Landscape evolution can be studied by morphometry which is the quantative measurement of the landscape shape. Three main geomorphic markers play a prominent role in the evaluation of tectonically active dip-slip normal fault zones; these are drainage basins, triangular facets and mountain front lineaments (Table 6.2 - e.g Keller, 1986; Mayer, 1986; Keller & Pinter, 2002). The study of those indices contributes to the quantification and the evaluation of a landform that is influenced by the active tectonics. Previous studies have discussed the basic geomorphological features in Attica, Greece region and especially at it’s western part . No focus has been placed at the Penteli escarpment with respect to the geomorphologic signature of the tectonic activity along the Dionysos fault. It’s the first attempt in order to evaluate the results of morphometry in the Dionysos fault , with the conjuction of the field work and geomorphological analysis to strengthen the
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convincement that the Dionysos fault is tectonically active. Also, geomorphic indices can demonstrate with their results the continuity whether or not, of the Dionysos fault.
Table 6.2 Summary of the morphometric parameters used in tectonic landform analysis of the Northern Penteli slopes, ( modified from Wells et al., 1988 and Ramirez-Herrera, 1998) Sources: (1) Bull, 1977;
(2) Bull and Mc Fadden, 1977; (3) Keller and Pinter, 2002; (4) Keller, 1986; (5) Silva et al. 2003; (6) Pérez-Peña et al. 2010; (7) Wells et al. 1988; (8) Ramirez-Herrera, 1998; (9) Petit et al. 2009; (10) Hare and Gardner, 1985.: Lmf—length of mountain front along the mountain–piedmont junction, Ls—straight- line length of the front, Lf—cumulative lengths of facets, Γh—local relief between the scarp base and the upper vertex of the triangle, α—slope of the triangle's altitude measured between the base and the vertex, Ar—area of the basin to the right (facing downstream)of the trunk stream,
At— the total area of the drainage basin.
Morphometric
parameter Mathematical
derivation Measurement
procedure Explanation Source
AF, asymmetry factor
AF=100(Ar/At)
&
AF=|50-(Ar x 100/ At )|
Define the ratio of the area basin to the right (Ar),
facing downstream of
the trunk stream to the
total area of the drainage basin (At). The
index was determined to detect tectonic
tilting transverse to
the flow at drainage
basins.
(3,6,10)
Smf, mountain
front sinuosity Smf=Lmf/Ls
Reflect a balance between the
tendency of stream and
slope processes to
produce irregular (sinuous) mountain front and
(1,2,3,4, 5,6)
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vertical active tectonics that
tend to produce a prominent straight front (Keller,
1986)
Morphometric
parameter Mathematical
derivation Measurement
procedure Explanation Source
Percentage
faceting Lf/Ls
Define the proportion of
a mountain front that has
well defined triangular facets, using
the ratio of the cumulative
lengths of facets (Lf) to
overall mountain front length
(Ls).
Tectonically active fronts
display prominent, large facets that are generated
and/or maintained by
recurrent faulting along
the base of the escarpments,
i.e. high percentage
faceting
(7,8)
82 Asymmetry factor (AF)
The categories of the absolute AF values are three, showed by an arrow (Fig.
6.14) that indicates the direction of the assymetry. The classes are as follows:
AF<50 means asymmetric eastward tilted basin,
AF=50 means symmetric basin,
AF>50 means asymmetric westward tilted basin.
As the Af value departs from the central value of 50 the influense of tectonic tilting increases accordingly ( e.g Hare & Gardner, 1985). The assymetry factors of the western part of the Dionysos fault ( N.1, N.2, N.3, N.4, N.5- in Fig. 6.14, 6.15) where the main streams flow northwards show systematic asymmetries with a general trend of uniform tilting towards the W – NW. Af values get increased towards the western drainage basin , in the fault tips, that is a powerfull element that the fault continues to the west. This element in conjuction with the slope distribution analysis , that showed the existance of slope anomalies in the western extension of the Dionysos fault ,shows that this
Facet slope to
height ratio α/Γh
Define the ratio of the facet slope (α) to facet height (Γh).
Tectonically active fronts display higher values of facet slope, height, length and smaller values of facet slope to height ratio
(9)
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major tectonic structure cross-cuts the Kifissos detachment fault and enters in the intermediate zone of higher deformation.
Figure 6.14 Asymmetry factor map (GGRS87) that defines the ratio of the area basin to the right (Ar- blue colored area), facing downstream of the trunk stream to the total area of the drainage basin (At).
The index was determined to detect tectonic tilting transverse to the flow at drainage basins. The direction of the purple arrow indicates the westward and eastward tilting. The drainage basins N.6 &
N.12 indicate symmetric basins. This map is presented in the Appendix II.
Figure 6.15 Graphical representation of the measured Af values. Af values get increased towards the western tip of the Dionysos fault while drainage basins N.7, N.11, N.13 suggest a symmetric pattern surrounded by middle or high asymmetry basins. The Af calculations are presented in the Appendix I.
0.0 10.0 20.0 30.0 40.0 50.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Af (Perez-Pena et al.,2010)
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The Af values for the drainage basins N.7 and N.13 ( Fig. 6.14, 6.15 ) suggest a predominatly symmetric pattern however, the Af values for the other basins which are independent of scale and distance show different trends of uniform tilting. Thus, drainage basins (N.6, N.10, N.11, N.12, N.15) show a trend of uniform tilting to the east with varying asymmetries. Basins N.6, N.10, N.11 ennvolve low asymmetry basins while N.12 and N.15 represent high asymmetry basins. The Af values for drainage basins (N.8, N.9, N.14, N.16, N.17) show a trend of uniform tilting to the west that envolve middle asymmetry basins with the exception of dranage basin N.8 that represents a high aassymetry basin.
From the interpretation of the Af values in Penteli region we suggest that the Dionysos fault is tectonically active with assymetries along it’s entire length.
We note a more active part in the central and the western part of the Dionysos fault and a decresment of the tectonic potential into the eastern part of the fault zone.
Mountain front sinuosity (Smf)
The mountain front in the western part of the Dionysos fault is geomorphologically expressed as a trace of north-facing scarps bounding the northern slopes of the Penteli Mt. Mountain fronts that had been created by faults with active character and uplift are relative straight and show low Smf
value (Bull, 1977; Bull & Mc Fadden, 1977; Silva et al. 2003; Pérez-Peña et al.2010). If the uplift decreases or ends, then the erosion processes create abnormalities in the foothills and the Smf values are increased. The most active mountain fronts have minor Smf values that vary between 1.0 - 1.6. The middle activity mountain fronts have Smf values that vary between 1.6- 3.0, thus when the Smf values outsprit this limit the mountain front is inactive. The calculation of the Smf index for the northern Penteli slopes were not performed ( Fig. 6.16 ) at its entire length, due to the non–existence of the elevation contours that operate to the Smf calculation.
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Figure 6.16 Hillshade map (GGRS87) showing the mountain fronts where the Smf index was measured.
Generally the Smf values for the northern Penteli slopes are low and vary between 1.00 in the western part (western sector in Fig. 6.16, 6.17) and 1.052 to the eastern measured mountain front (eastern sector in Fig. 6.16, 6.17). The mean Smf value of the Dionysos fault is 1.026 that indicates a very active mountain front, with high tectonic activity. We note a further decreasing of the Smf values towards the eastern tip of the Dionysos fault, that coincides with the Af interpretation. Thus, these elements strengthen the possibility of the western continuation of the Dionysos fault into the Kifissos drainage basin.
From the interpretation of the Smf index we suggest that Dionysos fault is tectonically active with an activity reduction to the east.
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Figure 6.17 Graphical representation of the measured mountain front sinuosity (Smf). The Smf values are low indicating the active character of the Dionysos fault. A decrement of the Smf values is noted towards the west-tip of the fault. The Smf calculations are presented in the Appendix I.
0.980 1.000 1.020 1.040 1.060
Western sector
Central sector
Eastern sector
1.004 1.023
1.052
Smf- Penteli Mt.
87 Triangular facets
The mountain front in the western part of the Dionysos fault is geomorphologically expressed as a trace of north-facing scarps bounding the northern slopes of the Penteli Mt. In particular, the strength of bedrock allows the further development of many well – developed triangular facets, providing evidence of Quaternary uplift in the footwall along the Dionysos fault plane.
A study by Petit et al. 2009; that based on statistical analysis of triangular faceted scarps, idicated a direct correlation between slip rate and facet slope.
Thus, a statistical analysis was carried out in the northern slopes of Penteli where triangular facets are developed (Fig. 6.18, 6.19, Table 6.3).
In the Penteli foothills seven triangular facets where detected, developing at the center of the Donysos fault where the maximun displacement is observed.
Those faceted surfaces comprising marbles that dip into the north at angles between 25°to 35° (from field observations). The maximum slope values are present in the triangular facet N.1 ( Table 6.3-A ) with an average slope of 36
° while the facet slope values decrease towards the fault tips (to the west).
Figure 6.18 The seven triangular facets that develop in the northern slopes of Penteli. (Google Earh photo, view from north.
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Table 6.3 Results of the triangular facet measurements in the mountain front of the Penteli, showing statistical relationships of the triangular facet attributes. The values are presented in Appendix I.Histograms show:
A: mean facet slope
B: facet height
C: facet slope- to height ratio
D: facet length
The differences between the observed slope values that vary from 36° in the facet N.1 to 24° towards the western facet N.7, and the measured dip angle from the fault surface (55° to 85° at the lower scarp from the field work) can
0 10 20 30 40
1 2 3 4 5 6 7
Mean facet slope ( °)
Facet number
0 100 200 300 400 500
1 2 3 4 5 6 7
Facet height (m)
Facet number
0 0.05 0.1 0.15
1 2 3 4 5 6 7
Slope /height ratio %
Facet number
0 100 200 300 400 500
1 2 3 4 5 6 7
Facet length (m)
Facet number
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be explained by the tilting of the footwall of the Dionysos fault to the south.
Height values ( Table 6.3-B ) seem to have a uniform distribution with though, a clear decreasement of the height values in the triangular facets N.6, N.7 in 273 m from heights up to 350m (facet N.1). So far ,the facet denotes as f5 is the highest at 368 m, while f1 has the speepest slope, at 36°.Mean slope- to-height ratios ( Table 6.3-B ) regularly display decrease towards the edges as the mean facet slope as has already been pointed. The measured facet length varied between 211m and 448m ( Table 6.3-D).
Figure 6.19 The seven triangular facets that are present in the Penteli area. The triangular facets are developed in the Penteli marbles.
Our statistical analysis of the trianular facets that are developing in the northern slopes of the Penteli are in agreement with Petit et al. 2009; thus, we idicated a direct correlation between slip rate and facet slope, that means that the characterists of the triangular facets are altered from the center of the fault towards to the tips. As the slip rates minimized along strike like as ,the development of triangular facets is lesser.
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A study by Tsimi & Ganas, 2015; correlated with an empirical relationship the Mean facet slope values ( Table 6.3-A ) with the fault slip rate. Thus, using this empirical relationship, for the central part of the Dionysos fault, and specifically in triangular facet N.1 where the maximum displacement is observed we calculated the fault slip rates for the central section of the fault.
Those results are in accordance with the estimated slip rates from the topographical profile (see Fig. 5.41 ) and in particular from a scarp that that had been discovered in the fied work with height up to 2 m. Summarizing, from the interpretation of the Af values, Smf mountain front sinuosity and the development of many well – developed triangular facets, that provide evidence of Quaternary uplift in the footwall along the Dionysos fault plane in the Penteli region we suggest that the Dionysos fault is tectonically active with assymetries along strike its entire length.We note a more active part in the central and the western part of the Dionysos fault and a decrement of the tectonic potential into te eastern part of the fault zone. Two of these analyzed indeces show that the major tectonic structure might cross-cuts the Kifissos detachment fault and enters in the intermediate zone of higher deformation (Fig. 6.20).
Figure 6.20 Slope distribution map of the Kifissos drainage basin that is denoted with N.7 (GGRS87) representing the possible continuation of the Dionysos fault zone to the west.
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7 Seismic Hazard Assessment
Instrumental and historical records show that the last 2.300 years (Ambraseys & Psycharis, 2012) with the exception of 1999 event, no major shaking events hosted in Athens Metrolopitan area (M>6.5 Galanopoulos, 1955). Following though the historical catalogue two events have been recorded in NE Attica ( Ambraseys & Jackson., 1997; Burton et al.,2004) the 1705 and Oropos 1938 event. The active Dionysos fault is a low slip rate fault that due to its long recurrence interval is absent from the historical catalogues. According to Papanikolaou I, 2015; seismic hazard assessement is still preodominaly based on the instrumental and historical catalogues of seismicity . However, these catalogues ae incomplete regarding both their temporal and spatial coverage becase they are generally too short and they are associated with large uncertainities regarding the epicentre localities. Thus, the methodology that has been performed aims to constract a seismic hazard map for the Dionysos fault, of the maximum expected seismic intensities which are based exclusively in the regional geological pattern as well as in the specific fault characteristics.
The worse case scenario for the Dionysos fault implies the rupture of the entire 16 Km of its length. Based on the equation between Magnitude (M) and surface rupture length (SRL) of the worldwide dataset of Wells &
Coppersmith, 1994; the Dionysos fault can generate a M=6.5 event.
Μ= a + b log *(SRL), where SRL corresponds to the fault length and a ,b are parameters that change due to the fault slip type. Furthermore the second parameter that has been examined for the construction of the seismic hazard map (Fig. 7.3) was the ratio of the depreciation rate of the macroseismic intensities (Theodoulidis, 1992) between the Dionysos fault and the area that will be affected in a possible future rupture (Fig. 7.1).
lnag = 3,88+1,12 Ms – 1,65 ln (R+15) + 0,41S+ 0,71P (1)
92 lnag = 0,28+0,67 IMM+ 0,42S + 0,59P (2)
For the system solution the above S,P parameters considered to be zero.
(1) = (2) => ln (R+15)= [3,60- 0,67 IMM+1,12MS]\1,65 (PGA Horizontal).
Thus, we calculated the epicentral distances (Table 7.1) for the intensities of the modified Mercali scale.
Table 7.1 Summary of the calculated Mw, Macroseismic intensities and their epicentral distances.
Fault Length(Km) Mw Intensity a
Epicentral distance
(Km) a
Intensity b
Epicentral distance
(Km) b
Dionysos 16 6.5 IX 3.39 VIII 13.39
Figure 7.1 Macroseismic intensity map (GGRS87) between the Dionysos fault and the area that will be affected in a possible future rupture.
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This methodology has been performed in previous studies such as Roberts et al. 2004; Papanikolaou et al. 2013; where they demonstrated a direct correlation between fault scaling parameters and local geology with the seismic hazard assessement. We have to note, that for the final construction of the macroseismic intensity map ( Fig. 7.1) the hypothetical epicentres were placed at a distance equal to the 4/5 of the maximum expected macroseismic intenisty ( here IX) in the hangingwall of the Dionysos fault. Furthermore, were taken into account both the fault dip ( 55°) and the focal depth (8 Km).For the constuction of the risk hazard map a geological categorization (Fig. 7.2) of the formation that are located in the affected area was performed in order to their geotechnical characteristics (Degg, 1992). These are:
Figure 7.2Geological categorization map (GGRS87) of the formations that are located in the area that will be affected in a possible Dionysos fault rupture, (Degg, 1992).
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A (purple color in Fig. 7.2): This category contains the geological formations that present an adequate seismic behavior, and correspond to the alpine basement that outcrops in the wider affected region.
B (green color in Fig. 7.2): This category contains the geological formations that exhibit an intermediate type of seismic behavior such as solid granular formations with medium coherence and in general medium cohesion soils.
C (brown color in Fig. 7.2): This category contains the geological formations that exhibit poor seismic behavior such as loose sandy soils that in the study area correspond to the Holocene basin formations.
Seismic Hazard Assessment Map (Fig. 7.3) presents the areas that will be affected if Dionysos fault zone activates in all of its 16 Km length. We estimated and visualized the macroseismic intensities that will come from the Dionysos fault zone in a possible future rupture.
The highest macroseismic intensities in a possible future rupture will be recorded in the hangingwall of the Dionysos fault (Fig. 7.3 – intensities X in red color). These areas evolve among others the villages of Zouberi, Nea Makri, Agia Marina, Agios Pandeleimonas and Vranas that are located in the coastal zone and Kryoneri in the Kifissos drainage basin. Despite the fact that Dionysos town is located closer to the Dionysos fault, it is interesting to note that this town will suffer only moderate damage (intensity VIII). Thus, the damage pattern varies over distances due to the bedrock geology changes. Four Km west of the Dionysos town, Drosia village will suffer higher damage (intensity IX) than Dionysos town due to the geological background (Holocene deposits). Other towns that will suffer among others from macroseismic intensities IX are Irakleio, Chalandri and Gerakas in the footwall of Dionysos fault and Kokinovraxos, Marathonas that lie many Km north of the Dionysos fault.
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Figure 20 Seismic hazard assessment map (GGRS87) from a possible rupture of the Dionysos fault. The measured intensities vary from VII to X. This map is also presented in the Appendix II.
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Several towns in the immediate footwall of the Dionysos fault such as Nea- Erythraia, Nea Penteli, Drafi and Anthoussa among others will experience minor to moderate damages (intensities VII- VIII). Such minor damages will be present also in towns that are located in the hangingwall of the Dionysos fault such as Rapentossa, Varibobi, Agia Triada and Vothonas among others.
From the aforementioned we realize that a future rupture of the Dionysos fault might cause an economical loss of many thousands of euros and above this might cost human lives. For this purpose, the Dionysos fault zone must be present in the potential fault threats, in the Attica region.
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8 Results of the study
Dionysos Fault zone is a low slip - rate fault that bounds the small Dionysos graben, trending NW-SE with length up to 16 km. The presence of this fault zone near Athens results high risk especially after the recent neighboring Athens 1999 moderate (Ms=5.9) event that ruptured an unknown since then, fault that resulted 143 fatalities.
The main work involves the mapping of the fault trace in order to find post-glacial scarps and study the geometry and other fault features. Further, geological mapping in Dionysos small graben carried out since January- February 2013 including stratigraphy and tectonic data in the wider area of Penteli Mt, from Ekali at the western Penteli area, Nea Makri at the eastern, in order to give a better perspective of the tectonic setting in the overall area.
The footwall of Dionysos fault zone includes substantially the entire Penteliko Mt. delimited to the west in Ekali, Nea Erythraia, Kastri , to the east in Pyrgari and to the south in Pendeli area. As in the hanging-wall thus and in the footwall of Dionysos fault the alpine formations that are present, are marbles and schists. Dionysos fault zone bounds the small Dionysos graben which develops between central Penteliko Mt. and Dionysos-Dionysovouni hills, consisting of Holocene and Pleistocene post-alpine formations. Holocene formations represent alluvial and river deposits consisting mainly of loose, brown- colored clayey sandy material with dispersed cobbles-rubbles and in places breccioconglomerate intercalations that are originated from Penteli alpine formations, specifically marbles and schist’s. The postalpine sediments that are present in the footwall of Dionysos fault, are part of Mesogea basin that extends from South Penteli Mt. to the North , and Koropi to the South.
Sedimentation started in the late Miocene and has continued through the Pliocene with continental and fluvioterrestrial sediments.
Dionysos fault has a clear geomorphic expression which is reflected in the elevation data across the fault showing two distinct parts. A flat area to the