Geological, geomorphological and archaeoseismological observations along the Cibyra fault and their implications for the regional tectonics of SW Turkey

In this paper, the Cibyra segment of the Fethiye-Burdur Fault Zone (FBFZ) is investigated using direct field evidence, which helps clarify the controversial behaviour of this zone. The remains of the ancient city of Cibyra which is located on the Cibyra Fault provide invaluable data in documenting traces of fault deformations and related palaeoearthquakes.

Turkish Journal of Earth Sciences (Turkish J Earth Sci.), Vol 20, 2011, pp 429–447 Copyright ©TÜBİTAK V KARABACAK doi:10.3906/yer-0911-51 First published online 21 March 2010 Geological, Geomorphological and Archaeoseismological Observations Along the Cibyra Fault and Their Implications for the Regional Tectonics of SW Turkey VOLKAN KARABACAK Eskişehir Osmangazi University, Department of Geological Enginneering, TR−26480 Eskişehir, Turkey (E-mail: karabacak@ogu.edu.tr) Received 04 November 2009; revised typescript receipts 01 February 2010; accepted 21 March 2010 Abstract: In this paper, the Cibyra segment of the Fethiye-Burdur Fault Zone (FBFZ) is investigated using direct field evidence, which helps clarify the controversial behaviour of this zone The remains of the ancient city of Cibyra which is located on the Cibyra Fault provide invaluable data in documenting traces of fault deformations and related palaeoearthquakes Slickensides on fault planes, deflected stream beds and terraces, fault-parallel elongated ridges along the Cibyra Fault are the clearest surface evidence for left-lateral faulting Archaeoseismological evidence in the ancient city is consistent with the geological and geomorphological observations along the fault Based on detailed field observations, it can be concluded that the Cibyra Fault reactivated in AD 417 and probably after the 7th century AD, causing extensive damage in Cibyra Based on geological and geomorphological field evidence, fault offsets and deformed archaeological relics, it can be suggested that the Cibyra Fault is an active left-lateral fault capable of producing earthquakes of considerable magnitude Furthermore, as the trend of the Cibyra Fault is consistent with the FBFZ it is concluded that NNE–SSW-trending faults in southwestern Turkey are active and the motion on them is left lateral Key Words: palaeoearthquake, archeoseismology, Cibyra ancient city, Fethiye-Burdur Fault Zone, SW Turkey Cibyra Fayı Boyunca Jeolojik, Jeomorfolojik ve Arkeosismolojik Gưzlemler ve Bunların GB Türkiye’nin Bưlgesel Tektoniği Hakkındaki Ưnemi Özet: Bu makalede, Fethiye-Burdur Fay Zonu (FBFZ)’nun Cibyra segmenti, zonun tartmal davrann aỗkla kavuturmaya katk salayacak dorudan arazi kantlar ile incelenmiştir Cibyra Fayı üzerinde bulunan Cibyra antik kentinin kalıntıları fay deformasyonunun ve ilişkili eski depremlerin izlerini belgelemede değerli veriler sağlar Cibyra Fay boyunca fay dỹzlemlerindeki kayma ỗizikleri, ửtelenmi dere yataklar ve teraslar, paralel olarak uzamış sırtlar sol yanal faylanmaya yönelik belirgin arazi kanıtlarıdır Ayrıntılı arazi gözlemleri temelinde, arkeosismolojik kanıtların jeolojik ve jeomorfolojik gözlemlerle uyumlu olduğu ve Cibyra Fayı’nın 417 ve olaslkla yy sonrasnda yeniden harekete geỗerek Cibyra antik kentinde bỹyỹk ửlỗekli hasara neden olduu sonucuna varlabilir Jeolojik ve jeomorfolojik arazi kanıtları ve arkeolojik kalıntılardaki yerdeğiştirme ve deformasyonlar gözönünde bulundurulduğunda, Cibyra Fayı’nın sol yanal aktif bir fay olduğu ve hatırı sayılır büyüklükte deprem üretebileceği ileri sürülebilir Dahası, Cibyra Fayı’nın yöneliminin FBFZ ile uyumlu olduğu gözönünde bulundurulduğunda, güneybatı Türkiye’de KKD–GGB uzanımlı fayların aktif olduğu ve üzerlerindeki hareketin sol yanal olduğu sonucuna varılabilir Anahtar Sözcükler: eski deprem, arkeosismoloji, Cibyra antik kenti, Fethiye-Burdur Fay Zonu, GB Türkiye Introduction Western Anatolia is one of the most actively extending regions in the world, mainly characterized by nearly north–south stretching (Şengör et al 1985) (Figure 1a) This extended terrain is bounded by the Aegean Arc to the south and the strike-slip Pliny-Strabo Trench to the southeast (Le Pichon et al 1979; Barka & Reilinger 1997) (Figure 1a) The northeast continuation of the Pliny-Strabo Trench is characterized by NE–SW-trending faults which bound the western Anatolia extensional region on land (Dumont et al 1979; Karaman 1989; Barka et al 1995; Barka & Reilinger 1997; Gürer et al 2004; Hall et al 2009) (Figure 1b) This NE–SW-trending fault zone, the Fethiye-Burdur Fault Zone (FBFZ), between the Mediterranean Sea and Burdur, is thought to be 429 CIBYRA FAULT, SW TURKEY a EURASIAN PLATE BLACK SEA NORTH ANAT OL IAN FAU LT EF ANATOLIAN BLOCK KF DF LT IAN F Z AEGEAN SEA Western Anatolia extension region R DU UR ST FAU L TO NA A FE TH İY E -B EA h nc e Tr Figure 1b bo tra -S y lin ARABIAN PLATE AN - CYPR P GE US AE AR C MEDITERRANEAN SEA 29 14 AFRICAN PLATE 200 o 31 19 b N 400 km o Burdur 19 71 Denizli Cibyra Antalya 57 study area Antalya Basin 19 de Fethiye Bay s h nc bo St y- in Pl e Tr Finike Basin MEDITERRANEAN SEA Rhodes Basin N 40 36o o Rh 37o Muğla 80 km Figure (a) Neotectonic setting of Turkey (Şaroğlu et al 1992; Barka & Reilinger 1997; McClusky et al 2000; Bozkurt 2001; Reilinger et al 2006) (yellow arrows show plate motions and yellow dashed line shows the boundary of the western Anatolia extensional region) (b) Fethiye-Burdur Fault Zone and major Neotectonic structures around the study area (Şaroğlu et al 1992; Yağmurlu et al 2005; Hall et al 2009) (red dashed lines show the boundaries of the FBFZ, red solid lines indicate surface ruptured earthquakes during the 20th century) (fault plane solutions are taken from Taymaz & Price 1992; Yılmaztürk & Burton 1999; Benetatos et al 2004) 430 V KARABACAK a regionally important tectonically active zone (e.g., Dumont et al 1979; Karaman 1989; Barka et al 1995) along which GPS measurements indicate at least 15 mm/yr left-lateral movement (e.g., Barka et al 1995; Barka & Reilinger 1997; Kahle et al 1998; Reilinger et al 2006) The FBFZ is defined by NE–SW-trending major faults with numerous NW–SE-trending shorter faults representing extensional features in a 50-km-wide shear zone (ten Veen et al 2008; Hall et al 2009) (Figure 1b) Akyüz & Altunel (2001) documented offset archaeological relics on a NNE–SSW-trending segment of the FBFZ, indicating that the fault is active, but their data were not sufficient to display the sense of motion on the fault zone However, although fault plane solution of a recent large earthquake (1957 [M= 7.1] Fethiye earthquake) indicates that the motion is left-lateral, some recent moderate earthquakes (e.g., 1971 [M= 6.2] Burdur earthquake) indicate normal motion predominantly (Figure 1b) (e.g., Taymaz & Price 1992; Yılmaztürk & Burton 1999; Benetatos et al 2004) and Koỗyiit (2000) suggests that these faults extending between the Mediterranean Sea and Burdur are normal faults As is clear from previous studies, there is an agreement on the existence of the NE–SW-trending active FBFZ between the Mediterranean Sea and Burdur, but the motion, which is mainly based on recent GPS data and fault plane solution, is under debate Segments of the FBFZ have ruptured during major earthquakes in the historical (e.g., Ergin et al 1967; Soysal et al 1981; Ambraseys 1989; Guidoboni et al 1994; Ambraseys & Finkel 1995; Tan et al 2008) and instrumental periods (Ergin et al 1967; Erinỗ et al 1971; Ambraseys 1989; Taymaz & Price 1992) For example, historical earthquake catalogues (e.g., Ergin et al 1967; Guidoboni et al 1994) report that the ancient city of Cibyra, which was located on a segment of the FBFZ, was destroyed by large earthquakes in 23 AD and 417 AD Akyüz & Altunel (2001) observed offset archaeological relics in Cibyra and attribute the offset to the 417 AD earthquake Since the beginning of the 20th century there have been also several significant (Ms>6) earthquakes along the FBFZ The first destructive earthquake of the 20th century in southwestern Turkey was the October 1914 Burdur earthquake (Ms= 7.0) The submergence of 20–40 km of the southeast shore of Burdur Lake indicated that significant normal faulting occurred along this part of the shore, the downthrow being as much as 1–2.5 m (Ambraseys 1989; Ambraseys & Jackson 1998; Yağmurlu et al 2005) Furthermore, Ambraseys (1989) reported that the 25 April 1957 Fethiye earthquake (M= 7.1) occurred with an offshore epicentre between Rhodes and the southwest coast of Turkey and caused extensive damage around Fethiye Bay (Figure 1b) The last major earthquake in the FBFZ was the 12 May 1971 Burdur earthquake (M= 6.2) which caused ground rupture near the northeastern end of the FBFZ (Erinỗ et al 1971) Damage was concentrated along the valley southwest of Burdur, where a surface rupture about 1–10 km long trends at 50° (Ambraseys 1989; Ambraseys & Jackson 1998; Yağmurlu et al 2005) The presumed tectonic fracture follows the contact between Quaternary and Neogene deposits with a downthrow of between 30–70 cm (Ambraseys & Jackson 1998; Yağmurlu et al 2005) Although previous studies agree that the FBFZ is a regionally important seismogenic zone between the Mediterranean Sea and Burdur, the sense of motion remains controversial Despite the abundance of investigations (e.g., Koỗyiit 1984; Karaman 1989; Taymaz & Price 1992; Price & Scot 1994; Barka et al 1995; Barka & Reilinger 1997; Temiz et al 1997; Koỗyiit 2000; Akyỹz & Altunel 2001; Gỹrer et al 2004; Yağmurlu et al 2005; ten Veen et al 2008; Hall et al 2009), there is a lack of direct field observations of the Holocene faulting on the FBFZ This paper presents geological and geomorphological field evidence of Holocene faulting on a segment called the Cibyra Fault of the FBFZ Most importantly, this paper re-examines archaeological evidence of seismogenic faulting on a ~2000 years old archaeological site, attributes faults to certain events and hence contributes to a better assessment of the recent activity on the Cibyra Fault Furthermore, results of this study demonstrate that the central section of the FBFZ is important in terms of earthquake faulting Thus, it is believed that documented field observations in this study would cast significant light on the sense of motion on the NE–SW-trending FBFZ in southwestern Turkey 431 CIBYRA FAULT, SW TURKEY Field Observations Geological and Geomorphological Observations Along the Cibyra Fault The NNE–SSW-trending Cibyra segment of the FBFZ (Figure 2a) extends between İbecik Village in the south and Çamkưy Village in the north (Figure 2b) The Cibyra Fault was mapped using aerial photographs, geological field evidence and offset physiographic features The Cibyra Fault does not exhibit clear field evidence for active faulting south of İbecik and north of Çamkưy, probably as a result of major stepovers in these locations (Figure 1b) Thus, towards both ends it was extended as probable faults (Figure 2b) The general trend of the Cibyra Fault is N20°E and the visible length of the fault is at least 35 km The main geological units in the study area are ophiolitic melange, pre-Pliocene limestones, Pliocene conglomerates and Quaternary deposits The fault offsets the limestones, conglomerates and Quaternary deposits (Figure 2b) The fault extends in limestones in the south where there are wellpreserved fault planes (Figure 3a) The dips of fault planes in the limestones vary between 70° and 85° WNW and nearly horizontal lineations on the planes indicate lateral motion (Figure 3b) Pliocene units consist of unconsolidated lacustrine deposits including conglomerates, sandstones and siltstones (Figure 4a) They are bedded and dips vary between 15° and 45° E (Figure 4a) Although fault planes are not well preserved in these unconsolidated Pliocene units, faults are visible in aerial photographs (Figure 4b), and are nearly vertical where exposed by erosion (Figure 4c) Quaternary deposits cover large areas around the Gölhisar Basin, and faulted Quaternary deposits are found north of Yusufỗa Village (Figure 2a) Faults in the Quaternary deposits are vertical (Figure 5a) Nearly horizontal lineations on fault planes in Quaternary deposits (Figure 5b) indicate horizontal motion, consistent with the motion indicated in limestone fault scarps (Figure 3b) Typical fault-induced geomorphological features include deflected stream beds, offset terrace deposits, elongated ridges and changes of stream channel pattern Detailed geomorphological observations 432 along the Cibyra Fault showed that stream beds are left-laterally offset up to 400 m along this extension (Figure 2b) As Figure 4b shows, a SE-flowing stream bed is left-laterally deflected south of Cibyra Detailed mapping of the terrace deposits in the northern flank of the river shows that the terrace is also displaced (Figure 4b) An approximate 30 m left-lateral offset on the fault line is seen both in the river and terrace (Figure 4b, c) Thus it can be concluded that the river and terrace are offset by the fault On a NNE–SSW-trending ridge about 300 m long and 70 m wide in Pliocene units in west of Yusufỗa Village (Figure 6), both margins are linear, and there are left-laterally offset stream beds on alignment of the western margin (Figure 6) Considering the lateral faulting, it can be concluded that the NNEtrending ridge is an elongated ridge within the fault zone Further north of Yusufỗa Village, the Cibyra Fault extends into Quaternary alluvium and cuts the NW-flowing Dalaman Stream (Figures 2b & 7), where meandering (sinusoidal) channel pattern of the stream bed in the western block of the fault strand become straight in the eastern block (Figure 7) This observation suggests that the change of stream channel pattern can be related to the inclination change of each block as a result of faulting Archaeoseismological Observations The ancient city of Cibyra, which was one of the earliest and the most important cities of Karia, is located on the NNE–SSW-trending Cibyra Fault segment of the FBFZ (Figures & 2) Although the detailed history of Cibyra is not as well-known as other ancient Anatolian cities, Akurgal (1995) and Ekinci et al (2008) stated that the city was established around the 10th century BC near Gölhisar Lake (Figure 2) and was moved to its present place around the 3th century BC The city became part of the Roman Empire in 84 BC and was abruptly abandoned at the beginning of the 5th century AD (Akurgal 1995; Ekinci et al 2008) Detailed field investigations in the ancient city of Cibyra showed earthquake damages affecting ancient ruins Archaeological observations were first reported by Akyüz & Altunel (2001) who recognized that surface ruptures of historical earthquakes offset the stadium rows, and blocks had fallen in domino- V KARABACAK Figure 4a, b 29037' Quaternary deposits Çamkưy Pliocene conglomerates km 40 limestones pre-Pliocene bed rocks Figure7 N 100 modern settlement Figure ancient settlement 100 active fault Yusufỗa 355 probable fault 180 Figure8b offset (m) CIBYRA stream 290 150 Figure GÖLHİSAR 375 road GƯLHİSAR BASIN 220 Gưlhisar Lake 30 Figure 380 140 b 300 150 400 360 120 Da 37008' 40 a lam re St an am 300 Evciler GƯLHİSAR 310 ÇAVDIR 110 ÇAMELİ Da pra Ya klı m D 300 a al m an St re am 37 00' DİRMİL Figure 2b İbecik Figure 4c, b km N 29 30' Figure (a) Major faults around the study area (redrawn from Alỗiỗek et al 2005) (b) Cibyra Fault, drawn on the basis of field observations around the Gölhisar Basin 433 CIBYRA FAULT, SW TURKEY E W a NNE SSW b Figure (a) Nearly vertical fault plane in limestones around İbecik Village (b) Nearly horizontal slickensides on the limestones indicate strike-slip motion in this area style Although Akyüz & Altunel (2001) observed left-lateral slip on the rows of the stadium, they did not provide detailed information for the amount of offset Since new measurement techniques (such as LIDAR – Light Detection and Ranging System) allow us to make precise measurements, detailed field studies have been conducted in the stadium to determine the style and amount of deformation In addition, recent archaeological excavations (2006– 2009) provided additional evidence for earthquake 434 damage, which support that the city is located on an active fault Earthquake evidence at Cibyra is characterized by collapsed walls, tilted and rotated blocks and faulted ruins The city was mainly built around the E–Woriented Sacred Road (Figure 8a) and except for the bouleuterion (senate house) all major buildings such as the theatre, bath, agora and stadium (Figure 8b) have partly collapsed For example, the theatre, constructed in 27–14 BC and renovated in 41–54 AD V KARABACAK E W Gölhisar a c b ro ad terrace terrace N 60 120 m Figure (a) A general view from the southern entrance of the stadium towards the south (red arrows show trace of the fault zone and blue line shows a left-laterally displaced stream bed) (b) A stream bed and its terrace are sinistrally offset about 30 m (red arrows indicate fault trace in the field and yellow dashed lines show edges of the stream bed) (satellite image is taken from Google Earth software) (see Figure 2b for location) (c) Trace of the fault (red arrows) in a road cut near the offset stream bed 435 CIBYRA FAULT, SW TURKEY E W a NE SW b Figure (a) Nearly vertical subparallel fault planes in Quaternary deposits north of Yusufỗa Village (b) Horizontal slickensides (yellow arrow) indicate strike slip motion on the fault plane (Ferrero 1974), is one of the well-preserved building in the city, but its walls are partly collapsed and some seats are displaced (Figure 9) Other major buildings including the agora, temple etc., are completely collapsed and their ruins are barely preserved The most exciting evidence of earthquake damage was observed near the southern and northern entrance of the stadium (Figure 8b) The stadium, constructed in 190 AD (Ferrero 1974), was in use until the city was abruptly abandoned after the 417 AD earthquake (Akurgal 1995; Akyüz & Altunel 2001) The long axis of the U-shaped stadium, constructed on an east-facing slope, trends N15°W; the semi-circular end is to the south (Figure 8b) It has 20 rows of seats in the western auditorium and or rows on the opposite side (Ekinci et al 2007) The eastern side and the northern end of the auditorium are completely collapsed (Figure 8b) but the western part is well preserved Near the southern entrance passage, seat rows are ruptured by a N23°E436 trending sinistral fault (Figure 10a, b) A new archaeological trench was excavated on the extension of the fault at the base of the stadium (Figure 10a) and this showed that the stadium floor was made of compressed limestone pebbles laid on flattened bedrock (Pliocene conglomerate) The fault cuts both the bedrock and stadium floor and the eastern side is upthrown (Figure 10c) The deformed part of the stadium was scanned by LIDAR to make precise measurements on displaced rows and the stadium floor (Figure 11a) Detailed quantitative assessment of LIDAR measurement in the hand-made trench indicates that the eastern side was upthrown about 20 cm and the flat surface was folded near the fault (Figures 11b) Analysis on coordinated point cloud data of seat rows indicates that seating blocks are in alignment and they keep their original positions on the western side of the N23°E-trending fault However, seats are completely disturbed and blocks are out of alignment on the V KARABACAK N 100 m Yusufỗa 115 230 m Figure Deflected stream beds on the Cibyra Fault west of Yusufỗa A NNESSW-trending elongated ridge (thin yellow lines) extends parallel to the fault in the eastern block (red dashed line shows fault trace, blue lines show left-laterally displaced stream beds, yellow dashed line shows possible eastward continuation of the stream bed and yellow arrows show the elongated ridge) (Satellite image is taken from Google Earth software) eastern side of the fault (Figure 10a, b) There are no systematic inclinations Analysis on point cloud indicates that the northern edges of 20 seat rows are displaced left-laterally between 60 and 67 cm (Figure 11c, d) Another sub-parallel fault branch extends next to the northern corner of the stadium but does not affect it (Figure 8) The fault is clearly exposed as a shear zone on the road-cut of the modern road north of the stadium (Figure 12a) Detailed observations at this location show that Pliocene units are strongly deformed by nearly vertical sub-parallel faults The long axes of ceramic pieces in the shear zone are parallel to the fault planes (Figure 12b) An ancient wall crossing the shear zone is displaced by the fault (Figure 12a) According to archaeologists (Ş Özüdoğru, personal communication 2008) the outer side of the wall was made of high quality marble blocks but the inner part was made of ordinary stones A plan view of the wall was constructed with the help of archaeologist (Ş Özüdoğru) (Figure 12c) Notably, the eastern continuation of the outer wall is missing and the inner wall aligns with the exposed part of the major outer wall (Figure 12c) This observation 437 CIBYRA FAULT, SW TURKEY l patt anne ht ch straig N ern ceramic pieces (Figure 13a) Ordinary houses, dated to the 6–7th century A.D (Ş Özüdoğru, personal communication 2008), were built on this filled material and their walls are well preserved (Figure 13a) Walls of ordinary houses are covered by colluvium and, as Figure 13b shows, some blocks collapsed in domino-style on the colluvium Similar damage was observed in the northern entrance of the stadium (Figure 14) The floor of the stadium is covered by a sedimentary package including ceramic pieces, and columns were toppled on this cover material (Figure 14a) Archaeological excavation showed that lower part of the columns is well preserved but blocks above the sedimentary package are displaced (Figure 14b) ea m g in er nd Discussion rn te at lp ne an ch 350 700 m Figure Dalaman Stream is offset by the Cibyra Fault Faulting changes the channel pattern from meandering to straight north of Yusufỗa (dashed red line shows fault trace) (Satellite image is taken from Google Earth software) probably suggests that the wall was sinistrally offset and the eastern part of the major outer wall was eroded It is also notable that blocks of the wall are tilted up to 10° According to archaeologists (Ş Özüdoğru, personal communication 2008) these blocks should be horizontal; thus, disordered blocks suggest that the fault was reactivated The portico area and northern entrance of the stadium provide additional data for historical earthquakes Archaeological excavations in the portico area showed that the stadium floor is blanketed by a 15-cm-thick layer, including clastic sediments derived from upper hillside and broken 438 Actively deforming Western Anatolia extension region is bounded to the south by the strike slip Pliny-Strabo Trench (Le Pichon et al 1979; Barka & Reilinger 1997) (Figure 1a) Onshore, in southwest Anatolia, the NE–SW-trending FBFZ is considered to be the northeastern continuation of the PlinyStrabo Trench (Dumont et al 1979; Barka et al 1995; Barka & Reilinger 1997; Gürer et al 2004) (Figure 1) Considering previous studies which mainly include geodetic results and instrumental records, the existence of the FBFZ and its motion are under discussion The Cibyra Fault is a segment of this controversial fault zone and field evidence along it helps clarify this discussion Geological and geomorphological field data indicate that the Cibyra Fault is about 35 km long Investigations of aerial photographs (Figures 4b, 6, & 8) and detailed field studies along the fault showed (Figures 2b, & 6) stream beds deflected by up to 400 m and faulted Quaternary deposits (Figures 4b, & 6), which are clear surface evidence for the activity of the Cibyra Fault Fault-parallel elongated ridges along the Cibyra Fault (Figure 6) are characteristic evidence for strike-slip faulting Slickensides on fault planes (Figures & 5) and sinistrally offset stream beds and terraces (Figures 2b, 4, & 7), suggest leftlateral motion on the Cibyra Fault The remains of ancient city of Cibyra, which is located on the Cibyra Fault, provide invaluable V KARABACAK modern road N ancient road Figure 12 Figure 14 a Cibyr Stadiu m portic Figure 13 o 11 Figure 10 Figure 11 10 90 11 area 12 30 N trench site Figure 4a 0 1230 250 500 m a 35 70 m b Figure (a) The plan of the Cibyra ancient city (major buildings: [1] stadium, [2] bouleuterion, [3] theatre, [4] agora, [5] bath, [pink line] sacred road) (Uygun & Dökü 2008) (b) Satellite image of the Cibyra stadium (taken from Google Earth software) evidence to help document traces of fault deformations and related palaeoearthquakes Detailed field observations in the ancient city showed that large historical earthquakes struck the city on several occasions and caused extensive damage characterized by faulting, systematically collapsed columns, broken monuments and tilted and rotated blocks (Figures 9, 13 & 14) Without analytical dating, deformation features cannot be correlated with recorded historical events, but offset archaeological ruins indicate that the damage observed in the ancient city of Cibyra is related to two different events Akyüz 439 CIBYRA FAULT, SW TURKEY a b c Figure (a) A general view of the theatre in Cibyra Its walls are partly collapsed and some seats are displaced (yellow arrows) (b, c) Close-up views of rotated blocks in the theatre & Altunel (2001) observed deformation on seat rows of the stadium in Cibyra and attributed them to the A.D 417 earthquake In this paper we re-examined archaeoseismogical evidence of the A.D 417 event, with supporting observations in the light of the new hand-made trench on the stadium floor (Figures 10a & 11a) The stadium was built on an east-facing slope A hand-made trench on the stadium floor showed that the eastern side is upthrown about 20-cm-along fault (Figures 10a, c & 11b) which deformed the southern seat rows The upthrown eastern side on the east facing slope suggest that the deformation on the stadium floor is of tectonic origin rather than ground failure The above observations suggest that the first event was the A.D 417 earthquake which caused left-lateral surface faulting of about 63.5 cm on southern seat rows (Figures 10b & 11c, d) and 440 an upthrown stadium floor by about 20 cm (Figures 10a, c & 11b) The up to 63.5-cm left-lateral offset on seat rows indicates horizontal motion but the 20cm up-thrown on the flat stadium floor suggests a considerable vertical component Since striated fault planes along the Cibyra Fault indicate horizontal slip (Figures & 5), it is likely that the upthrow on the stadium floor resulted from the local geometry of the fault Historical earthquake catalogues (e.g., Ergin et al 1967; Soysal et al 1981; Ambraseys 1989; Guidoboni et al 1994; Ambraseys & Finkel 1995; Tan et al 2008), show no large historical earthquake after the city was abruptly abandoned in 417 A.D However, the following observations in the ancient city of Cibyra suggest a later large earthquake on the Cibyra Fault: (1) There is another sub-parallel fault branch which V KARABACAK a NE SW stadium floor stadium floor b 4m N c Figure 10 (a) Faulting in the southern entrance of the Cibyra stadium Red dashed line indicates fault trace (b) Aerial photo from seat rows (Ekinci et al 2007) (c) A hand-made excavation on the extension of the fault in the floor of the stadium Deformation is obvious on the stadium floor which was made of compressed limestone pebbles on flattened bedrock (arrows show the fault) formed a strongly deformed shear zone north of the stadium An ancient wall crossing this shear zone is displaced by reactivation of the fault (Figure 12a) (2) The faulted floor is covered by a sedimentary deposit in the stadium In the northern entrance of the stadium, blocks of the columns are well preserved in the lower 441 CIBYRA FAULT, SW TURKEY NE SW a SE NW sedimentary fill compressed limestone pebbles conglomerate b c 4m N d 4m N Figure 11 (a) Point cloud of the southern entrance of the Cibyra stadium taken by LIDAR Blue dashed rectangle is the hand-made trench, red solid line indicates fault in the trench, red dashed line is the fault trace (b) LIDAR scan of the trench wall shows vertical displacement in the base of the stadium (c) Point cloud of seat rows (d) Analysis of point cloud data indicates left-lateral displacements on 20 seat rows of about 63.5 cm mean value by the N23°E trending sinistral deformation zone (red line shows deformation zone, successive blue and yellow lines show line of seat rows) (half arrows show motion on the fault) 442 V KARABACAK a 70 100 25 50 70 50 cm b c inner wall major wall probably eroded probably eroded N 25 50 cm Figure 12 (a) A roadcut near the northern entrance of the Cibyra stadium Dashed black lines show shear zones which reflect successive faulting, dashed red lines show relatively younger faults which offset the wall Yellow lines indicate tilting degrees with numbers (b) Man-made materials (ceramics) filled the shear zone (c) Plan view of the offset wall (in Figure 2a) shows about 30 cm sinistral offset by faulting 443 CIBYRA FAULT, SW TURKEY E W ordinary house ents clastic sedim a E W b Figure 13 (a) A general view of the western side of the stadium Excavation on this side showed that ordinary house walls were not disturbed Yellow arrows show the upper surface of colluvium on which columns of portico fallen in domino style (Figure 13b) (b) Side columns of portico area have fallen towards the west in domino style on colluvium 444 V KARABACAK W a E N S b Figure 14 (a) Fallen columns near the northern entrance of the stadium (photo taken from Akyüz & Altunel 2001) (b) Archaeological excavations revealed that blocks above the deposits are displaced, but the buried part of the column was not disturbed (below blue arrow) part of the sedimentary deposit, but blocks above it have collapsed in domino style (Figure 14a) (3) In the portico area of the stadium, colluvium covering 6–7th century ordinary houses was immediately overlain by blocks collapsed in domino style (Figures 13 & 14) The above observations suggest that a second large earthquake took place after the 7th century AD and caused extensive damage in the ancient city of Cibyra Thus geological and geomorphological field evidence and offset archaeological relics on the Cibyra Fault, suggest that this fault can produce earthquakes of considerable magnitude Wells & Coppersmith (1994) stated that about 60–65 cm offset or a 35 km surface rupture implies a magnitude between 6.5 and 7.0 Thus, it can be suggested that the Cibyra Fault reactivated with a magnitude of about in historical time and it can happen again The FBFZ consists of NE–SW-trending major faults and numerous NW–SE-trending shorter faults representing extensional features in a 50-km-wide shear zone (ten Veen et al 2008; Hall et al 2009) (Figure 1b) In the central part of the zone, offset archaeological relics of Cibyra suggest left-lateral faulting The trend of the Cibyra Fault is consistent with the general trend of the FBFZ Thus, field evidence including geological, geomorphological and archaeoseismological observations support the Holocene activity of the FBFZ and documented field evidence suggests that the dominant motion along the fault zone is sinistral Conclusions The NNE–SSW-trending Cibyra Fault is a segment of the FBFZ in its central part Slickensides on fault planes, deflected stream beds and terraces, and faultparallel elongated ridges along the Cibyra Fault are the clearest surface evidence for left-lateral faulting The ruins of the ancient city of Cibyra, located on the Cibyra Fault, provide invaluable archaeoseismological data consistent with the geological and geomorphological observations, and show that the reactivation of the Cibyra Fault in 417 and probably after the 7th century AD caused extensive damage in the ancient city of Cibyra Considering the geological and geomorphological field evidence and offset and deformed archaeological relics, it can be suggested that the Cibyra Fault is an active left-lateral fault capable of producing earthquakes of considerable magnitude Furthermore, as the trend of Cibyra Fault is consistent with the FBFZ it is concluded that the NNE–SSW-trending faults in southwestern Turkey are active, with left-lateral motion 445 CIBYRA FAULT, SW TURKEY Acknowledgments This research is supported by the Eskişehir Osmangazi University Research Foundation (project no: 200815006) Ưnder nlü assisted in the field The author is grateful to Erhan Altunel (Eskişehir Osmangazi University) for his comments on an earlier version of this manuscript Special thanks go to Şükrü Özüdoğru and Eray Dökü (Department of Archeology, Mehmet Akif Ersoy University) who provided an in press document and made useful discussions about the ancient city of Cibyra The author is grateful for helpful comments and constructive reviews by two anonymous reviewers which improved the manuscript References Akurgal, E 1995 Anadolu Medeniyetleri [Anatolian Civilisations], 5th Edition Net Publications, İstanbul [in Turkish] Akyüz, H.S & Altunel, E 2001 Geological 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in southern Turkey Journal of Seismology 3, 6181 Soysal, H., Spaholu, S., Kolỗak, D & Altinok, Y 1981 Türkiye ve Çevresinin Deprem Katalogu, MƯ 2100–MS 1900 [Earthquake Catalogue of Turkey and Surroundings, 2100 BC– 1900 AD] TÜBİTAK Publications No TBAG 341 447 ... southwestern Turkey 431 CIBYRA FAULT, SW TURKEY Field Observations Geological and Geomorphological Observations Along the Cibyra Fault The NNE–SSW-trending Cibyra segment of the FBFZ (Figure 2a)... (Figure 1b) In the central part of the zone, offset archaeological relics of Cibyra suggest left-lateral faulting The trend of the Cibyra Fault is consistent with the general trend of the FBFZ Thus,... related to the inclination change of each block as a result of faulting Archaeoseismological Observations The ancient city of Cibyra, which was one of the earliest and the most important cities of Karia,