Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

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1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology TECTONIC MORPHOLOGY OF THE LAKE OHRID BASIN (FYROM, ALBANIA) N. Hoffmann (1), K. Reicherter (1); C. Grützner (1), T. Wiatr (1) and T. Fernández‐Steeger (2) (1) Neotectonics and Natural Hazards, RWTH Aachen University. Lochnerstr. 4‐20. 52056 Aachen, GERMANY n.hoffmann@nug.rwth‐aachen.de (2) Chair of Engineering Geology and Hydrogeology, RWTH Aachen University. Lochnerstr. 4‐20. 52056 Aachen, GERMANY Abstract: The Ohrid Basin is a major N‐S trending graben structure located on the border of Macedonia (FYROM) and Albania, associated with other basins in the Dinaride mountain belt. Within the basin an “ancient lake” developed since the Late Miocene/Pliocene. Since the beginning of basin formation around 700 m of sediment accumulated in the lake, the initial stage of basin formation is a strike‐slip movement followed by extension. The general geodynamic setting of the Lake Ohrid area can be described with a “basin and range” situation. The multidisciplinary ICDP‐SCOPSCO initiative is currently investigating Lake Ohrid and its environs in order to launch a deep drilling project. Key words: Palaeostress, Palaeoseismology, Macedonia, Albania INTRODUCTION The evolution of the Lake Ohrid Basin is not only discussed by geoscientist but is also a big issue to the biology community because of the high number of endemsim. In order to understand the mechanisms that affected the Western Macedonian area the multidisciplinary ICDP‐SCOPSCO project will be carried out at Lake Ohrid. Within this a structural and tectonomorphological study is accomplished at the vicinity of the lake. Lake Ohrid considered as an “ancient lake” was tectonically formed during the late Tertiary in the extensional periods of the Palaeogene and Neogene (Dmurzdanov et al. 2004). The main objective of the observation is to delineate the (neo)tectonic history of the lake. We combined the data of the palaeostress analysis with those of the joint and fracture analysis to track the assumed lineations which we derived from satellite images and old geological maps. In addition features such as fold axes of older deformation phases were measured to accomplish the picture of different deformation phases. The area which was investigated stretches from the east of the Lake including the Mountains of the Galicica National Park to the west where up to now only the areas close to the shore lines were taken into account. Because of rapidly rising relief on both sides of the lake, data taken on top of the mountains are not representative for the given research topic. The northern tip of the investigated area is the outlet of the river Crni Drim and the south is limited by the end of the aggradations plain. Therefore the investigation focused on fault planes with striae close to the western and eastern shorelines of the lake. The lithologies are mainly Triassic limestones and Mesozoic ophiolites. The origin of the lake formation is unclear, possibly an older tectonic transtensional phase or reactivation led to a pull‐apart like opening of the basin, followed by E‐W directed extension. A certain influence of basement 56 structures on the present‐day fault patterns has been assumed, but not proven. Present‐day slip rates of faults estimated by GPS are smaller than 1 to 2 mm/a (Burchfiel et al., 2006), but the uncertainties are larger than the observed movement. GPS data are still not precise enough to discriminate active faulting. GEOGRAPHY AND GEOLOGY OF THE LAKE OHRID BASIN The Lake Ohrid Basin (40°54’ ‐ 41°10’ N, 20°38’ ‐ 20°48’ E) is located at the border of the Former Yugoslavian Republic of Macedonia (FYROM) and Albania stretching over a length of c. 30 km and a width of c. 15 km. Within the basin an “ancient lake” developed. The time frame of the lake evolution ranges between 2‐10 Ma depending on different authors. Clear is that the lake formed in an early stage of the basin development and probably as an initial small river crossing the graben. (Albrecht and Wilke 2008). The lake at an altitude of 693 m a.s.l. is surrounded by the horst of the Galicica Mountain Range with a height of 1750 m ‐ 2200 m to the east and the Mokra Mountain Chain with a height of c. 1500 m to the west. With almost Fig.1: Geological Map of the Lake Ohrid region (Wagner et al 2008)
290 m water depth the basin accumulated about 700 m of sediments since the Late Miocene/Pliocene. The basin morphology is relativeley simple with a general N‐S orientation and straight shorelines. The region of the Ohrid Basin is part of the Alpine orogeny that formed during late Jurassic to Miocene and belongs to the N‐S trending system of the Dinarides and Hellenides. The orientation of the basin matches the general orientation basins of the Balkan region. All graben structures like the Korce Basin are bent to N‐S trending faults. In Fig. 1 it is clear that the strike of the basins (N‐S) does not correspond to the strike of the major tectonic units (NW‐SE). This already gives evidence that the basins are formed due to a younger deformation stage. Through the Pliocene the Ohrid and the Korce Basin belonged to the same lake‐river‐system and have been disconnected lateron. NEOTECTONICS AND SEISMICITY FYROM and Albania are seismic active regions. Especially, the intramontane basins of Late Neogene age form highly active seismic zones in the region with a high risk of moderate earthquakes. Several moderate earthquakes have been reported during the last few centuries (Muco 1998; NEIC database, USGS). Major earthquakes occurred during historical times. Lychnidos (the ancient city of Ohrid) was destroyed completely by an earthquake in 526 AD. It was rebuilt by Emperor Justinian (527‐565), who Fig.2: Seismicity Map NEIC database was born in the vicinity, and was called by him Justiniana Prima, i.e. the most important of the several new cities that bore his name. The last prominent earthquake took place in on 18 th February 1911 at 21.35 close to Lake Ohrid Basin, (M 6.7, corresponding to EMS X; 15 km depth, N 40.9°, E 20,8°). The last earthquake occurred on Jan 8 th 2009 with a magnitude of MW=4.9 close to the lake. Hypocenter depths scatter between 10 and 25 km but some deeper earthquakes occur between 25 kmand 50 km depth. Very rarely intermediate earthquakes around 100 km depth are observed. Small and moderate earthquakes (< M 5.5) take place predominantly along major fault zones, and are concentrated along the margins of the Ohrid Basin. The Ohrid‐Korça Zone is 1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology 57 considered to be the region of the highest seismic hazard in the Albanian‐Macedonian Corridor based on present‐ day seismicity (see Fig. 2) (Aliaj et al., 2004). The present day tectonic regime can be divided in a coastal domain of compression at the Adriatic coast followed by an interior domain of extension dominated by north striking normal faults to the west (see Aliaj et al. 2004). TECTONIC GEOMORPHOLOGY The Ohrid Basin meets all criteria of an active, seismogenic landscape: linear step‐like fault scarps occur in the landscape and under water in the lake. Wineglass shaped valleys, wind gaps, triangular facets and well preserved scarps are exposed (see Fig. 3). Also rotated blocks can be inferred by the geomorphology. Postglacial (or Late Pleistocene) bedrock fault scarps at Lake Ohrid are long‐lived expressions of repeated surface faulting in tectonically active regions, where erosion cannot outpace the fault slip. Generally, the faults and fault scarps are getting younger towards the basin center, as depicted on seismic and hydroacoustic profiles. Additionally, mass Fig.3: Topography at Lake Ohrids east coast. View from N. movement bodies within the lake and also onshore (rockfalls, landslides, sub‐aqueous slides, homogenites, turbidites) are likely to be seismically triggered, eventually damming the outflow of Lake Ohrid temporarily. Fig.4: Shaded relief of the Lake Ohrid area (based on Shuttle Radar Topographic Mission data, light from east). Morphological lineations (white lines) frame the lake (Wagner et al 2008) Lake Ohrid is situated in a zone controlled largely by extension in the Peshkopia‐Korça belt, trending N‐S in the eastern part of Albania. It is framed in the W by N‐S trending lineations and in the E by sets of N‐S and NNE‐ SSW trending lineations (see Fig. 4). Active faults displace
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Abstracts Volume Archaeoseismology
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Edita e imprime: Sección de Public
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Page 9 and 10: the importance of performing absolu
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Page 13 and 14: DISCUSSION Based on detailed field
Page 15 and 16: Table 1: Source parameters used in
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Page 21 and 22: continuous support. N. Shalev, G. G
Page 23 and 24: The tumulus initial morphology show
Page 27 and 28: our understanding and prediction of
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Page 31 and 32: Nevertheless the lack of research,
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Page 35 and 36: The site reconnaissance successfull
Page 43 and 44: Table 1: Surface faulting extent re
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Page 57: RISK ANALYSIS The created hazard ma
Page 61 and 62: CONCLUSIONS From the palaeostress a
Page 63 and 64: Fig. 2: Geological map of the study
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Page 99 and 100: Fig. 8: Seismic ground shaking may
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Page 103 and 104: CONCLUDING REMARKS The exposed exam
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In addition, the Database of histor
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were published by IGME (1983) and E
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1 st INQUA‐IGCP‐567 Internation
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The city collapsed and it was aband
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Fig. 4: Slipped lintel in a window
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to overcome shear resistance and in
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Newmark, N.M. (1965). Effects of ea
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associated with various tsunamis (<
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this method, the magnitude of the e
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etween pieces. Fragments have not f
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on the exact date of the earthquake
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THE ARCHAEOLOGICAL ZONE COLOGNE Aft
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affecting the Alcázar was about 50
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archaeoseismology could instead bec
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In the area later occupied by Vilni
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Fig. 2: Assemblage of landforms ass
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Fig. 8: Data and regression for mag
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Fig. 3: Relationships among magnitu
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CONCLUSIONS 1. Spatial distribution
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The stratigraphic sequence (Fig. 4)
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especially in the zones of hanging
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interpretations are based on relati
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Fig. 5: Gray‐scale value laser im
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(~40 cm), F‐ rotated building str
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Index 1 st INQUA‐IGCP‐567 Inter
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