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2 nd INQUA-IGCP-567 International Workshop on Active Tectonics, Earthquake Geology, Archaeology and Engineering, Corinth, Greece (2011) EARTHQUAKE ARCHAEOLOGY INQUA PALEOSEISMOLOGY AND ACTIVE TECTONICS THE DISCONTINUITY OF A CONTINUOUS FAULT: DELPHI (GREECE) Wiatr, Thomas (1), Klaus Reicherter (1), Ioannis D. Papanikolau (2, 3) & Tomás Fernández-Steeger (4) (1) RWTH Aachen University, Neotectonics and Natural Hazards Group, Lochnerstr. 4-20, 52056 Aachen, Germany (2) Laboratory of Mineralogy & Geology, Department of Sciences, Agricultural University of Athens, 75 Iera Odos Str., 11855 Athens, Greece. papanikolaou@ucl.ac.uk (3) AON Benfield UCL Hazard Research Centre, Department of Earth Sciences, University College London, WC 1E 6BT, London UK. papanikolaou@ucl.ac.uk (4) RWTH Aachen University, Department of Engineering Geology and Hydrogeology, Lochnerstr. 4-20, 52064 Aachen, Germany. fernandez-steeger@lih.rwth-aachen.de Abstract (The discontinuity of a continuous fault: Delphi (Greece): We used a terrestrial laser scanning system for the reconstruction and analysis of the morphotectonic features on a fault segment near the ancient Delphi, Greece. Delphi is located on the northern part of the Gulf of Corinth and embedded in a seismic landscape and is the major onshore fault north of the Corinth Gulf. This paper concentrates on the LiDAR long range investigation of the fault for generating a digital elevation model. The model was used for estimating the natural fault plane height by using several vertical profiles. The results show a high horizontal and vertical variability on a 120 m long fault scarp. Key words: long range LiDAR, discontinuous fault, Corinth Gulf, Delphi INTRODUCTION The ancient Delphi with its oracle dedicated to the god Apollo, was the most popular place of worship around 700 B.C. to 400 A.D. in ancient Greece. Delphi is located at the southern flank of Mount Parnassus that is compiled by thick-bedded neritic Mesozoic limestones with significant bauxite deposits. The mountain range is situated on the northern coast of the Gulf of Corinth and bounded by an active fault zone that dips southwards (Fig.1). Ambraseys & Jackson, 1998; Papazachos & Papazachou, 2003 and Pavlides & Caputo, 2004). Piccardi (2000) described the 373 B.C. earthquake, which partly destroyed the ancient Delphi showing that there is a post-earthquake reconstruction phase at the shrine of Athena (located around 500 m east of the Temple of Apollo) (Fig.2). Fig. 2: Setting of the archaeological site of the shrine of Athena Pronaia (modified from Piccardi, 2000). Fig. 1: Tectonic and topographic overview map of the Delphi area. The Gulf of Corinth is a graben like tectonic structure with E-W trending normal faults and characterized one of the fastest extending regions worldwide with up to 20 mm/yr rate (Billiris et al., 1991; Briole et al., 2000). Destructive historical earthquakes in the area are reported for 373 B.C. (Piccardi, 2000), 515 A.D. (Ambraseys & Jackson, 1998; Papazachos & Papazachou, 2003) and 1870 (1 st of August, Ms=6.7, Furthermore, he postulates that the Temple of Athena was relocated from its original position. Aim of our study was the reconstruction of the morphotectonic features of the Delphi fault scarp, located 2 km west of the Temple of Apollo by using a terrestrial remote sensing technique. We scanned a 120 m long fault scarp with long range LiDAR. We used a terrestrial LiDAR (Light Detection and Ranging) system from Optech Inc. METHODS The ground-based LiDAR (Light Detection And Ranging) or TLS (terrestrial laser scanning) remote sensing method has been established as a versatile data acquisition tool in photogrammetry, engineering technologies, atmospheric studies and as a good data acquisition tool in geosciences and geological engineering in difficult accessible areas. 276
2 nd INQUA-IGCP-567 International Workshop on Active Tectonics, Earthquake Geology, Archaeology and Engineering, Corinth, Greece (2011) EARTHQUAKE ARCHAEOLOGY INQUA PALEOSEISMOLOGY AND ACTIVE TECTONICS As the TLS has a high spatial and temporal resolution it is an effective remote sensing technology for reconstruction, monitoring and observation of geosciences phenomena. The fundamental principle of ground-based LiDAR is to generate coherent a laser beam with little divergence by stimulated emission. LiDAR is a contact- and destructionless, non-penetrative active recording system which is stationary during the recording. The electromagnetic waves are reflected by surfaces and the receiver detects portions of the backscattered signal. All scan sequences were mapped with first pulse detection mode. The laser ranging system is based on measuring the time-offlight (two-way travel time) of the short wave laser signal. Advantages of the terrestrial method are the flexible handling, a relatively quick availability of an actual dataset, and a very high spatial resolution of the object with information about intensity, x-y-zcoordinates and range. The combination with a digital camera allows combining the point cloud with panchromatic information in order to achieve additionally the RGB colour-coding. For the morphological analysis twelve vertical profiles with 10 m distance to each other were generated from the LiDAR data. All profiles start on the same height level on top of the fault zone on the foot wall and are perpendicular to the fault plane towards the hanging wall. Results show a variation of the free face fault height with a difference of 5.4 m. The height data range of the natural bedrock fault plane is between 4 and 9.4 m. Furthermore, horizontal variations of 3 m were detected (Fig.5). Laser scanning allows 3D surface data acquisition, which is specifically characterized by a digital data record and a computerized data analysis. Furthermore, an implementation of the dataset in a geographical information system (GIS) is uncomplicated with accurate digital elevation models (DEM) or digital terrain models (DTM) sourced directly from the raw dataset. The infrared laser scanner detected the monochromatic information of the backscattered intensity in 256 grey values. The information of the monochromatic wavelength, the detected backscattered intensity, reflexes the surface properties in the near infrared range. This wavelength is invisible for human eyes. Hence, the results show a different kind of view of the surface conditions. The quality of the reflection depends on the inclination angle of the laser beam, the range between the object and scanner, the material, the colour, the surface condition (weathering/roughness), and the spatial resolution. DISCONTINUOUS FAULT SEGMENT ON A CONTINUOUS FAULT NEAR DELPHI The scan position for the long range investigation was 250 m south of the fault plane (Fig.3). The raw data point cloud includes around 6.2 million points for an 130 m long and 60 m wide scan window. For this study a point resolution of 2 cm was chosen. Moreover, we recorded 9 close range scans with 4 mm point resolution (Fig.4). After data validation and cleaning, the point cloud has been geo-referenced and imported into a GIS. In the GIS, the scans have been converted in a triangulated irregular network (TIN) and in a raster format. With the grid format it is possible to calculate the basic applications for morphologic specifications. Fig. 3: Panchromatic images of the investigation area. a) The geoeye satellite image shows the continuous fault by Delphi and includes the long range LiDAR study area (modified from Google earth). b) Photo of the south dipping continuous fault zone 2 km westward from the ancient shrine of Apollo by Delphi. We found that the Delphi fault has an oscillation in three dimensions (horizontal and vertical displacement), across the 120 m long scan sequence. The absolute elevation range of all profiles on the natural free face fault surface (vertical range of the fault plane) is between 574.5 m and 588.7 m above sea level (see Fig.5 right site number 1) and the horizontal variation is between 11.2 m and 21 m (see Fig.5 number 1 under the profiles). The horizontal and vertical value range (number 1) includes the variation of the knick point on top of the free face to the foot wall (number 2) and the knick point of the bottom of the free face to the hanging wall (number 3). Furthermore, the results had shown a variability of the alluvial deposits (vertical and horizontal displacement) of the hanging wall (number 3). It turns out that the long term slip rate for postglacial scarps (last 19 ka ± 3 ka) in this case ranges between 0.21 ± 0.04 mm/yr and 0.49 ± 0.09 mm/yr 277
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