Turkish earthquakes reveal dynamics of fracturing along a major strike-slip fault zone

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During the last 5 months of 1999, northwestern Turkey experienced two major earthquakes along the North Anatolian Fault Zone (NAFZ). The first earthquake struck the country at 3:01 A.M. local time on August 17, and caused extensive damage in the
  EOS Eos,Vol.  81,  No.  28, July 11,  2000 Turkish  Earthquakes  Reveal Dynamics  of  Fracturing  along a  Major Strike-Slip  Fault  Zone PAGES  309,313 During the  last  5 months of 1999, northwestern Turkey experienced two major  earthquakes along the North Anatolian Fault Zone  NAFZ). The  first  earthquake struck  the country at 3:01 A.M.  local time on August  17,  and caused extensive  damage in the towns  ofYalova, Gol cuk,  Izmit,  Adapazari,  and Duzce (Figure 1). The  second  earthquake  occurred at 6:57  PM. local  time on November 12 and caused dam age  mostly in Duzce and Kaynasli. The  7.4-Mw  main shock of the August 17 Izmit  earthquake  was centered at 40.702°N, 29.987°E and srcinated at a depth of 17 km. The  center was about 11 km  southeast  of Izmit,  a major  industrial  town (Figure  l).The earthquake  was a devastating  natural  disaster that  claimed close to 20,000  lives  and  left more  than  100,000 people homeless. The  earthquake  caused extensive surface rupture  along a 130-km segment of the western  part  of the North Anatolian Fault Zone NAFZ),  from the town of Golyaka in the  east to the town of Golcuk in the west (Figure 1). We  measured two large displacements at two geographic  locations  that  are about 70 km apart.The first one is 4.1 m and was measured at Golcuk in the vicinity of the Turkish  Navy Headquarters, and the second is 4.86 m and was measured  east  of  Arifiye. Displacements generally decrease to the east  and west of  these  two locations.  A  1.3-m displacement was measured  near  the  village of  Suadiye, about 40 km  east  of Golcuk  (Fig ures  1 and  2). An  inversion analysis of seismic VOLUME 81  NUMBER  28 JULY  11 2000 PAGES  309-320 data  suggests a two-stage fracturing (Figure 3). The  field  measurements correlate  very well with  the seismic  results.  One fracture and the associated  earthquake  occurred first.This event  probably triggered  another  fracture, which  led to  another  earthquake.The two-stage  rupture  process may also be related to witness accounts.  All  of the survivors inter viewed felt  two shaking events separated by a brief silent period. The  November 12 Duzce  earthquake  was probably triggered by the one in August.  The epicenter of  this  Mw  = 7.1  earthquake  was located  at  40.79°N,31.1  l°E,and its focus was at a depth of 10 km.The  earthquake  claimed about 1,000  lives  and  left  more  than  10,000 people  homeless. This second  earthquake created a 40-km long surface  rupture  from south  of Golyaka to the Bolu Mountain  tunnel (Figure  l).We  measured a maximum of 4.5 m of  right-lateral displacement in Dagdibi,a  vil lage  southeast  of  Duzce.  Displacement gener ally  decreases to the  east  and west of the location  of maximum displacement (Figure 1). Liquefaction  and ground-shaking were the dominant  causes  of damage during both earthquakes. Liquefaction generally caused 28.80 29,00 29,20 29.40 29.60 29.80 30.00 30.20 30.40 30.60 30.80  31.00 31.20 31.40 31.60 31.80  3200 Fig. 1.  Structural map showing the summary of measured  right-lateral  surface displacements along the surface trace of the fracture  zones  associated with  the Aug. 17 and  Nov.  12,1999 earthquakes red lines) in northwest  Turkey  along the  western  part of the North Anatolian  Fault  Zone.  Fracture zones  associated  with  Aug. and  Nov.  earthquakes, respectively,  shown  by green and blue lines. A two-stage fracturing process for the  Aug.  earthquake is suggested by two large displacement values measured about 70 km apart. A 4.1-m displacement is measured in the  Golcuk  area and a  4.86-m dis placement is measured in a village  called  A.  Kirazca,  east  of the  town  of  Arifiye.  The insert  shows  the major structural features  of  Turkey.  The rectangle within the insert  shows  the location of the enlarged area. Abbreviations:  NAFZ  =  North Anatolian  Fault  Zone;  EAFZ  =  East Anatolian  Fault  Zone; EFZ  = Ecemis Fault Zone; BSZ - Bitlis Suture  Zone;  A =  Ankara; I  =  Istanbul; Iz  =  Izmir; K  =  Karliova.  Original color  image appears at the back of  this  volume. TRANSACTIONS,  AMERICAN  GEOPHYSICAL UNION  Eos,Vol.  81,  No.  28, July  11,2000 C destruction in the areas underlain by thick, semi-consolidated recent river deposits. Its effects  can be observed in many areas; the town  of  Adapazari (Figure  1) suffered exten sive  damage.  Liquefaction  especially  affected the buildings  that  were  taller  than  four stories and  poorly  built. In many areas, damage to buildings has been controlled by distance from  the surface fracture and the thickness of the alluvium deposits on which they  were built. For  example,  buildings  that  were only about 100 m  from  the fracture, but built on bedrock,  survived  the earthquake with little damage.The  buildings  that  were  built on thick alluvium deposits up to 10 km away  from  the ruptured segment experienced major, and in some cases total, damage because of the  effect of  liquefaction. In general, one- to two-story buildings,  especially  the  wooden  ones,  survived both earthquakes with little or no damage. Tectonic Setting Turkey  is located in a technically active region  and contains two of the world s major active  strike-slip fault  zones,  the North  Anato lian and East  Anatolian.The  North Anatolian Fault  Zone  NAFZ),  first recognized by  Ketin [1948],  has a 24 ± 4 mm/yr slip rate  [Stein  et al,  1997]  and controls the northern margin of the westward  escape  [Sengoret  al,  1985]  or lateral extrusion  [Qemen  et al,  1993]  of the Turkish Plate,  due to the  collision  between the Arabian and Eurasian plates in southeastern Turkey The  NAFZ  extends westward  from  Karliova, where  it meets the East  Anatolian  fault zone at a triple junction.  It  branches into two splays  just west  of the Dokurcan  Valley  (Figure  l).The northern branch reaches to the Sea of Marmara at the eastern end of the  Gulf  of  Izmit.  It  follows the northern margin of the depressions  below the Sea of Marmara and reaches to the  Gulf  of Saroz.The  southern branch  follows  the southern margin of the  Iznik Lake  and extends into the  Aegean  Sea via the  Gulf  of  Gemlik  and  Biga 8 Fig.  2.  Photographs  showing a) the maximum 4.86-m  right lateral  displacement  along  a paved  road  just outside the  village of  A. Kirazca,  east of the town  of Arifiye; b)  a  1.30-m  right lateral  displacement of displaced  poplar trees near the Suadiye village;  and c) a4.1-m displacement in the Golcuk  area.  Original color  image appears at the back of this volume. Peninsula.The earthquakes  that  have occurred along the  NAFZ  since 1939 suggest  that  there is a westward migration of earthquakes along the fault zone  [Ambraseys,  1970;  Toksoz et al,  1979;  Stein  etal.,mi}. The  western  part  of the fault zone has ruptured in several earthquakes since 1939: 1943,  Ladik-Tosya,  Ms  = 7.6; 1954,  Bolu-Gerede, Ms =7.3;  1957,Abant,Ms=  7.0; and 1967,Mudur-nu,Ms = 7.1.The August  17,  Izmit  earthquake occurred along the  Duzce-Yalova  segment of the  NAFZ,  which  is the westward continuation of  the segment  that  ruptured during the 1967 Mudurnu earthquake  (Figure  1).  There  fore,  one might speculate  that  the next big earthquake might occur along the segment of the fault west  of the town  ofYalova  under the Sea of Marmara  (Figure  l).Okay  et al.  [1999]  suggest that  an active strike-slip basin exists along the NAFZ  under the Sea of Marmara.  However,  the geometry  of the northern and southern  strands of  the  NAFZ  under the sea and their earthquake potential are, at best,  poorly  known at present. It is therefore  very  difficult, with  the available  data,  to estimate the location of the active  fault zones of the sea  floor  that  could be a major  threat  to Istanbul,Turkey s largest  city, which  has a population of  over  10  million. Surface  Rupture Associated with the Izmit Earthquake The  eastern termination of the August 17, Izmit  earthquake surface  rupture  is about 10 km  south  of the town of  Duzce.The  western termination is probably in the Sea of Marmara to the west of the town of Karamursel  (Figure 1).Therefore,the  fracture zone is about 130 km long on land, but may continue under the Sea of Marmara for as much as another 50 km.The  largest surface displacements associated with the surface  rupture  are best  devel oped  in the  Arifiye  and Golcuk areas  (Figure l).We  measured  over  4.0-m right-lateral displacements in several locations in the area east of  Arifiye  with respect to different references (that  is,  displaced roads, trees, walls,etc.).  A  maximum displacement of 4.86 m  was measured along a paved road  just  outside the  village  of  A.  Kirazca (Figure 2a).The surface  rupture  strikes north  85°W  and shows a 3.45-m right lateral displacement near the collapsed  overpass of the Trans-European Highway.  From  Arifiye  to the west, it extends westward  and continues into Sapanca  Lake. The  fracture surfaces again west of the lake near the Suadiye, where we measured  1.30-m right-lateral displacement with reference to the regularly displaced poplar trees  (Figure  2b). To  the west of Suadiye, the fracture  truncates the  Izmit-Yalova  highway about 5 km  south  of the town of  Izmit,  where it strikes north  85°W In  this  area,  we  measured 1.80-2.64-m right-lateral displacements in walls separating private properties and in  fields  separating plants and trees.The fracture extends into the sea with an orientation of north 60°E in the Basiskele area of the  Gulf  of  Izmit,  where  it contains synthetic and antithetic fractures. From  the Basiskele area, the fracture extends westward  into the town of  Golcuk.  It can be observed  in the grounds of the Turkish  Navy s headquarters.  We  measured a total of 4.1-m right lateral displacement in the Western  wall separating the Turkish  Navy  grounds  from  private land,  where  the fracture strikes north 86°W  (Figure  2c).The  earthquake  rupture probably extends parallel to the coastline in the Golcuk area, where there is an observed collapse  of about a 100-m  wide  and 500-m long  segment of the coastline into the Sea of Marmara. From there, it extends westward into the Sea of Marmara. In  summary, the eastern termination of the Izmit  earthquake fracture zone can be observed  about 10 km  south  of  Duzce.  Its displacement gradually increases westward and reaches 4.86 m around the town of  Arifiye, decreases to about 1-1.3 m about 5 km west of  Sapanca  Lake,  and then increases to about 4.1 m in Golcuk.These displacement values thus  reveal a bi-model slip distribution with one  lobe  of high slip in the  Arifiye  area, and the other in the Golcuk area  (Figure  1). The  western termination of the fracture zone suggests  that  the segment of the fault to the west ofYalova  in the Sea of Marmara  (Figure  1) might have been loaded by the  Izmit  earthquake. Therefore,  the geometry and earthquake potential of the faults under the Sea of Marmara should be studied to locate the active fault zones that  may be a major  threat  to Istanbul. Inversion  Analysis of Seismic Record To  evaluate the source process of the  Izmit earthquake,  we  inverted the teleseismic body waves  recorded at 24 stations located at  differ ent azimuths, using the method of Kikuchi and Kanamori  [1991].We  used a time  window  of 50 s because no events  were  recorded by the accelographs  deployed  nearby the  rupture zone  beyond  this  time  span  after the first P onset on the accelograms.The inversion analysis  followed  the approach described by Pinar et al.  [1996]  and Pinar  [1998].  Eos,Vol.  81,  No.  28July 11,  2000 Observed  Right-Lateral Displacements on Surface Rupture in  m, 4,1 2.64 1*3 3,45 436 2.28 Golcuk Izmit MHU»  M**,7 Arifiye Distance  Along  Strike  Km) n ria  Nm 10 18Nfft S *10 18  Nm 4?1<ri8Nm 3 10 18 Hm 1 *tr  18Nm Rg.  5.  Diagram showing the moment release along the fault  between  Golcuk  and Arifiye. The legend indicates the scalar seismic moments. The magnitude values indicate the  size  and location  of the major aftershocks. The hypocentral locations of the two  subevents  are  shown  as 1 and 2. Inversion  analysis suggests  that the second subevent began 7 s after the first subevent Original color  image appears at the back of  this  volume. The  analysis (Figure 3) suggests  that  the major moment release is associated with two subevents  that  are separated in space and time.The  centroid toment Tensor  CMT)  location of the first subevent is  near  Arifiye  at a depth of 12  km,  with a scalar moment of  Mo  = 1 x  KPNm  taking place on a fault  that  strikes north  84°E,dips  86°,  and has a rake of 176°. During  this  subevent, the  rupture  propagated eastward, which is inferred from the pulse width  of the initial portion of the seismograms. In  case of a unilateral  rupture  propagation, the pulse width recorded at the  stations  located at the side where the  rupture  propagated is narrower compared to the  stations  located at the opposite  side.  The  observed seismograms show  that  the pulse width at the  stations  located to the  east  of the source region are narrower compared to those recorded at  stations  located west  of the source region,suggesting eastward rupture  propagation.The  CMT  location of the second subevent is beneath the town of Golcuk at a depth of 20  km,  the scalar seismic moment of  which is  Mo  = 0.7 x  lO^Nm.The  strike,dip, and rake of the second subevent were found to be  north  87°E,87°,and 176°, respectively .The inversion analysis also indicates  that  the  rupture associated with the second subevent began about 7 seconds later and propagated eastward. The  inversion analysis of the seismic record of  the Izmit earthquake, combined with the geometry  of the surface  rupture,suggest  that the  earthquake  was a two-stage  rupture  event along the  Diizce-Yalova  segment of the  NAFZ. Future  analysis of additional seismic  data  from the  earthquake  should reveal more information about the process of two-stage fracturing along a major strike-slip fault zone  such  as the  NAFZ. Surface Rupture Associated with the Duzce Earthquake The  November 12 Duzce  earthquake  caused a 40-km long surface  rupture  extending from south  of Golyaka to the  east  and the Bolu Mountain  tunnel  to the  west.  Southeast  of Diizce,  we measured a maximum of 4.5-m right-lateral displacement in the  village  of Daggdibi.  Displacement generally decreases to the  east  and west of the location of the maximum displacement (Figure  l).The  western 10 km of the  rupture  contains  substantial vertical  component of movement generally equal to the amount of horizontal component.  However,  the remaining 30 km of the  rupture  to the west is almost entirely strike-slip and contains little or no vertical component of movement. Acknowledgments Fieldwork  following  the Izmit  earthquake was supported by the Turkish Science Foundation.  We  thank John  Goff,Sue  Hugh, and an anonymous reviewer for helpful comments about the  manuscript.  We  also  thank  Omer Emre and Nizamettin Kazanci for valuable discussions during our fieldwork. Authors Ibrahim Qemen, School of  Geology,Oklahoma State  University, Stillwater,  USA;  Ergun Gokten, BakiVarol,Recep  Kilig,  Volkan Ozaksoy,  and Cenk Erkmen,  Department of  Geological  Engineering, Ankara  University, Turkey;  Ali Pinar, Department of  Geophysics,  Istanbul  University, Turkey For  more information, contact Ibrahim Qemen,  School of  Geology,  Oklahoma State University, Stillwater,  Okla. 74078,  USA;  E-mail: icemen@okway.okstate.edu  References Ambraseys,  N.N.,  Some characteristic features of  the Anatolian fault zone,  Tectonophys., 9, 143-165,1970. Qemen,  I., M. C.  Goncuoglu,  A.  Erler,  H.  Kozlu,  and D.  Perincek,  Indentation tectonics  and  associated lateral extrusion  in  east,  southeast  and  central Ana tolia,  Geol.Soc.  Am.  Annual  Meeting,  Abstracts with Programs,  25, All  6,1993. Ketin,I.,Uber  die tektonish-mechanischen Folgerun- gen  aus den  grossen anatolishen Erdbeden  des  letzen Dezenniams,  Geol.  Rundschau, 36, 77-83,1948. Kikuchi,M.,and H.Kanamori, Inversion of complex body  waves-Ill, Bull. Seismol. Soc.Am.,  81,  2335-2350,1991. Okay,  A., E.  Demirbag,  H.  Kurt,  N.  Okay, and  I.  Kuscu,  An  active, deep marine strike-slip basin along  the  North Anatolian fault in Turkey,  Tectonics,  18,  129-147,1999. Pinar,  A.,  Source inversion  of the  October  1,1995  Dinar  earthquake  Ms  =  6.1): A  rupture  model with implications for seismotectonics  in  SW Turkey,  Tectonophysics,  292,255-266,1998. Pinar,A.,YHonkura, and  M.Kikuchi.A  rupture  model  for  the 1967 Mudurnu  Valley,Turkey  earthquake  and  its  implication for seismotectonics  in the  western  part  of the North Anatolian Fault  Zone,  Geophys. Res. Lett., 23, 29-32,1996. Sengor,  A. M.  C,  N.  Goriir,  and FSaroglu, Strike-slip faulting  and  related basin formation  in  zones of tectonic escape:Turkey  as a  case  study,  in  Strike-slip Faulting  and  Basin Formation, edited  by  K.T. Biddle  and  N.  Christie-Blick, Society  of  Economic Paleontologists  and  Mineralogists, Spec. Publ.,37, 227-64,1985. Stein,  S. R., A.  Barka, and  H.  J. Dietrich, Progressive failure  on the  North Anatolian fault since 1939  by  earthquake  stress  triggering,  Geophys.  J Int.,  128,  594-604,1997.  Toksoz,  M. N., A.  FShakal, and  A. J.  Michael,  Space-time migration of  earthquakes  along  the  North Anatolian fault zone  and  seismic gaps,  Aire  and  Applied Geophysics, 117, 1258-1270,1979.   Eos,Vol. 81, No. 28,July 11,2000 41. B L A C K S E A • 41. Silc ..... 41. Turkish Navy HQ. (4.1 m) 40. Gebze 。 セ SEA OF MARMARA Izm fr'r:, Cma rc.t ll AL 40. 40 . , , o 20 Ian .1 --- 28.80 29.00 29.20 29.40 29.60 29.80 30.00 3O.ro 30 . 40 30 . 60 30 . 80 31. 00 31.20 31.40 31.60 31.80 3200 Fig. J. Structural map showing the summary o measured right-lateral surface displacements along the surface trace o he fracture zones associat ed with the Aug. 17 and Nov. J 2, J 999 earthquakes red lines) in northwest Turkey along the western part of the North Anatolian Fault Zone. Fractur e zones associated with Aug. and Nov. earthquakes, respectively, shown by green and blue lines. A tW<rs age fracturing process for the Aug. earthqu ak e is suggested by two large displacement values measured about 70 km apart. A 4. Jom displacement is measured in the G6lciik GreG and a 4.86-m dis - placement is measured in a village called A. Kirazca, east o he town o Arifiye. The insert shows the major structural features o Turkey. The recta ngle within the insert shows the location o he enlarged area. Abbreviations: NAFZ = North Anatolian Fault Zone; £AFZ = East Anatolian Fault Zone; EFZ = Ecemis Fault Zone; BSZ = Bitlis Suture Zone; A = Ankara; I = Istanbul; Iz = Izmir; K = Karliova. Page 309 Page 313 Fig. 2. Photographs showing a) the maximum 4.86-m right lateral displacement along a paved road just outside the village o A. Kirazca, east o t he town o Arifiye; (b) a 1.3IJ.m right lateral displacement o displaced poplar trees near the Suadiye village; and (c) a 4.1 -m displacement in the G6lcilk area.  Eos,Vol. SI, No. 2S,July 11,2000 Observed Right-Lateral Displacements on Surface Rupture (in m.) 4 1 2.64 1 3 3.45 4 86 2.28 Golcuk bmit Arifiye -10 10 Distance Along Strike (Km) Q Nm 11'11r'18Nm ェ   セ Nm U Nm l 110 H 18 Nm h   Q   Q Nm Q Nm 0 18 Nm Fig. 3. Diagram showing the moment release along the fault between G6/ciik and Arifiye. The legend indicates the scalar seismic moments. The magnitude values indicate the size and location o he major aftershocks. The hypocentrallocations o he two subevents are shown as 1 and 2.lnuersion analysis suggests that the second subevent began 7 s after the first subevent. Page 313
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