Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

Fig. 4: Differential liquefaction of a stratified bed at a
paleoseismic event at 12,400 BP (Mörner, 2003, 2008; site
Hunnestad).
The material vented behaves quite differently with
respect to grain sizes. In a large mushroom structure a
laminar flow is recorded by the stratification of sand and
course silt grains, a turbulent flow by the totally random
distribution of fine grains measured by the anisotropy of
magnetic susceptibility (AMS), and a free flow of the very
small particles controlling the magnetic polarity (ChRM)
allowing a firm orientation with respect to the
geomagnetic pole.
The spatial distribution of liquefaction as to a single event
is more or less linearly related to the magnitude of the
earthquake.
Liquefaction structures are usually formed at earthquakes
of a minimum magnitude in the order of M 5–5.5. This
implies that the recording of past liquefactions tells us
that the magnitude ought to be, at least, above ~5.5 (with
respect to the Richter‐scale).
Because earthquakes usually are not single events, but
rather a cluster of shocks and after‐shocks, one would
expect to see not just one phase of liquefaction, but
multiple phases (Mörner, 2003).
Fig. 5: Five successive phases (1–5) of liquefaction of the 9663
vBP paleoseismic event, interpreted to represent shocks and
after‐shocks of the same earthquake event (from Mörner,
2003, 2008; site: Myra West). Five successive phases were also
recorded at a site (Hög) located 35 km away.
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology)
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Liquefaction and venting of liquefied material are
recorded at numerous sites in Sweden (Mörner, 2003,
2005, 2008). The spatial distribution of one and the same
liquefaction event – 320x100 km for the 10,430 vBP event
and 80x40 km for the 9663 vBP event – gives evidence of
high‐magnitude events.
At several events, we were able to record multiple phases
of liquefaction. At the 9663 vBP paleoseismic event, we
recorded 5 successive phases (Fig. 5), interpreted in terms
of shock and after‐shocks.
The size and type of liquefaction structures have a bearing
on the magnitude. In some cases we have recorded the
venting of gravel, even course gravel and pebbles (Figs. 6–
7). This calls for magnitudes in the order of M>8. One
such event is dated at 10,388 vBP and another at 6100 BP
(Mörner, 2003, 2008).
Fig. 6: Venting not only of sand but also gravel and pebbles at
the 10,388 vBP event (Mörner, 2003, 2008; site Turinge
grusgrop). This calls for a high‐magnitude paleoseismic event
(M>8).
Fig. 7: Structureless liquefied sand as a part of the venting of
sand‐gravel‐pebbles in the Fig. 6 site.
Seismic shaking may also generate wavy patterns of
previously horizontal sand and clay beds (Fig. 8; cf.
Mörner, 2003; Mörner and Sun, 2008).
Extensive turbidites are often formed by the sediment
masses set in motion by slides, liquefaction and tsunami
waves. Their spatial distribution is 300x200 km for the
10,430 vBP event and 320x90 km for the 9663 vBP event.
They stick out as distinct “marker‐varves” (Fig. 9). This
allows a very precise dating as to the Swedish Varve
Chronology.