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Ganas, A., Pavlides, S. & Karastathis, V., 2004

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Ganas, A., Pavlides, S. & Karastathis, V., 2004 5th International Symposium on Eastern Mediterranean Geology Thessaloniki, Greece, 14-20 April 2004 Active fault mapping in Attica, Greece, using image processing of Digital Elevation Models Ganas A.1, Pavlides S.2 & Karastathis V.1 Geodynamics Institute, National Observatory of Athens, PO Box 20048, Athens 11810, Greece, [email protected] Department of Geology, Aristotle University of Thessaloniki, Thessaloniki 54 124, Greece, [email protected] Keywords: Active faults, Attica, DEM, shaded relief, earthquakes This paper presents a structural analysis of digital elevation models (DEMs) and shaded relief images of Attica, Greece in order to map active faulting patterns. The analysis is based on fundamental properties of DEMs such as aligned tonal variations, artificial illuminations and elevation profiles. Our results are compared against recent geological data. Five (5) normal fault segments have been recognized on the basis of structural and geomorphological inter- pretation. The faults are from west to east: Thriassion, Fili, Afidnai, Avlon, and Pendeli and range in length between 10 to 20 km. All of them show geomorphic evidence for recent activ- ity; can rupture in magnitude 6-6.5 earthquakes and present considerable hazard for Athens. Introduction – data processing This study was prompted by the 7/9/1999 Athens earthquake (Ms5.9) that occurred in a “low- seismic hazard” area of Greece (Papazachos and Papazachou, 1997, page 126). This unex- pected earthquake indicated that the seismic potential of Attica may be greatly underesti- mated, thus, making necessary new investigations for seismic sources near Athens. We sug- gest that digital processing of elevation data may provide evidence for recent activity along fault segments, thus supply information on a) lengths of faults b) scarp heights and c) relative degree of activity. This methodology is complementary to current methods of seismic hazard assessment. Following the Athens earthquake, recent field work (Pavlides et al., 2002) has indicated that rupture occurred along the Phyle fault segment, located in Parnitha mountain (Figure 1). Here we show that by applying the digital processing techniques, we may charac- terize four (4) other segments as active. The same methodology has been applied to a coarser DEM, including the Thiva grabens to the North of Attica, and some observations on the eastern propagation of the Gulf of Corinth rift are made. Topography can be used as a proxy for fault displacement within rift systems (Roberts and Ganas, 2000). Digital Elevation Models (DEMs) are modelled analogues of topography origi- nating mainly from Photogrammetry (either air photography or spaceborne stereo images) and/or radargrammetry. As the vertical error of those models usually does not exceed 10% of actual topography in areas of rugged relief (i.e. a mountain peak of 1000 metres would ap- pear as 900 or 1100 metres on the DEM) many workers have used DEMs as a new tool to observe and map landforms and structural boundaries, to display and to understand spatial relationships of structures. In this study the elevation data originated from the elevation con- tours on the general use maps of the Hellenic Army Geographical Service (HAGS) at scales 1:50,000 (two sheets) and 1:250,000 (one sheet). The contours have been extracted from airborne Photogrammetry and their accuracies are well within the requirements for fault seg- ment mapping (vertical accuracy is fourth of the contour, 5 m and 25 m respectively for the datasets used). In order to map fault segments accurately we produced a 20-m DEM. Then, we constructed a 60-m DEM in order to apply our analysis on a larger-area using the same concept. We were able to map a dozen of normal fault segments which presents a profound Ref: T7-9 5th International Symposium on Eastern Mediterranean Geology Thessaloniki, Greece, 14-20 April 2004 Figure 1. 20-m Intensity image showing elevations of Northern Attica, Greece. High elevations are shown as bright pixels while low elevations as dark. T is Thriassion basin, F is Fili basin, A is Avlon basin, Af is Afidnai basin and P is Pendeli moun- tain. Black arrows point to the termination of active faults. Inset: Field Photograph of the Fili Fault Segment, Attica, Greece. The fault scarp is developed in hard, resis- tant limestone. The fault strike/dip is N120/70°SW. Notice the elliptical shape of the footwall. View is to the East. difference in terms of strain localization when compared to the structures to the east (Gulf of Evia) and west (Gulf of Corinth). The 60-m DEM smoothes topography more than the 20-m one because contour spacing and interval are both larger. However, the long wavelength fea- tures of topography (mountain ranges, valleys) remain clear. Data analysis and interpretation methods The use of digital elevation images in fault mapping is a well established technique (e.g. Ga- nas et al., 2001). The first property of an elevation image is the tonal variation of pixels usu- ally spread out on a 16-bit scale. The convention is to use a normal look-up table for heights, i.e. increasing brightness levels correspond to higher elevations. It follows that continuous, linear tonal boundaries on the intensity image for distances more than 100 pixels indicate the positions of mountain fronts (see Figure 1). Many times the tonal boundaries are quite sharp, i.e. the values of pixels on either side can differ by more than 500 metres, depending on DEM resolution. Their image locations can be easily matched with long breaks in slope (seen in general use maps) and are assumed to indicate the positions of fault scarps, which in turn indicate long-term neotectonic deformation (hundreds up to several millions of years). In Fig- ure 1 we show five (5) such areas, where tonal boundaries are sharp and imposing. One of those areas (upper right inset in Figure 1) has already been mapped by Pavlides et al., (2002) as the Fili segment. It is reasonable to suggest that the other four (4) image features corre- spond to active, normal faults. We elaborate further on this below. The second property of a DEM image is that we can extract longitudinal profiles of mountain ranges so that to observe how topography varies along strike. A third tool is to draw transverse profiles to locate signifi- cant breaks and measure escarpment heights. All large segments have escarpment heights in excess of 400 metres. Smaller escarpments are probably due to small normal faults inside the Parnitha Mt. Using similar arguments we constructed the fault map of the greater area on the 60-m shaded relief image (Figure 2). The following segments have been identified using the methodology described above: APFS Agioi Apostoloi, OFS Oropos, MR Maliza Rupture, Ref: T7-9 5th International Symposium on Eastern Mediterranean Geology Thessaloniki, Greece, 14-20 April 2004 Figure 2. 60-m Elevation model of Attica and surrounding areas with superimposed fault pattern. The model is illuminated from the North at a low angle to accentuate topography. Red lines are interpreted normal faults. Blue lines are major rivers. Cyan colour indicates lakes. Dark yellow pattern indicates urban areas. Note that all faults have lengths less than 20 km as defined by the systematic reduction of eleva- tions towards their ends. LTFS Loutropolis, MEFS Megara, KSFS Kakia Skala, PLFS Pyli, AIFS Aigosthena, ABFS Agios Basilios, ERFS Erithres, KAFS Kaparelli, and DAFS Dafnoula. We note that segments SAFS and ERFS have been recognized by Roberts and Ganas, 2000; MEFS segment by Goldsworthy and Jackson, 2000 (the same authors have named KSFS as Saros Fault), and segments THFS and FIFS by Ganas et al., (2001). We have estimated magnitudes of future events in case there is an large earthquake along any of the remaining four (4) segments near Athens from empirical relationships. For simplic- ity, we assume surface rupture lengths equal to the extracted fault lines. The relationships of Ambraseys and Jackson (1998, Ms=1.14*logL+5.13) and Pavlides, et al., (2000, Ms=1.03*logL+5.30) provide almost identical results whilst that of Papazachos and Pa- pazachou, (1989, Mw=(1.85+Log(L))/0.51) 0.4-0.5 magnitude units less. Despite the notable differences between the estimates it is clear that events between 6.0 and 6.5 Ms are to be expected. Another use of images like Figure 2 emerges when it is compared to focal mecha- nism data. We note that both the 1981 earthquake sequence and the 1999 Athens earth- quake focal plane solutions match satisfactorily the strike of mountain ranges. This implies that perhaps we could make use of lineaments on shaded relief images to infer the strike of seismogenic faults inside other rift systems. The interpretation of the fault pattern in Figure 2 (60-m DEM) shows first that the normal faults of Attica are more closely spaced in compari- son to the Corinth Rift, further west, a pattern that implies lower slip rates (Cowie and Rob- erts, 2001). If lower slip rates are indeed true this explains both the long recurrence intervals of earthquakes in Attica and the geomorphic expression that is only a fraction of the huge Quaternary offset across the southern coast of the Gulf of Corinth. Secondly, west Attica is dominated by normal faults striking N60 to N90 whereas both North Attica (Parnitha) and east (Pendeli) is dominated by normal faults striking N120 degrees (on average). This marked dif- ference in active fault strikes shows that the Gulf of Corinth rift cannot propagate through Parnitha Mt and it has to circumvent it from the north. It seems reasonable to suggest that the reason for that is the Quaternary volcanism of the area, which has weakened the Attica crust.
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