5. Pieces of flint are found in the covering washed gravel zone.
These flints originates from Denmark and suddenly appeared
on the Swedish West coast at a tsunami 12,700-12,400 C14yrs BP.

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Structures in the still active gravel pit juts to the east of the old one (as recorded
in 2013 at the reconnaissance for this excursion).
Fig. 5. Strongly deformed and liquefied large-scale beds.
Fig. 6. Liquefied and faulted beds.
Note the “swimming” stones in the liquefied sand (to the left).

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Fig. 7. A liquefied wedge has moved in from the right to the left;
over-turned and crumbled beds, liquefied sand penetrating the gravel unit.
Fig. 8. Stones have sunken (vibrated) down into the liquefied sand,
giving evidence of ground motions and vibrations.
This has previously been recorded at an 8000 BP beach in the Stockholm region
(Mörner, 2003, p. 250).

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Fig. 9. Shoreline diagram of the Swedish West Coast (Mörner 2003, modified
from Mörner, 1969).
Arrows up mark tsunami events. Arrows down mark earth slides.
Cubs = Halland-1 paleoseismic event related to the 12,400 BP shoreline.
Triangles = Halland-2 paleoseismic event related to the 11,600 BP shoreline.
Some sites (dots) = the Halland-3 paleoseismic event and 11,250 BP shoreline.
The Hunnestad site lies at +65 m between the 12,700 and 12,400 BP shorelines.
The Båstad-Torekov area lies at about -43-45 km where the shorelines bend.
At Eskilstorp (Stop 1), there are multiple earth slides and a tsunami bed.
Sea level fell to a submarine level at 965-9300 BP
And then rose again in an oscillatory way

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The BÅSTAD-TOREKOV area
and Holocene seismotectonics
Fig. 10. The chronology of the paleoseismic events in the Båstad-Torekov area
is referred to an unusually detailed reconstruction of sea level changes
(from Mörner, 1980).

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Fig. 11. Stop 1: earth slide dated 4800 BP
Fig. 12. Stop 1: shoreline profile of 1968 (Mörner, 1968, 2008).

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Fig. 13. Stop 2: strongly liquefied Holocene marine beds (Mörner, 2003, 2008).
The liquefaction event must post-date the PTM-5A sea level at 5000 BP.
Gravel and stones at the top represent the extremity of the Stop 1 slide.
The structure-less sand layer is the main liquefied (fluidized) unit.
An age of about 4800 BP is inferred (Mörner, 2003, 2009).

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Fig. 14. Stop 3: the bedrock constituting the shore cliff from the PL maximum
(PTM 4 and 5A dated at about 6000-5000 BP) is heavily fractured.
This fracturing must post-date the 5000 BP sea level.
An age of about 4800 is inferred (Mörner, 2003, 2009, 2011)
Fig. 15. Stop 3: angular blocks cover rounded beach gravel of the PL beach.

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Fig. 16 (A & B). Stop 4: beach ridges and the cover of angular talus blocks
(from Mörner, 2003, 2008. 2009) providing three main phases of block falling;
viz. just after beach 9 (i.e. about 4800 BP), after beach 6 but before beach 5 (i.e.
around 3500 BP), and just after beach 2 (i.e. about 900 BP).

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Fig. 17. Stop 5a: view of the Råle Fault area.
Fig. 18. Stop 5: the general beach ridge succession is just south of Råle down-
faulted in three steps at 4800, ~3500 and 900 BP (Mörner, 2003, 2008, 2009).

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Fig. 19. Additional Stop 6 with three sea levels seen in the road profile.
Fig. 20. Dated subsurface profile of Fig. 18 (Mörner, 2008).
Fig. 21. Multiple transgression phases of the PL beach at Torekov.

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THE 4800 BP PALEOSEISMIC EVENT
PRIMARY NORMAL FAULT
Reactivation of Hallandsåsen Horst Fault
LATERAL-SYMPATHETIC FAULT
The Råle Fault 6 km SW of the main fault
(moving 4800, 3500 and 900 BP)
Vertical throw: 1.1-1.4 m
BEDROCK FRACTURING
A: several sites along the main fault
B: dated to post-date 5000 BP shoreline
TALUS SHUTTERING
A: at several sites along the fault (recorded for 2 km)
B: shuttering talus on top of the 5000 BP shoreline
EARTH SLIDES
A: major earth slides along the fault slope
B: with sediment wedges on top of 5000 BP shore deposits
LIQUEFACTION
A: recorded along the River Stensån banks
B: sea level dating at 4800 BP
Fig. 22. Synthesis of the records of a paleoseismic event at 4800 BP
(from Mörner, 2011).
References
Mörner, N.-A., 1969. The Late Quaternary history of the Kattegatt Sea and the Swedish West Coast:
deglaciation, shorelevel displacement, chronology, isostasy and eustasy. Sveriges Geol.
Undersökn., C-640, 1-487.
Mörner, N.-A. 1980. The northwest European “sea-level laboratory” and regional Holocene eustasy.
Palaeogeogr. Palaeoclim. Palaeoecol. 29, 281-300.
Mörner, N.-A., 2003. Paleoseismicity of Sweden – A Novel Paradigm. A Contribution to INQUA from
its Sub-commission of Paleoseismology. Reno 2003, ISBN 91-631-4072-1, 320 pp.
Mörner, N.-A., 2004. Active faults and paleoseismicity in Fennoscandia, especially Sweden: primary
structures and secondary effects. Tectonophysics, 380, 139-157.
Mörner, N.-A., 2009. Late Holocene earthquake geology in Sweden. Geological Society of London,
Spec. Publ. 316, 179-188.
Mörner, N.-A., 2011. Paleoseismology: The application of multiple parametres in four case studies in
Sweden. Quaternary International, 242, 65-75.
Mörner, N.-A., 2013. Patterns in seismology and paleoseismology, and their application in long-term
hazard assessments. The Swedish case in view of nuclear waste handling. Pattern Recognition in
Physics, 1, 75-89.

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POSTSCRIPT
Paleo-Earthquakes in Southern Sweden – http://www.seismo.com/iaspei/
Field trip to Paleo-Earthquakes in Southern Sweden with guides: Professor Nils-Axel Mörner
from Stockholm and adjunct state seismologist Søren Gregersen from Copenhagen.
!!One-day excursion to Hallandsåsen JUL 27, the day after closure of the IASPEI assembly.
!!Signs of 5 paleo-earthquakes younger than 5000 years are (1) faulting - displacements of
coastline sediments, (2) talus, (3) bedrock fracturing, (4) rock slides, (5) liquefaction -
disturbances of water-filled sediments, (6) tsunami sediments.
!!
Participants in IASPEI July 27 Excursion in Sweden
Sören Gregersen DK organizer
Bob Engdahl US former secretary-general of IASPEI
Peter Suhadolc IT present secretary-general of IASPEI
John Adams CA consultant in planning
James Dewey US
John Ebel US
Ketil Haarstad NO
Hong Li CN
Aruna Kithsiri Nandasena JP
Masatako Ando TW
Gerassimos Papadopoulos GR
Alexander van der Gusev RU
Olga Pavlenko RU
Ronald van Nooyen NL
Nils-Axel Mörner SE excursion leader
A few responses to the excursion
I have a hard time accepting earthquakes as late as 1000 or 3-5000 years ago in an area that
has smaller and smaller stresses as time goes since the Ice left the area 12000 years ago. But
you, Niklas did show us exciting nature and arguments both by your publications, pictures
and on site. It was appreciated by us all that we had an educational trip.
Sören Gregersen
I would also like to thank - on behalf of IASPEI and personally - both Soren and Niclas for
the excellent and instructive field trip!
Peter Suhadolc
It was a wonderful field visit with Prof. Morner's explanations and Prof. Soren's excellent
coordination.
Aruna Kithsiri Nandasena
Thanks indeed! It was excellent!!
Gerassimos Papadopoulos