Th e western termination of the 1999 İzmit earthquake still remains as an intriguing problem for researchers and the people residing around the Sea of Marmara. There have been numerous off shore mapping and modelling studies performed in the Gulf of İzmit.
Turkish Journal of Earth Sciences (Turkish J Earth Sci.), Vol 20, 2011,ET pp.AL 359–378 Copyright ©TÜBİTAK Ư KOZACI doi:10.3906/yer-0910-45 First published online 15 October 2010 The North Anatolian Fault on the Hersek Peninsula, Turkey: Its Geometry and Implications for the 1999 İzmit Earthquake Rupture Propagation ÖZGÜR KOZACI1,2, ERHAN ALTUNEL3, SCOTT LINDVALL2, CHARLIE BRANKMAN2,4 & WILLIAM LETTIS2 İstanbul Technical University, Eurasian Earth Sciences Institute, Maslak, TR−34469 İstanbul, Turkey now at Fugro William Lettis & Associates, Inc., Walnut Creek, 94596 California, USA (E-mail: kozaci@lettis.com) Eskişehir Osmangazi University, Engineering Faculty, Department of Geological Engineering, TR−26040 Eskişehir, Turkey now at Department of Earth & Planetary Sciences, Harvard University, Cambridge, Massachusetts, 02138, USA Received 02 November 2009; revised typescripts receipt 23 June 2010 & 04 August 2010; accepted 03 September 2010 ‘We dedicate this study to Aykut Barka who devoted his life to understanding the earthquake phenomenon He was a brilliant scientist, a true friend and a giving advisor besides his humble personality He will be remembered as a source of inspiration and kindness.’ Abstract: The western termination of the 1999 İzmit earthquake still remains as an intriguing problem for researchers and the people residing around the Sea of Marmara There have been numerous offshore mapping and modelling studies performed in the Gulf of İzmit However, the main debate about the western termination of the 1999 İzmit surface rupture is linked to the Hersek Peninsula and corresponding fault geometry We focused our efforts at resolving the fault geometry on the Hersek Peninsula by applying geological mapping, geomorphic analyses, palaeoseismic trenching and geophysical surveys Our studies reveal that the North Anatolian Fault forms a restraining stepover and did not experience surface rupture during 1999 İzmit earthquake in the vicinity of Hersek Peninsula We tested this fault geometry with a finite element model in half elastic space and correlated the results successfully with the existing topography In addition, we ran a simple Coulomb model to explain the possible cause of surface rupture termination at this specific location Our studies, combined with detailed offshore bathymetry data, suggest that the restraining step of the North Anatolian Fault on the Hersek Peninsula is capable of creating an efficient earthquake rupture barrier Key Words: North Anatolian Fault, Hersek Peninsula, fault geometry, rupture termination, active tectonics Kuzey Anadolu Fayı’nın Hersek Deltası’ndaki Geometrisi ve 1999 İzmit Depremi Kırığının İlerlemesine Etkileri Özet: 1999 İzmit depreminin batıda sonlandığı yer araştırıcılar ve Marmara Denizi civarnda yaayanlar iỗin ửnemli bir sorun oluturmaya devam etmektedir zmit Kửrfezini konu alan pek ỗok ky ửtesi haritalama ve modelleme ỗalmalar yaplmasna ramen 1999 zmit depremi yỹzey krnn sonlandığı yerle ilgili tartışmalar Hersek Deltası’na ve Kuzey Anadolu Fayı’nın buradaki geometrisine dỹỹmlenmitir Bu sorunu anlamak ỹzere ỗalmalarmz Hersek Deltasndaki fay geometrisini anlamamıza yardım edecek şekilde jeomorfolojik analizler, paleosismik hendek kazıları, ve jeofizik araştırmalar üzerinde yoğunlaştırılmıştır Çalışmalarımız Kuzey Anadolu Fayı’nın bu bölgede sıkışma oluşturan bir geometriye sahip olduğunu ve 1999 İzmit depremi sırasında yüzey kırığı meydana getirmediğini ortaya koymuştur Yarı uzayda sonlu elemanlar yửntemiyle modellenen bu fay geometrisi ỗalma alannn güncel topoğrafyası ile uyum göstermektedir Ayrıca, basit bir Coulomb modellemesi ile yỹzey krnn neden burada sonuỗlanm olduu aỗklanmtr Deniz ỗalmalar ile karada yaptmz ỗalmalarn biraraya getirilmesi Kuzey Anadolu Faynn Hersek Deltasnda skmal bir sỗrama yaptn ve bu fay geometrisinin etkin bir deprem kırığı engeli oluşturduğunu ortaya koymaktadır Anahtar Sözcükler: Kuzey Anadolu Fayı, Hersek Deltası, fay geometrisi, kırık sonlanması, aktif tektonik 359 NORTH ANATOLIAN FAULT ON THE HERSEK PENINSULA Introduction th On August 17 1999, the M7.4 İzmit earthquake struck the Marmara region of Turkey causing much devastation The İzmit earthquake is the seventh surface rupturing, large-magnitude earthquake in a westward migrating earthquake sequence on the North Anatolian fault (NAF) during the 20th century (e.g., Barka et al 2000, Figure 1a) The section of the NAF within the Sea of Marmara remains as a seismic gap between the 1912 Saros and 1999 İzmit earthquake ruptures and the probability of a surface rupturing earthquake event is heightened for this region (e.g., Parsons 2004) The ~1500-km-long dextral transform NAF is one of the major tectonic structures of Anatolia, accommodating ~90% of the deformation between the Eurasian Plate and Anatolian Block (McClusky et al 2000; Reilinger et al 2006) During the İzmit earthquake, four segments (Karadere, Sakarya, Sapanca, and Gölcük) of the NAF experienced surface rupture with right-lateral displacements of up to five metres The ~126-kmlong surface rupture terminated near Gölyaka in the east (Figure 1b), but the western termination of the İzmit earthquake is more uncertain since it lies offshore in İzmit Bay According to some geodetic models (i.e Wright et al 2001; Reilinger et al 2000; Bürgmann et al 2002) and seismicity analysis (i.e Pınar et al 2001) it was suggested that the 1999 surface rupture extended 10–30 km west of the Hersek Peninsula Offshore studies within the Gulf of İzmit demonstrated the presence of underwater fault scarps (Polonia et al 2004; Cormier et al 2006; Uỗarku et al 2008), but these were somewhat inconclusive in addressing the location of the 1999 rupture termination Understanding where earthquake ruptures terminate has fundamental implications for Probabilistic Seismic Hazard Analysis (PSHA) and earthquake physics Structural complexities along faults (i.e asperities, stepovers, bends, and structural junctions) may arrest rupture propagation and cause perturbation of the state of stress on adjacent fault segments The first and most vital step is documenting the characteristics (i.e hypocentre, extent, geometry, and slip distribution) of individual ruptures Documenting earthquake rupture endpoints and understanding what caused a rupture to terminate at 360 that specific location are essential for estimating the location and potentially the magnitude of future large earthquakes Researchers (e.g., Barka & KadinskyCade 1988; Stein et al 1997; Wesnousky 2008) have convincingly demonstrated that fault geometry and Coulomb stress loading can significantly affect the initiation point of the next large earthquake on a fault system Furthermore, it has been noted that rupture end points usually coincide with discontinuities on faults, such as stepovers (e.g., Segal & Pollard 1980; Sibson 1985) Thus, gaining insights into the western extent of the 1999 İzmit earthquake rupture is essential to estimate the magnitude of the expected Marmara earthquake The Hersek Peninsula is central to the debate on the western termination of the 1999 surface rupture because it is the westernmost locality where the NAF can be observed directly before it enters the Sea of Marmara (Figure 1b) This paper aims to describe the geometry of the NAF on the Hersek Peninsula and discusses its implications on the fault rupture of the 1999 İzmit earthquake In this study we employed a comprehensive, multi-technique approach on the Hersek Peninsula Specifically, we performed geomorphic analyses, geological mapping, palaeoseismic trenching, geophysical surveying, modelling of deformation in half-elastic space with finite elements and Coulomb stress change modelling We also combined our on land results with the existing offshore data in order to present a complete fault model for the Hersek Peninsula We then present a detailed discussion of the implications of fault geometry at our study area Study Site A Historical Background The Hersek Peninsula is a triangular fan-delta with an area of ~25 km2 in the Gulf of İzmit (Figures 1b & 2) The tip of the Hersek Peninsula extends ~5.5 km northward into the Gulf of İzmit creating the shortest distance (~2.7 km) between the northern and southern shores The location and physiography of the Hersek Peninsula not only allows for a shortened gulf crossing but also controls the entrance to the gulf and the route to İzmit (Nicomedia) and İznik (Nicaea) while providing a suitable landfall area with its beaches and delta plain Consequently it has been 40 45’ W W S S N 29 30’ 29 30’ Hersek E 1912 E 1957 İzmit 30 00’ 44 1999b Bolu 19 32 30 00’ Gölcük segment Sapanca segment Karamürsel 17 August 1999 epicenter Mw 7.4 196 1999a İstanbul Gulf of İzmit 28 Lake Sapanca Sakarya Niksar 30 30’ 194 30 30’ segment Adapazarı Ilgaz 1943 360 d Kara 1939 eg ere s Suşehri BLACK SEA t men 31 00’ 10 km 31 00’ Erzincan 400 Figure (a) Simplified map of the North Anatolian fault and westward migrating earthquakes since 1939 (from Barka 1999) (b) Location map of the study area (dashed square) on LANDSAT image Star symbol shows the epicentre of the 17 August 1999 earthquake White bold lines are 1999 İzmit earthquake surface rupture (from Lettis et al 2000) Dashed lines in the Gulf of İzmit are our interpretation of the fault geometry based on Kuỗu et al (2002) bathymetry b a 400 N 40 45’ Ö KOZACI ET AL 361 NORTH ANATOLIAN FAULT ON THE HERSEK PENINSULA occupied for centuries as a strategic location in the Gulf of İzmit (Supplementary figure 1) The settlement on the Hersek Peninsula has been known as various names by different cultures throughout history It was known as Drepanon until 318 A.D when Byzantine emperor Constantine renamed Drepanon as Helenopolis after his mother who was born there By 1087, the name Cibotos and/or Civetot were used by Europeans However, with the effects of repetitive earthquakes and battles Helenopolis was, sometimes, called ‘Eleinou Polis’ meaning ‘the wretched town’ (The Catholic Encyclopaedia 1910) Later in the 16th century during the Ottoman Empire it was called Hersek after Hersekzade Ahmed Paşa Today, it is still called Hersek Village The settlement on the Hersek Peninsula has undergone three major construction phases during history The first major construction took place after Constantine renamed Drepanon as Helenopolis Constantine stayed in Helenopolis on the way back to İstanbul (Constantinople) from the Yalova thermal baths, especially during his last years After Constantine, especially during Justinian’s time, Helenopolis gained more importance when the gulf crossing traffic was shifted between here and Dakibyza (Gebze) Justinian rebuilt Helenopolis by adding an aqueduct, a second public bath (a rare situation for the time), churches, a palace and other buildings (Supplementary figure 2) He also cleared the entrance of the Drakon River (currently known as Yalakdere), built bridges and widened the road to Nicaea (İznik) During this period the Drakon River valley was used as the route connecting Constantinople (İstanbul), Helenopolis (Hersek) and Nicaea (İznik) Later, in the 16th century, Hersekzade Ahmed Paşa built a small harbour, 700 houses, a mosque with two minarets named after him, two inns, and a care house for the poor and a school of Islamic theology Many great earthquakes (Supplementary table 1) as well as battles throughout history affected the study site During the palaeoseismic excavations by Witter et al (2000) following the 1999 İzmit earthquake two destruction horizons were identified within the trenches In addition, many graves and bones were recovered The remnants of an aqueduct (Justinian 362 era), baths, a cistern, and the Hersekzade Ahmed Paşa Mosque can still be readily observed in the vicinity of Hersek Village The Hersekzade Ahmed Paşa Mosque experienced extensive damage only one year after its construction during the great 1509 earthquake It experienced less extensive damage in other large earthquakes affecting the region including the 1999 İzmit earthquake Geology/Geomorphology of the Study Site The Hersek Peninsula has four main geologic/ geomorphic units; (1) delta plain deposits, (2) marine terrace deposits, (3) beach ridge deposits, and (4) lagoon deposits (Figure 3) The oldest deltaic unit is the Upper Pleistocene Altınova formation (Chaput 1957; Akartuna 1968; Saknỗ & Bargu 1989), which includes sand with widespread Ostrea shells, clayey sand, silty sand, marl and sandy marl, and uncomformably overlies the Yalakdere and Taşköprü sandstone Dedeler Hill, 28 m a.s.l (above sea level), is the most prominent geomorphic feature on the peninsula Uplifted marine terraces on its flanks indicate it is an area of active uplift Dedeler hill is a NE–SW-trending ridge, bounded by a steep scarp on its south-eastern flank (Figure 2) and more gentle slopes on its northwestern flank The delta is ~2–3 m a.s.l and constitutes most of the Hersek Peninsula (Kozacı 2002) (Figure 2) It is formed by the north-flowing Yalakdere River The headwaters of Yalakdere in the Samanlı Mountains are ~480 m a.s.l and ~17 km south of the Hersek Peninsula Recent deposition occurs in the northwest portion of the delta (Figure 2) The youngest marine terraces are composed of marine sand with loose fabric and coarse Gastropod packages, which in some places uncomfortably overlie the Altınova formation They are exposed approximately 400–500 m inland near Hersek Village at an average elevation of about 1–2 m a.s.l (Figures & 3) The middle and youngest marine terrace deposits overlie the oldest marine terrace deposits with angular unconformity Although all marine terrace deposits have a similar lithology they can be easily differentiated on aerial photographs by their elevation difference The oldest marine terrace Ö KOZACI ET AL Figure Map showing the vicinity of the study area Coloured contours are extracted from the 20X exaggerated digital elevation model (DEM) and overlaid on the aerial photo Colour-coded contour intervals represent 5-m elevation changes Note that Dedeler Hill has a NE–SW-trending elongated shape located at the north of the peninsula with an elevation of 28 m (a.s.l.) The delta morphology with its active and passive lobes became easily recognized as a result of using 1/1000 scale survey data Trench locations are shown as yellow lines (T4, T5, T6…) Seismic reflection profile location is shown as white bold line (SRP) Very Low Frequency-Electromagnetic profile locations are shown as a white box (VLF) Previous palaeoseismic study site by Witter et al (2000) is shown as a yellow box (1999) Dashed white box shows the area of Figure and yellow box north of Hersek Lagoon shows the location of Figure The DEM was created using 1/1000 scale topographic survey of T.C İller Bankası 363 NORTH ANATOLIAN FAULT ON THE HERSEK PENINSULA deposits, about 5–6 metres thick and 10–15 m a.s.l, represent the shore facies with sand lenses and local Ostrea rich zones Beach ridges of well-rounded pebbly sands are well exposed west of Hersek Village Modern rounded pebbly beach sand is well exposed on both the east and west shores of the Hersek Peninsula Modern basin deposits and tidal marsh is composed of sandy silts and can be observed around Hersek Lagoon (Figure 3) Palaeoseismic Trenching Following the 17 August 1999 İzmit earthquake, Witter et al (2000) excavated several palaeoseismic trenches ~250 m northwest of the Hersek Lagoon (Figure 2) in an effort to document the rupture history of the North Anatolian Fault on the Hersek Peninsula However, this trench site unearthed remnants of an ancient settlement (Witter et al 2000) Walls, foundations, clay water pipes, graves, bone fragments, and evidence of destruction were documented during these excavations and the site was abandoned During the summer of 2000, we performed additional palaeoseismic trenching in two different locations on the Hersek Peninsula (Figure 2) The first set of trenches was located across the tonal and vegetation lineaments that were mapped on the delta plain as a result of our aerial photography interpretations We excavated six, approximately north–south-oriented slot trenches (T-4, T-5, T-6, T-7, T-8, and T-9) on the delta plain west of the Witter et al (2000) site (Figure 2) The total length of these 1.5-m-wide trenches is ~604 m, with depths ranging between to 2.2 metres, depending on ground water conditions and trench wall stability The trenches located on the delta plain exposed laterally continuous and undeformed strata consisting of predominantly marine sand overlying silty sand, sand, and clay of deltaic and lagoonal origin, but no faults were exposed Nevertheless, these trenches provide a spatial constraint for the fault locations on the delta plain The second set of palaeoseismic trenches (T-10, T-12, T-14, T-15, T-16 and T-17) were excavated 364 across a south-facing scarp forming the southern flank of Dedeler Hill and the shore of the lagoon (Figures & 4) Trench T-10 was excavated as a series of short trenches down the southern flank of Dedeler Hill (Figure 4) It exposed a marine terrace that abruptly thickened and a drop of the abrasion platform, most probably indicative of fault deformation Strands of the North Anatolian fault and related deformation were exposed in Trenches T-12, T-14, and T-16 Trench T-15 was excavated perpendicular to T-16 and parallel to the NAF (Figure 4), and exposed secondary strands of the NAF at this locality There was no compelling evidence of deformation within trench T-17 Trench T-12 Trench T-12 was excavated across a N70°E-trending dilatational crack that was formed during the 17 August 1999 İzmit earthquake in south of Dedeler Hill (Figures & 5a, b) Trench T-12 is 16 metres long, 1.5 metres wide and 2.5 metres deep, and exposes the North Anatolian fault at station eight The fault strikes N70°E with a near vertical-dip and extends to the surface (Figures 5b, c & 6) South-dipping (30°), shell-rich units south of the fault and massive clay with sand and gravel are juxtaposed along the main fault Secondary deformation is expressed as a N65°W-trending near-vertical fissure at station two The tilting of the units south of the fault indicates north-side-up deformation Trench T-14 Trench T-14, 22 metres long, 1.5 metres wide, and approximately metres deep, was excavated east of trench T-12 (Figure 4) The fault zone is exposed between stations six and seven with an orientation of N70°E Marine terrace deposits and fluvial units are juxtaposed along the fault zone (Figure 7a) Units south of the fault zone dip gently to the south consistent with T-12 stratigraphy (Figure 7b) Radiocarbon samples T14-6, T14-9, and T14-14 yielded calibrated (2-sigma) ages of 2215 (+133,-65) ybp (years before present), 1562 (+129,-39) ybp, and 3785 (+174,-93) ybp, respectively These ages indicate faults in the trench have experienced recurrent late Holocene ruptures Ö KOZACI ET AL N Qhb E W Qhpk S Qhb Qmt1 Gulf of İzmit Qmt3 Qhpk Qmt1 Qhpk Qpu Qhp Qmt T10 T10 Qmt2 Qmt1 Hersek Lagoon hA Nort nato l aul t ian F Qhb Qha km Explanations Qhpk Qhb modern beach sand Qmt2 modern basin and tidal marsh Qmt3 oldest marine terrace Qpu Pleistocene Altınova formation middle marine terrace Qhp Holocene beach ridge deposits Qha Holocene alluvium terrace riser Qmt1 young marine terrace drainage system Figure Geomorphic and geologic map of the Hersek Peninsula Trench T-16 N E W T-14 S T-17 T-16 E N68 T-12a T-15 Hersek Lagoon fractures T-12b 30 m Figure Detailed map showing trench locations (T12, T14, T15, T16, and T17) and mapped fault traces on the south-facing scarp of Dedeler Hill Trench T-16, 27 m long, 1.5 m wide, with its deepest section reaching 2.2 metres in depth, is located between trenches T-12 and T-14 (Figure 4) The fault zone was observed between stations zero and eight (Figure 8a, b) A vertical fault juxtaposes horizontal units in the south against north dipping units in the north at station 0.5 The main fault zone, however, is oriented ~N65°E and exposed between stations five and eight This north-dipping reverse fault is accompanied with almost vertical antithetic deformation around station seven Furthermore, the stratigraphic units north of the main fault zone are folded and uplifted as a result of transpression in this area Radiocarbon samples T16-1, T16-2, and T1611 yielded calibrated (2-sigma) ages of 6662 (+117,365 NORTH ANATOLIAN FAULT ON THE HERSEK PENINSULA Fault: N70 °E Fig ur e5 c a c b Figure (a) A fracture formed during the August 1999 earthquake (b) Two different units are juxtaposed on both sides of the North Anatolian fault in Trench T-12 (c) Close-up view of the fault on the western wall of the trench 164) ybp, 6131 (+146,-139) ybp, and 5922 (+68,152) ybp, respectively The ~6.6 ka-old T16-1 was recovered from marine terrace deposits (Unit K) The ~6.1 ka-old T16-2A, however, was recovered from a large anthropologic excavation (Unit L) cutting and postdating units F, I, K, and J These dates suggest that the marine terrace deposits emerged above sea level some time between ~6.6 and 6.1 ka years before present The units north of the fault zone are older than the buried soil horizon (Unit D) south of the fault zone, where Sample T16-11 was recovered 366 These results suggest that units north of the fault zone did not override the units south of the fault as a result of reverse faulting until ~5.9 ka years before present The trench exposures provided direct confirmation of the location of fault strands of the NAF and demonstrated that the style of deformation (right-lateral with a considerable north-side-up reverse component) is consistent with the long-term style of deformation produced from repeated surface rupturing earthquakes reflected in the uplift and tilt of Dedeler Hill Ö KOZACI ET AL T-12a S N root B ? F ? carbonate lining contact D F carbonate lining contact F E fissure N65ºW, sub-vertical F F F C D 0m A A FAULT ZONE N70ºE, sub-vertical 10 12 A very coarse shell hash; minor sand minor amount of recrystalized fibrous material, weakly cemented; localized alterations 14 B medium coarse shell hash to shell rich zone; upper 20 cm of unit contains fewer shell fragments and greater clay content C medium coarse shell to shell rich zone; upper 30-40 cm of zone contains very few shells, fine sand to silty sand, weakly cemented D medium coarse shell hash and some sand, fining upwards, fine sand to silty sand matrix supported E thin lens of fine sand; no shell fragments F clay (bedrock) interbedded with sand, gravel, cobbles; clay is massive, mottled; sands range from fine to well sorted and well 16 m Figure Log of trench T-12 (western wall) Geophysical Surveys Seismic reflection and Very Low Frequency – Electro Magnetometer (VLF-EM) surveys were performed on the delta plain in order to locate the westward continuation of the North Anatolian Fault (Figure 2) The north–south-oriented, 650-m-long seismic reflection profile is located ~600 m west of the lagoon (see Figure 2) A sledge hammer was used as the energy source A 12-channel recording system was used with five-metre geophone spacing Interpretation of the low-fold stacked profile indicates the presence of a discontinuity 200 metres north of the southern end of the seismic profile (Figure 9) VLF-EM surveys, which have been successfully used for non-mineralized shallow fault zone investigations (e.g., Jeng et al 2004), were focused on the area of deformation in seismic reflection data (Figure 2) Four parallel, 90-metre-long profiles were performed five metres apart in order to confirm this deformation both laterally and vertically Data were collected using an ENVI Scintrex VLF instrument with metre intervals The in-phase (IP), outof-phase (OP), and TILT values were measured simultaneously in three different frequencies between 15 kHz to 30 kHz (16.0, 23.4, and 26.8 kHz) All measurements were stacked into three dimensional plots and demonstrate a structural anomaly between metres 50 and 70, in agreement with the observed deformation on the seismic reflection profile (Figure 10) Model Combination of our palaeoseismic investigations on Hersek Peninsula and offshore geophysical surveys (Kuỗu et al 2002 and Cormier et al 2006) revealed a left-stepping geometry for the North Anatolian Fault (Figure 11) As a further test, we utilized finite element modelling in half elastic space for comparing the resultant deformation of this fault geometry with the present day geomorphology (Figure 12) In addition, a simple Coulomb model (Figure 13) was employed to provide a plausible physical explanation on how this restraining stepover might have affected the 1999 rupture propagation Finite Element Modelling in Half Elastic Space We tested the fault geometry documented during our field studies by using finite element modelling in half-elastic space (Figure 12) Coulomb 2.0 (King et al 1994 and Toda et al 1998) was used to correlate the modelled deformation patterns of various fault 367 NORTH ANATOLIAN FAULT ON THE HERSEK PENINSULA C B a A1 A2 C D F N 14-T14/14 F H H F F G C A2 E D 14-T14/9 12 10 A1 Ahorizon, similar toA2 but includes some silt 14 16 18 C 14-T14/6 m Trench T-14 S 20 22 m b A2 clay with sand and gravel, organic rich B sandy clay to clayey sand with gravel, many gravel clasts with diameters up to cm; few tile fragments, weakly disseminated carbonate concentrated along roots and pores C similar to unit B but no carbonate concentration; fewer and smaller tile fragments; amount of sand increases towards the bottom of the unit D terrace deposits composed of gravelly sand to locally clayey sand; upper contact is formed by well-rounded cobbles with diameters up to cm; common shell fragments E palaeo-soil; sandy gravel with rounded clasts up to cm in diameter, common shell fragments, weakly disseminated carbonate along the roots and pores F marine terrace deposits composed of interbedded medium to very coarse sand and fine gravel lenses; common to many shell fragments concentrated along beds G clay, gravely clay, gravely sand and clay interbedding H palaeo-trenches by human activity charcoal samples C14-T14/6 C 14-T14/9 C 14-T14/14 Figure (a) Photo showing two different units (white and black arrow heads) juxtaposing both sides of the fault (red arrow) (b) Log of trench T-14 (western wall) geometries with the current morphology of the Hersek Peninsula We ran different models with various plausible fault geometries (such as right stepping, left stepping, overlapping, no overlap) determined by onshore and offshore studies (see online data repository for results) In all models the 368 same fault parameters were applied: m of dextral and 0.3 m vertical slip for the segments to the east of the peninsula and on the peninsula These values are both compatible with the InSAR inversions (please see discussions for details) and a potential segmentation boundary-type deformation Assuming that the fault %Hs/%Hp %Hs/%Hp approx depth 10- 15 m approx depth 5-10 m approx depth 0-5 m -4.0 -2.0 0.0 2.0 4.0 6.0 8.0 10.0 -4.0 -2.0 0.0 2.0 4.0 6.0 8.0 -9.0 -7.0 -5.0 -3.0 -1.0 1.0 3.0 5.0 approx depth 10- 15 m approx depth 5-10 m approx depth 0-5 m % OP 3-D (16.0 kHz - 23.4 kHz - 26.8 kHz) -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 approx depth 10- 15 m approx depth 5-10 m approx depth 0-5 m TILT 3-D (16.0 kHz - 23.4 kHz - 26.8 kHz) -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5 5.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5 4.5 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 Figure 10 VLF-EM measurement results are shown (see Figure for location) The four profile results are combined and evaluated as a block diagram 3-D %IP (In Phase), %OP (Out of Phase) and TILT values are plotted (left to right) for 16 kHz, 23.4 kHz and 26.8 kHz frequencies representing approximately 0–5, 5–10 and 10–15 metre depth intervals (top to bottom) respectively The x-axis of the 3-D diagram is along the profile line, where the y-axis shows the width of the four parallel profiles Note that in these diagrams a significant change in the electrical conductivity can be observed approximately in the middle of the profiles, which are on the same trend as the anomaly on the seismic reflection profile Black arrows indicate fault location %Hs/%Hp %Hs/%Hp %Hs/%Hp %Hs/%Hp %Hs/%Hp %Hs/%Hp %Hs/%Hp % IP 3-D (16.0 kHz - 23.4 kHz - 26.8 kHz) Ö KOZACI ET AL 371 NORTH ANATOLIAN FAULT ON THE HERSEK PENINSULA 29º30´E 29º40´E 29º50´E N W strike-slip fault HEREKE E normal fault İZMİT Gulf of İzmit DERİNCE GÖLCÜK HERSEK PENINSULA km 40°42´N DEĞİRMENDERE 40°42´N 40º45´N 40°45´N S DARICA KARAMÜRSEL Figure 11 Interpretation of fault geometries (from Kozacı 2002) based on detailed bathymetry data by Kuỗu et al (2002) There is a 2-km-wide stepover west of Gölcük Note that there are two ~1-km-wide, rhomboidal releasing stepovers between Değirmendere and the Hersek Peninsula The North Anatolian fault makes a restraining stepover just east of the Hersek Peninsula and continues westwards with a trend of N70°E ratios also shows similarities Furthermore, the observed folding geometry on the seismic profiles, run in the east of the Hersek peninsula by Kuỗu et al (2002), can be correlated very accurately with the modelled deformation pattern (Figures 12b, f) Hersek onshore segment (Figure 13) The modelled stress shadow caused by a rupture on the NAF east of the Hersek Peninsula reaches –0.2 bars and creates an unfavourable Coulomb failure condition for the receiver fault on the Hersek Peninsula Coulomb Model Results and Discussion Stein et al (1997) demonstrated that calculation of static stress changes resulting from large earthquakes can help identify how earthquakes on adjacent fault segments interact Their study of the 20th century earthquakes along the NAF showed that nine out of ten epicentres struck within the area of increased stress (2–4 bars) caused by the preceding earthquake Most segments within decreased stress changes (–0.1 to –0.6 bars), however, did not experience rupture As presented above, the Hersek Delta is a flat plain ~2–3 m a.s.l and the NE–SW-oriented, oval-shaped Dedeler Hill is a prominent topographic high (28 m a.s.l.) at the tip of the peninsula (Figure 2) Thus, it is necessary to discuss its presence because the following observations suggest that Dedeler Hill was raised tectonically (1) The NE–SW-trending Dedeler Hill is an asymmetric high; its NW flank is gentle while its SE flank is steep (Figure 2); (2) Detailed geomorphologic mapping revealed marine terraces nestled around the hill at different elevations above sea level (Figure 3); (3) Trenching on the south-facing scarp exposed tilted (Figure 6) and folded units along a major fault (Figure 8), and (4) the presence of the Hersek Lagoon is evidence of subsidence south of Dedeler Hill (Figure 2) Based on these observations, it can be concluded that the Dedeler Hill has been actively uplifting in this part of the Hersek Delta and its existence provides key evidence for the style of tectonic deformation in the study area Coulomb stress change calculations depend on fault geometry, slip, and the coefficient of friction of a source fault, and on the fault orientation of a receiver fault We used the best fit fault model parameters (location, geometry, and physical fault parameters) from a finite element model to calculate a simple Coulomb static stress change The coefficient of friction value was set at 0.4, as suggested by Stein et al (1997) A model rupture with a m dextral and 0.3 m vertical slip on a source fault corresponding to the East–West-oriented offshore segment east of the peninsula generates a stress shadow area in the Hersek Peninsula area for faults oriented ~N70°E, like the 372 Offshore bathymetry and seismic data showed that the NAF trends nearly east–west east of the Hersek Ö KOZACI ET AL Figure 12 Modelling of deformation in the vicinity of the Hersek Peninsula using finite elements in elastic half-space The two dextral faults (left-stepping faults with no overlap) used for this model are located according to the findings of onshore (Receiver Fault - RF) and offshore (Source Fault – SF) studies (a) Topography of Dedeler Hill on the peninsula is correlated with the modelled deformation Plan view of the grid model showing the fault geometry and the NW–SE (A–A’) cross-section is compared to the morphology of the Hersek Peninsula The NW–SE-oriented cross-section (A–A’) across Dedeler Hill presents a NW facing gentle slope and a SE facing scarp (b) An offshore seismic profile (Kuỗu et al 2002) east of the peninsula is correlated with the modelled deformation The N–S seismic profile (B–B’) displays folded sediments with a sudden dip towards the fault (south) Plan view of the grid model showing the fault geometry and the N–S (B–B’) crosssection is correlated with the offshore seismic profile east of the peninsula Note that many geometries were run but only the displayed geometry provides an acceptable fit to the observed morphology 373 NORTH ANATOLIAN FAULT ON THE HERSEK PENINSULA The Western Extent of the 1999 İzmit Surface Rupture Geodetic measurements and models suggest that the rupture of the 1999 İzmit earthquake propagated 10–30 km west of the Hersek Peninsula, with displacements of up to metres at depth Feigl (2002) correlated the geodetic and seismic moment estimates and gave a detailed list of potential causes for the discrepancies between field and geophysical observations A lengthy discussion of these causes was provided by Feigl et al (2002) specifically for the İzmit earthquake SF RF bars -0.2 -0.1 -0.02 0.02 0.1 0.2 Figure 13 Coulomb stress change model for the faults in the vicinity of the Hersek Peninsula A source fault (SF), corresponding to the offshore fault in the east of the Hersek Peninsula with an east–west trend, and a receiver fault (RF– the fault on which the Coulomb failure is calculated) in the west corresponding to the N70°E-trending onshore segment, were placed according to the field observations and best-fit deformation model The source fault was divided into patches and the slip on this fault was assigned to be m in the middle and decreasing towards the ends to m In addition, a 0.3 m vertical slip was put in as a fault parameter Note that the receiver fault in the west that represents the fault on the Hersek Peninsula falls in the stress shadow area Peninsula (Figures 11 & 12) Our palaeoseismic trenches on the south-eastern flank of Dedeler Hill provide evidence for major strike-slip faulting with a north-side up reverse component (Figures 5–8) Although other trenches on the delta plain did not provide any evidence for faulting, geophysical surveys confirm the westward extension of this fault at depth (Figures & 10) Palaeoseismic trenches, in combination with the geophysical survey, indicate that the NAF trends N70°E in the study area (Figures & 4) Both field observations and existing data suggest that Dedeler Hill has been rising as a result of a restraining bend on this part of the North Anatolian Fault (Figures 11 & 12) 374 We would like to emphasize that the discrepancy between the geodetic and geologic techniques may very well be within the uncertainties of geodetic measurement techniques and modelling parameters The discrepancy between the geodetic studies and our observations may be the product of a few reasons: (1) methodological uncertainties, (2) some artefact of geodynamic models used to model deformation with geodetic data, (3) comparison of observations from different depths, and (4) the length of observation period The first reason for the apparent discrepancy between the field observations and the geodetic models is demonstrated by Feigl et al (2002) who report that the uncertainties of inferred slip from geodetic models for the 1999 İzmit rupture could be as high as metre Also, using smooth versus stepping fault geometry and, more importantly, using the most accurate fault geometry, are some of the important parameters used in geodynamic models with a direct effect on the modelling results (Feigl et al 2002) In addition, Hearn & Bürgmann (2005) demonstrated that using depth-dependent versus uniform geodynamic models may affect the seismic moment calculations up to a factor of three Thirdly, our field observations are limited to only very shallow depths (< 40 m), in contrast to the geodetic measurements And lastly, the extent of an earthquake rupture may continue to evolve after days, weeks or even years following the main shock Feigl et al (2002) pointed out that the GPS data and interferograms record 75 and 30 days of deformation and may contribute to the uncertainties up to 10% Hence, some of the deformation inferred from geodetic measurements could be the result of post-seismic deformation beyond rupture termination that was included within the measurement over an extended interval Ö KOZACI ET AL Our field observations agree with Barka et al (2000) that the Hersek Peninsula did not experience any surface rupture during the 1999 İzmit earthquake It is possible that the saturated and unconsolidated delta sediments may have masked minor slip hence making surface rupture recognition difficult Alternatively, the surface rupture may have ‘skipped’ the Hersek Peninsula However, we find these alternatives highly unlikely, based on our field observations In palaeoseismic trench exposures north of the lagoon, it is evident that the NAF has ruptured to the surface on the Hersek Peninsula during the late to middle Holocene Therefore, there is no mechanical reason why the rupture should bypass only the on-land location between the off-shore basins that are suggested to have evidence for 1999 surface rupture Although submarine investigations documented surface deformation west of the Hersek Peninsula (Uỗarku et al 2008), there are no piercing features that would suggest consistent dextral displacement We suggest that the observed ‘fresh looking’ surface deformation on the Yalova fault segment to the west of Hersek Peninsula, most likely represents secondary sympathetic deformation During our field observations following the 1999 İzmit earthquake we observed similar secondary deformation on the Taşköprü Delta to the west Although this deformation was parallel to the general strike of the North Anatolian fault in this area, the associated lateral displacements at the surface were in the order of a few centimetres only This kind of secondary soft sediment deformation can be explained by strong ground shaking induced slope failure or lateral spreading, depending on the morphologic location As a result, our studies on the Hersek Peninsula suggest that the surface rupture of the 1999 İzmit earthquake did not extend west of the Hersek Peninsula It is likely that the ‘required’ slip at depth to the west of the peninsula is the result of triggered aftershocks similar to the Düzce earthquake (Reilinger et al 2000; Langridge et al 2002; Cormier et al 2006) and may potentially indicate the nucleation point of the next earthquake or simply post-seismic deformation This section may also re-rupture during the expected Marmara earthquake similar to the M7.2 1999 Düzce earthquake What Stopped the 1999 Surface Rupture at Hersek Peninsula? Our field observations (the geomorphology of Dedeler Hill, uplifted marine terraces, depression of Hersek Lagoon, multiple exposures of the NAF within trenches on the south facing scarp of Dedeler Hill, and presence of a deformation zone within geophysical profiles) and field evidence-based simple models demonstrate that the stepover of the NAF on the Hersek Peninsula creates a restraining bend Our investigations suggest that this was efficient enough to terminate the 1999 rupture propagation Contrary to the geodetic models, the absence of a surface rupture beyond this stepover makes a compelling argument against extended rupture The effects of geometrical fault discontinuities are discussed in various studies as a candidate mechanism for stopping earthquake rupture propagation (e.g., Sibson 1985; KadinskyCade & Barka 1989; Harris & Day 1999; Harris et al 2002; Lettis et al 2002) Many factors may affect this process: (1) the type of step (releasing or restraining); (2) the width of this step; (3) the presence of transfer faults within the basin (Harris & Day 1993; Oglesby 2005); (4) the amount of remaining energy for the continuation of the rupture propagation; (5) the amount of accumulated slip on the neighbouring segment in the rupture direction, and (6) the direction of the source directivity Kozacı (2002) re-interpreted the fault geometry between Gölcük and the Hersek Peninsula, based on the very detailed bathymetry study results published by Kuỗu et al (2002) Unlike previous studies that had suggested a ~5-km-wide releasing Karamürsel stepover (Barka 1999; Lettis et al 2000) or a single east–west-trending Karamürsel segment (Harris et al 2002) based on previous bathymetry data, we propose that there are two relatively narrow (~1 km wide) releasing stepovers between the Hersek Peninsula and Değirmendere (Figure 11) As a result, a significant amount of energy should have been dissipated within this stretch of the fault The location of these geometrical discontinuities coincides with the sudden decrease in the slip amount anticipated by the models of Feigl et al (2002) and Çakır et al (2003) However, perhaps these releasing stepover basins are not wide enough to completely arrest the rupture Moreover, although the restraining step on the Hersek Peninsula is less than km wide, the fault 375 NORTH ANATOLIAN FAULT ON THE HERSEK PENINSULA segment on the peninsula remains within the stress shadow area of the adjacent fault rupture to the east (Figure 13) Our two-dimensional coulomb model may be used to explain why restraining stepovers, are equally, if not more, likely to arrest rupture than a wider releasing stepover (Figure 13) Another cause for the rupture propagation to run out of energy might be the 1894 earthquake, which occurred between the Hersek Peninsula and the Çınarcık Basin (Eginitis 1895; Ambraseys 2001; Harris et al 2002) Alternately, this geographical limitation itself, as suggested by Cormier et al (2006), suggests a structural discontinuity during a historic earthquake at the same location Lastly, the North Anatolian fault begins to bifurcate into two branches (Princes Islands segment in the north and Çınarcık segment in the south) at the Hersek Peninsula This structural junction on its own could be considered as a significant segmentation location capable of rupture arrest (Kame et al 2003) The Yalova segment exposed within our palaeoseismic trenches along the scarp north of the lagoon strikes ~N70°E and possibly is the continuation of the Çınarcık segment The structural connection between the Princes Islands segment and Karamürsel segment, however, is not well developed Cormier et al (2006) bathymetry data showed a ~250-m-wide, 5-km-long en-échelon style deformation zone between the Hersek Peninsula and where the Princes Islands segment becomes well defined further west In addition, two potential buried faults were observed on the northern extent of our seismic reflection profile west of Dedeler Hill These buried fault splays are potentially the continuation of the en-échelon fault structure observed by Cormier et al (2006) using bathymetry west of the Hersek Peninsula Based on the orientation and the degree of geomorphic expressions we suggest that a westwardpropagating rupture on the Karamürsel segment would preferentially propagate on to the southern (Yalova and then to Çınarcık) segments Conclusions Our study demonstrates that the pressure ridge (Dedeler Hill) located in the middle of the İzmit Bay is a product of a restraining stepover at this location In addition, it implies that the 1999 surface rupture did not extend west of the Hersek Peninsula We suggest that this conclusion corresponds to the location where the North Anatolian Fault begins to bifurcate into the Yalova and Çınarcık segments to the southwest and the Princes Islands segment to the northwest Although, the geodetic models suggest m of slip beneath the Hersek Peninsula at depth (i.e 10– 20 km) diminishing within 10–30 km to the west (Reilinger et al 2000; Wright et al 2001; Bürgmann et al 2002; Feigl et al 2002; Çakır et al 2003) the lack of surface rupture implies that the 1999 rupture did not extend beyond Hersek Peninsula to the west As a consequence we speculate that the next large earthquake in this area with a surface rupture will probably break the segment crossing the Hersek Peninsula, as well as the faults to the west with displacements similar to the 1999 earthquakes (3–5 m) at or near the surface We therefore propose that the restraining step over at the Hersek Peninsula presents an efficient structural barrier for earthquake rupture propagation at shallow crustal levels Acknowledgments We would like to thank PG&E and Lloyd Cluff for their support that made this study possible We also thank AK Kağıt A.S for their generous hospitality at their facilities in Yalova Emre Evren and Uğur Meray provided much appreciated help in the field and trenches ầalar Yalỗner performed the VLF 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Pull-apart (North Anatolian Fault) American Geophysical Union Fall Meeting, California, Abstracts, p T24A–07 Wesnousky, S.G 2008 Displacement and geometrical characteristics of earthquake surface ruptures: issues and implications for seismic-hazard analysis and the process of earthquake rupture Bulletin of the Seismological Society of America 98, 1609–1632 Witter, R., C., Lettis, W., Bachhuber, J., Barka, A.A., Evren, E., Çakır, Z., Page, W., Hengesh, J & Seitz, G 2000 Palaeoseismic trenching study across the Yalova Segment of the North Anatolian Fault, Hersek Peninsula, Turkey The August 17, 1999 İzmit earthquake, M= 7.4, Eastern Marmara region, Turkey: study of surface rupture and slip distribution In: Barka, A.A., Kozacı, Ö., Akyüz, S & Altunel, E (eds), The 1999 İzmit and Düzce Earthquakes: Preliminary Results İstanbul Technical University, İstanbul, 329–339 Wright T.J., Fielding, E.J & Parsons, B 2001 Triggered slip: observations of the 17 August 1999 İzmit (Turkey) earthquake using radar interferometry Geophysical Research Letters 28, 1079–1082 Yüksel, F.A 1995 Seismic Activity of the Gulf of zmit Region In: Meriỗ, E (ed), Quaternary Sequence in the Gulf of İzmit, ISBN 975-96123-0-5 Ö KOZACI ET AL SUPPLEMENTARY DATA A- Archeoseismology Figure Mapshowing historical towns, cities and locations Red dashed box indicates the study site of the Hersek Peninsula See next page for Table I NORTH ANATOLIAN FAULT ON THE HERSEK PENINSULA Table Table showing the historical earthquakes affecting Hersek Peninsula and its vicinity DATE M.Ö 19 24.11.29 33 02.01.69 120 121 128 170 268 269 ?.10.350 24.08.358 ?.11.359 02.12.362 26.01.446 08.12.447 448 467 25.9.478 488 500 551/554 15.08.553 16.8.554 26.10.740 25.10.989 23.09.1064 14.09.1509 01.10.1567 25.05.1672 25.05.1672 25.05.1719 02.09.1754 13.01.1871 23.11.1875 19.04.1878 10.05.1878 10.07.1894 20.06.1943 26.05.1957 18.09.1963 22.07.1967 INTENSITY VIII IX VIII VII VIII VIII VIII VIII IX, VI VIII VIII, VI VIII, VI IX, VIII VIII VI VII VIII VIII X VIII VIII IX VII VIII IX IX, VII VI VI VIII VIII, IX IX M= 6.4 M= 7.0 M= 6.4, 6.3 M= 7.1 LOCATION İznik, İzmit İznik, İzmit İznik, Kocaeli, Bursa and surroundings İznik, İzmit İznik, İzmit İzmit İzmit İzmit and surroundings İzmit and surroundings İzmit-Gebze İzmit, İznik Kocaeli, İznik, İstanbul İzmit İznik, İzmit, İstanbul İzmit Körfezi, İstanbul, İzmit İzmit Körfezi, İstanbul, İznik İzmit, Karamürsel İzmit Karamürsel (Helenopolis), İzmit İzmit-Yalova İzmit İzmit and surroundings İzmit, Kocaeli İzmit İstanbul, İzmit, İznik Doğu Marmara İstanbul-İzmit İstanbul, Edirne, İzmit, Bolu, Bursa Sapanca İzmit İzmit, İstanbul İstanbul, İzmit, Karamürsel İzmit Körfezi, İstanbul, İzmit İzmit, Erdek İstanbul İzmit, İstanbul, Bursa, Sapanca İzmit, İstanbul, Bursa İstanbul, İznik, Karamürsel, Tekirdağ, Lapseki Adapazarı, Hendek, Akyazı, Arifiye Abant Yalova, Princes Islands Mudurnu Yüksel 1995; Ambraseys & Finkel 1991; Ergin et al 1967 II RESOURCE 1, 1 1, 2 1 1 1, 1, 2, 1, 2, 1, 1, 3 1, 2 2, 3 1, 1, 1, 2, 3 1, 2, 1, 2, 3 2 2, Ö KOZACI ET AL Figure Some of the historical remnant locations shown on a 1/10000 scale aerial photo (a), and their photos (b–e) (photos by Kozacı, 2000) B– Roman aqueduct tower, C– Cistern, D– Public bath, E– Hersekzade Ahmet Paşa Mosque III NORTH ANATOLIAN FAULT ON THE HERSEK PENINSULA SUPPLEMENTARY DATA B- MODELS N N A A A’ a A’ b A A’ c B B’ N B B’ d e B f Figure Left-stepping model with gap or potential restraining bend Best-fit model IV B’ Ö KOZACI ET AL N N A A A’ a b A’ A A’ c B B’ N B B’ d e B B’ f Figure Right-stepping model Note that this fault geometry does not create a model that is comparable to the real geomorphology V NORTH ANATOLIAN FAULT ON THE HERSEK PENINSULA N N A A A’ b a A’ A A’ c B B’ N B B’ d e B B’ f Figure Left-stepping model east of peninsula with overlap on the peninsula Although this fault geometry creates a somewhat comparable onshore morphology the correlation between the model and offshore seismic profile is poor VI Ö KOZACI ET AL N N A A A’ a b A’ A A’ c B B’ N B B’ d e B B’ f Figure Left-stepping model with no overlap or gap This fault geometry also creates a comparable model to the recent geomorphology of the study area However, the offshore seismic profile does not exhibit a back-facing scarp as suggested by this model VII .. .NORTH ANATOLIAN FAULT ON THE HERSEK PENINSULA Introduction th On August 17 1999, the M7.4 İzmit earthquake struck the Marmara region of Turkey causing much devastation The İzmit earthquake. .. morphology 373 NORTH ANATOLIAN FAULT ON THE HERSEK PENINSULA The Western Extent of the 1999 İzmit Surface Rupture Geodetic measurements and models suggest that the rupture of the 1999 İzmit earthquake. .. of the Hersek Delta and its existence provides key evidence for the style of tectonic deformation in the study area Coulomb stress change calculations depend on fault geometry, slip, and the