Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

and warp palaeosols (Fig. 5) in the area south of Ohrid
city.
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FAULT‐SLIP ANALYSIS
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Palaeosol
Fault gouge
We studied 24 sites with suitable fault‐slip data for stress
inversion. These sites are spread on the east and west
coast of the lake. On each location we measured a
representative number of fault planes concerning the
spatial orientation of fault plane (dip direction, dip) and
striae (azimuth, plunge) and additional the sense of slip
(reverse, normal, dextral or sinistral). The data base was
heterogeneous (Fig. 6) so special separation techniques
needed to be applied to get good results.
The first step in processing the collected data was to
correct the datasets due to minimal measurement
failures. Therefore, the lineations are rotated along a
great circle which is defined by the lineation and the pole
of the fault plane to align on the fault plane (Tectonics FP
by Reiter & Acs 1996‐2003). Next we calculated the PT‐
axes from the fault plane datasets. The best results were
achieved with a theta angle of 30°. These heterogeneous
data were separated by hand. The separation on the PT‐
axes projection was performed with Tectonics FP. The
results already gave a tendency of three sigma1
directions: NW‐SE, NE‐SW and vertical. These first results
met our expectations drawn from the DEM observation.
After Sippel et al. 2008 good results have been achieved
by combining the multiple inverse method after Yamaji
(2000) with the sorting of the data by PT‐projection.
With this hypothesis we applied the multiple inverese
method after Yamaji (Yamaji et al. 2000). This method is
useful especially on datasets where a polyphase stress
history can be inferred. Thus we chose this method to
investigate the spatial and temporal variations of
palaeostress in the Ohrid Basin. The multiple inverse
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
833 m
b)
Fig.5: Fault with tilted Palaeosol south of Ohrid city. a)
values of dip direction and dip angle are displayed.
b) photograph of the fault with person as scale.
a)
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method is based on the Wallace‐Bott hypothesis. Wallace
(1951) and Bott (1959) stated that fault movements are
expected to occur in the direction parallel to the
maximum shear stress on fault planes. Finally, we used
different sets of striae, which could be relative dated and
data from foldaxes to develop a relative dating of the
stress states.
Fig. 6: Angelier plot of fault plane and slickenside lineation of
three representative outcrops at the east coast
JOINT MAPPING
The cliffs along the east coast of Lake Ohrid were mapped
structurally by identifying and measuring joints, bedding
and faults in mainly Triassic limestones. The data were
sorted, corrected and plotted as rose digrams for each
outcrop (see Fig. 7). In total four dominant strike
directions of joints were identified, these are N‐S, W‐E,
NW‐SE, and NE‐SW oriented. All sets are pervasive. The N‐
S set is related to the youngest deformation, according
the faults and geomorphology around the lakes,
substantiated by earthquake focal mechanisms (Wagner
et al., 2008). The older joint sets are related to the
Tertiary deformation phase. Remarkably, N‐S joints are
confined to N‐S faults, which we interpret as a very
localized deformation. Blocks in between those N‐S faults
remain unaffected by the recent deformation.
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Fig.7: Examples for the joint sets at the east coast of Lake
Ohrid
E‐W trending joint sets occur in the southeastern part of
the lake, very close to the large E‐W trending Galicica
wind gap. Faulting there is not obvious; however, the
Galicica ridge is displaced right‐laterally and also
vertically. The rigde crest heights vary from 1970 m in the
north and 2255 m in the south, resulting in 285 m of
vertical offset. Also, strike changes from NNE in the north
towards NNW in the south. The E‐W structures are also
cut by N‐S trending faults.
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