6600 years of earthquake record in the Julian Alps (Lake Bohinj, Slovenia)
2018
Sequences of lake sediments often form long and continuous records that
may be sensitive recorders of seismic shaking. A multi-proxy analysis of
Lake Bohinj sediments associated with a well-constrained chronology was
conducted to reconstruct Holocene seismic activity in the Julian Alps (Slovenia).
A seismic reflection survey and sedimentological analyses identified 29
homogenite-type deposits related to mass-wasting deposits. The most recent
homogenites can be linked to historical regional earthquakes (i.e. 1348 AD,
1511 AD and 1690 AD) with strong epicentral intensity [greater than ‘damaging’
(VIII) on the Medvedev–Sponheuer–Karnik scale]. The correlation
between the historical earthquake data set and the homogenites identified in
a core isolated from local stream inputs, allows interpretation of all similar
deposits as earthquake related. This work extends the earthquake chronicle
of the last 6600 years in this area with a total of 29 events recorded. The
early Holocene sedimentary record is disturbed by a seismic event
(6617 94 cal yr BP) that reworked previously deposited sediment and led
to a thick sediment deposit identified in the seismic survey. The period
between 3500 cal yr BP and 2000 cal yr BP is characterized by a major destabilization
in the watershed by human activities that led to increases in erosion
and sedimentation rates. This change increased the lake’s sensitivity to
recording an earthquake (earthquake-sensitivity threshold index) with the
occurrence of 72 turbidite-type deposits over this period. The high turbidite
frequency identified could be the consequence of this change in lake earthquake
sensitivity and thus these turbidites could be triggered by earthquake
shaking, as other origins are discarded. This study illustrates why it is not
acceptable to propose a return period for seismic activity recorded in lake
sediment if the sedimentation rate varies significantly.
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