Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

macroseismic magnitude M M=3‐4.9. However, these
magnitudes are not considered to be enough for creating
a relief (cf. McCalpin 1996). Thus, the presented research
using near‐fault investigation in artificial trenches in the
Czech portion of the SMF was carried out in order to
discover probable pre‐historic faulting responsible for the
mountain front morphology as well as to reveal the
kinematics of the fault.
In order to select a suitable site for trenching,
geomorphological investigation, analysis of the digital
elevation model, and geophysical sounding (multi‐
electrode method, GPR) were carried out. The results of
trenching in two localities (Vlčice and Bílá Voda) are
presented here.
Fig. 2: Strike‐slip fault zone with a feature of flower structure
documented in the trench in Vlčice site. A – older Miocene unit, B
– younger Miocene unit, C ‐ deformation zone, D ‐ early Holocene
colluvium. Note the dragged B unit layers.
The trenching across the SMF at the first locality Vlčice on
the N‐S trending mountain front revealed succession of
sedimentary units disturbed by tectonic features. The
trench exposed crystalline rocks, three units of early to
mid Miocene fluvial to limnic sediments with organic‐rich
beds (with graphite), Pleistocene alluvial fan deposits, and
three sequences of Holocene colluvium. Several faults
were documented: reverse faults (strike 160°, dip 40‐70°
to SW) displacing crystallinics over Miocene deposits, a
subvertical deformation structure (probably strike‐slip)
within the Miocene sediments (strike 35°, dip 75° ESE; Fig.
2), and minor normal faults within the youngest Miocene
unit (strike 145°, 45° to NE). Moreover, the three Miocene
sedimentary members, positioned vertically next to each
other, were probably dragged by the reverse fault. To
determine the age of the identified deformations
variously dated sediments were used. The dating
included: known age of mid Miocene deposits,
radiocarbon dating of charcoal and paleosols, and
supposed Late Pleistocene age of the last gelifluction.
Time constraint of the identified fault movements is given
in Table 1. Movements disturbing Neogene deposits and
pre‐dating last gelifluction involve the reverse faulting
that displaced crystalline rocks over the Miocene
deposits, and horizontal movements. The younger
movements occurred by reverse and normal faults.
One of the younger reverse fault within the crystalline
rocks probably created a coseismic relief step, which is
concealed by early Holocene colluvial deposits
(10,940±140 cal. yrs BP). This step was documented 0.6 m
high at the distance of 1.3 m and was covered by a
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
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colluvial wedge‐like form. As the step must have been
seen on the surface before the deposition of the
Holocene sediments and at the same time it is not
affected by Late Pleistocene gelifluction, the movements
occurred probably on the boundary
Pleistocene/Holocene. The youngest movements
probably occurred in the lowest part of the slope and
were related to normal faults. The faults cut the Miocene
sediments and divide two area with diverse
erosion/depositional regime. Moreover, this zone
coincides with spring area and low resistivity zone shown
by the geophysical sounding. This normal faulting
predates the soil sediment (410±80 cal. yrs BP) buried by
the youngest colluvium.
Trenching at the second locality Bílá Voda was carried out
across NW‐SE trending mountain front. The exposure
revealed strike‐slip fault zone (striking 135°‐150°) dividing
Paleozoic crystalline rock and Late Pleistocene colluvial
deposits derived from the fault zone (Fig. 3). The deposits
overlay Miocene sediments, which are warped. The fault
has a flower structure character and shows several
repeated movements. Kinematic indicators within the
older structures show a sinistral component. However,
the sense of the youngest movements will be able to
recognize based on further trenching in this locality, since
the recent stresses inferred from GPS and micro‐
displacements measuring are not unequivocal (see
Badura et al., 2007, Štěpančíková et al., 2008).
Table 1: Time constraint of faulting history within the SMF
identified at the trenching site Vlčice. The ages are based on the
age of deformed sediments, radiocarbon dating of charcoals, and
paleosols covering the deformed sediments.
type of deformation time limits of movements
maximum minimum
reverse faulting 15 Ma yrs BP 15‐11 ka BP
horizontal
movements
15 Ma yrs BP 15‐11 ka BP
reverse faulting 15‐11ka yrs BP 10,940 ± 140
cal. yrs BP
latest normal 10,940 ± 140 cal 410 ± 80 calBP
faulting
BP
Dating of movements is based mainly on the relative age
of the Late Pleistocene fault‐related colluvial deposits.
Besides clasts of tectonic breccia, they include pebbles of
eratic material coming from glacial deposits, whereas the
last continental glacier reached the study area in Elsterian
2 (400‐460 ka). Moreover, the Pleistocene deposits, fault
zone, and crystalline rocks are covered by geliflucted
layers of all the involved rock types and then by the
youngest Holocene colluvial deposits (800±50 cal. yrs BP).
Younger individual faults of the fault zone penetrate the
Late Pleistocene deposits but they are concealed by the
geliflucted layers. Since the coarse‐grained fault‐related
deposits are quite homogeneous at the whole thickness
(maximum confirmed 3.5m), no individual faulting events
could be recognized. The youngest movements (probably
horizontal with some vertical component) displaced even
the geliflucted layers, while the vertical displacement
made up around 35 cm (Fig. 4). These displaced layers are
concealed by the recent Holocene colluvium. To estimate
the magnitude of this last event, further trenching
focused on horizontal offset identification will be
necessary.