Selim Özalp

Slip Distribution, Fault Geometry, and Fault Segmentation of the 1944 Bolu-Gerede Earthquake Rupture, North Anatolian Fault, Turkey

The 1944 Bolu-Gerede earthquake ( M s 7.3) occurred along the North Anatolian fault in northern Turkey and ruptured for about 180 km of the fault. The surface slip distribution and fault geometry, which provide fundamental data on detailed fault segmentation and recurrence of faulting during a multisegment rupture, are poorly known for this event. We conducted geomorphological and geological investigations and eyewitness interviews to determine cumulative offsets through several earthquake cycles. The 1944 rupture comprised five fault segments based on slip distribution and fault step-overs and bends. From west to east, they are called the Bolu, Yenicaga, Gerede, Ismetpasa, and Bayramoren segments. The segments were 21 to 46 km long, and the average slip was 1.9–4.3 m, similar to well-examined segments of the 1999 Izmit rupture. In contrast, the fault jogs were relatively small, suggesting that the 1944 earthquake rupture process was relatively smooth and of short duration. Proposed fault segmentation is consistent with the extent of surface ruptures and damage intensity associated with historical earthquakes in 967, 1035, and 1050 a.d. Historical records indicate that the Bolu and Yenicaga segments ruptured in 967 a.d., and the Gerede, Ismetpasa, and possibly, Bayramoren segments ruptured in 1035 a.d. Therefore, the 1944 rupture occurred as a consequence of multisegment rupture, which consists of at least two distinct earthquake segments. Along the Gerede segment, which slipped 3 to 6 m in 1944, cumulative offsets of 6 to 25 m were observed. These offsets are double, triple, and quadruple multiples of the 1944 slip, implying a similar slip distribution during the four most recent large earthquakes along this segment and supporting the concept of characteristic slip, even though the rupture length varied greatly during the last four earthquake cycles.

Step-Over and Bend Structures along the 1999 Duzce Earthquake Surface Rupture, North Anatolian Fault, Turkey

The 12 November 1999 Duzce earthquake (M w 7.1), which occurred on the Duzce fault of the North Anatolian Fault Zone, formed a 40-km-long surface rupture, which extended between Golyaka in the west and Kaynasli in the east. The earthquake nucleated close to the Beykoy step-over, a 0.8-km-wide and 1.8-km-long pressure ridge, with bilateral rupture propagation both to the west and the east. The Beykoy, Kaynasli, and Cevizlik step-overs, through which the 1999 Duzce rupture propagated, are less than 2.5 km wide. The Eften and Bakacak step-overs, where the rupture was arrested, are more than 4 km wide. These exceptional dimensions of step-overs associated with the 1999 Duzce rupture were investigated for their influ- ence on rupture propagation. The earthquake broke three distinct geometric segments, from west to east, the Eften, Dagdibi, and Kaynasli fault segments, separated by left-restraining step-overs at Beykoy and Kaynasli. To the west of the main shock, 3.5 m of average slip on the Eften fault segment propagated into the 2-km-wide Cevizlik releasing double bend, where it was largely arrested, with smaller (0.5 m) slip continuing westward on the Golyaka intervening segment, west of Lake Eften. This is similar to the ter- mination of slip observed on the Karadere fault segment in the 1999 Izmit earthquake, where 1.5 m of slip was largely terminated by a releasing double bend at the Golyaka intervening segment, with minor slip continuing eastward near Lake Eften. To the east of the main shock, 5 m of slip decreased along the Dagdibi fault segment to about 3.5 m at the 1-km-wide Kaynasli step-over. From here to the east, the 2.5 m of average slip on the Kaynasli fault segment continued to decrease, reaching zero at the releasing step-overs to the Elmalik fault. The terminations of the 1999 Duzce surface rupture are defined by distinct struc- tural complexities. The rupture was arrested in the west at the 4-km-wide Eften releasing double bend, where the eastern termination of the 1999 Izmit rupture also occurred. To the east, the rupture terminated at the 4- to 5-km-wide Bakacak releasing double step-overs. Both eastern and western ends of the Duzce rupture have short fault segments that experienced minor rupture during the 1999 Duzce earthquake. The Golyaka intervening segment experienced minor rupture, during both the 1999 Izmit and Duzce earthquakes. The Duzce rupture propagated across 0.8- to 2-km- wide step-overs, but it was arrested at step-overs 4 km wide or wider. These results agree with the observations on ratios between step-over width (in kilometers) and strike-slip displacement (in meters) collected along strike-slip faults worldwide (Let- tis et al., 2002).

Active fault database of Turkey

We have updated the active fault map of Turkey and built its database within GIS environment. In the study, four distinct active fault types, classified according to geochronological criteria and character, were delineated on the 1:25,000 base map of Turkey. 176 fault segments not included in the former active fault map of Turkey, have been identified and documented. We infer that there are 485 single fault segments which are substantially potential seismic sources. In total 1964 active-fault base-maps were transferred into the GIS environment. Each fault was attributed with key parameters such as class, activity, type, length, trend, and attitude of fault plane. The fault parameters are also supported by slip-rate and seismogenic depth inferred from available GPS, seismological and paleoseismological data. Additionally, expected maximum magnitude for each fault segment was estimated by empirical equations. We present the database in a parametric catalogue of fault segments to be of interest in earthquake engineering and seismotectonics. The study provides essential geological and seismological inputs for regional seismic hazard analysis of all over Turkey and its vicinity.

Seismotectonic database of Turkey

Turkey is located in one of the most seismically active regions in the world. Characterizing seismic source zones in this region requires evaluation and integration of geological, geophysical, seismological and geodetical data. This first seismotectonic database for Turkey presented herein was prepared, under the framework of the National Earthquake Strategy and Action Plan—2023. The geographic information system (GIS)-based database includes maps of active faults, catalogues of instrumental and historical earthquakes, moment tensor solutions and data on crustal thickness. On the basis of these data, 18 major seismotectonic zones were delineated for Turkey and the surrounding region. The compilation and storage of the seismotectonic data sets in a digital GIS will allow analyses and systematic updates as new data accrete over time.

Evolution of seismic hazard maps in Turkey

A review on the historical evolution of seismic hazard maps in Turkey is followed by summarizing the important aspects of the updated national probabilistic seismic hazard maps. Comparisons with the predecessor probabilistic seismic hazard maps as well as the implications on the national design codes conclude the paper.

The Bekten Fault: the palaeoseismic behaviour and kinematic characteristics of an intervening segment of the North Anatolian Fault Zone, Southern Marmara Region, Turkey

The Bekten Fault is 20-km long N55°E trending and oblique-slip fault in the dextral strike-slip fault zone. The fault is extending sub-parallel between Yenice-Gönen and Sarıköy faults, which forms the southern branch of North Anatolian Fault Zone in Southern Marmara Region. Tectonomorphological structures indicative of the recent fault displacements such as elongated ridges and offset creeks observed along the fault. In this study, we investigated palaeoseismic activities of the Bekten Fault by trenching surveys, which were carried out over a topographic saddle. The trench exposed the fault and the trench stratigraphy revealed repeated earthquake surface rupture events which resulted in displacements of late Pleistocene and Holocene deposits. According to radiocarbon ages obtained from samples taken from the event horizons in the stratigraphy, it was determined that at least three earthquakes resulting in surface rupture generated from the Bekten Fault within last ~1300 years. Based on the palaeoseismological data, the Bekten Fault displays non-characteristic earthquake behaviour and has not produced any earthquake associated with surface rupture for about the last 400 years. Additionally, the data will provide information for the role of small fault segments play except for the major structures in strike-slip fault systems.

Paleosismolojik bulgular ışığında Orhaneli Fayının Holosen aktivitesi, Bursa, KB Anadolu

Orhaneli Fayi, Biga Yarimadasi’nda yer alan yaklasik 30 km uzunlugunda, ters bileseni baskin, sag yonlu dogrultu atimli bir faydir. Kuvaterner yasli Orhaneli Havzasi’nin guney sinirini bu fay kontrol etmektedir. Bununla birlikte soz konusu fay ayni zamanda Tavsanli Zonu’na ait metamorfik kayaclari, Kretase yasli ofiyolitik birimleri ve Neojen/Kuvaterner yasli sedimanlari kesmekte olup hava fotografl arinda ve arazide belirgin cizgiselligi ve fay sarpliklari ile kolayca takip edilebilmektedir. Fay boyunca gozlenen uzamis sirtlar, otelenmis vadiler ve topografik boyunlar, fayin aktivitesine isaret eden onemli jeomorfolojik unsurlardir. Bu calismada Orhaneli Fayi uzerinde gerceklestirilmis olan ilk paleosismolojik calismalarin sonuclari yer almaktadir. Bu kapsamda fay uzerinde iki adet hendek acilmistir. Serceler ve Kusumlar olarak adlandirilmis bu hendeklerde, Tavsanli Zonu’na ait metamorfik kayaclarin fay tarafindan Kuvaterner yasli sedimanlar uzerine itildigi belirlenmistir. Yapilan calismada, Kuvaterner’de yuzey yirtilmasiyla sonuclanmis dort buyuk depremin izlerine rastlanmistir. Serceler hendeginde, en eski depremin MO 22.000±3.200 ile MO 6.600±800 arasinda, son depremin ise MO 770-415 den once gerceklestigi belirlenmistir. Kusumlar hendeginde ise, en eski depremin MO 6.600 ile MO 3.085 arasinda, son olayin ise MS 650 tarihinden sonraki bir donemde gerceklestigi belirlenmistir. Soz konusu eski depremler arasinda tekrarlanma periyoduna iliskin bir yorum yapilamamistir. Bu calisma ile Orhaneli Fayi’nin Holosen aktivitesi kanitlanmistir. Toplam uzunlugu 30 km olan ve iki segmentten olusan bu fayin tek parca halinde kirilmasi halinde, Mw= 6,87 buyuklugunde bir deprem uretme potansiyeline sahip oldugu belirlenmistir.

HOLOCENE ACTIVITY OF THE ORHANELI FAULT BASED ON PALAEOSEISMOLOGICAL DATA, BURSA, NW ANATOLIA

Orhaneli Fayi, Biga Yarimadasi’nda yer alan yaklasik 30 km uzunlugunda, ters bileseni baskin, sag yonlu dogrultu atimli bir faydir. Kuvaterner yasli Orhaneli Havzasi’nin guney sinirini bu fay kontrol etmektedir. Bununla birlikte soz konusu fay ayni zamanda Tavsanli Zonu’na ait metamorfik kayaclari, Kretase yasli ofiyolitik birimleri ve Neojen/Kuvaterner yasli sedimanlari kesmekte olup hava fotografl arinda ve arazide belirgin cizgiselligi ve fay sarpliklari ile kolayca takip edilebilmektedir. Fay boyunca gozlenen uzamis sirtlar, otelenmis vadiler ve topografik boyunlar, fayin aktivitesine isaret eden onemli jeomorfolojik unsurlardir. Bu calismada Orhaneli Fayi uzerinde gerceklestirilmis olan ilk paleosismolojik calismalarin sonuclari yer almaktadir. Bu kapsamda fay uzerinde iki adet hendek acilmistir. Serceler ve Kusumlar olarak adlandirilmis bu hendeklerde, Tavsanli Zonu’na ait metamorfik kayaclarin fay tarafindan Kuvaterner yasli sedimanlar uzerine itildigi belirlenmistir. Yapilan calismada, Kuvaterner’de yuzey yirtilmasiyla sonuclanmis dort buyuk depremin izlerine rastlanmistir. Serceler hendeginde, en eski depremin MO 22.000±3.200 ile MO 6.600±800 arasinda, son depremin ise MO 770-415 den once gerceklestigi belirlenmistir. Kusumlar hendeginde ise, en eski depremin MO 6.600 ile MO 3.085 arasinda, son olayin ise MS 650 tarihinden sonraki bir donemde gerceklestigi belirlenmistir. Soz konusu eski depremler arasinda tekrarlanma periyoduna iliskin bir yorum yapilamamistir. Bu calisma ile Orhaneli Fayi’nin Holosen aktivitesi kanitlanmistir. Toplam uzunlugu 30 km olan ve iki segmentten olusan bu fayin tek parca halinde kirilmasi halinde, Mw= 6,87 buyuklugunde bir deprem uretme potansiyeline sahip oldugu belirlenmistir.

The Manyas fault zone (southern Marmara region, NW Turkey): active tectonics and paleoseismology

The Manyas fault zone (MFZ) is a splay fault of the Yenice Gönen Fault, which is located on the southern branch of the North Anatolian Fault System. The MFZ is a 38 km long, WNW–ESE-trending and normal fault zone comprised of three en-echelon segments. On 6 October 1964, an earthquake (Ms = 6.9) occurred on the Salur segment. In this study, paleoseismic trench studies were performed along the Salur segment. Based on these paleoseismic trench studies, at least three earthquakes resulting in a surface rupture within the last 4000 years, including the 1964 earthquake have been identified and dated. The penultimate event can be correlated with the AD 1323 earthquake. There is no archaeological and/or historical record that can be associated with the oldest earthquake dated between BP 3800 ± 600 and BP 2300 ± 200 years. Additionally, the trench study performed to the north of the Salur segment demonstrates paleoliquefaction structures crossing each other. The surface deformation that occurred during the 1964 earthquake is determined primarily to be the consequence of liquefaction. According to the fault plane slip data, the MFZ is a purely normal fault demonstrating a listric geometry with a dip of 64°–74° to the NNE.

Active tectonic and palaeoseismological features of the western section of Mustafakemalpaşa Fault; Bursa, NW Anatolia

The Mustafakemalpaşa Fault (MF), located among Manyas, Ulubat and Orhaneli faults, is a right lateral strike-slip and 47 km in length. The MF begins with a pressure ridge at the west and exhibits complex jog terminations at east ends in restraining left stepovers. The western section of the fault bounds Miocene and Quaternary units and continues towards İlyasçılar. The central segment of the fault, starts with approximately 750-m leftward stepover, exhibits a sinusoidal geometry between Kapaklıoluk and Kabulbaba. In this section, MF traverses mountainous terrain and cuts Ophiolite, Jurassic limestones and Miocene detritals, forming dextral faulting features and gaining reverse component. The eastern section exhibits left stepping en-echelon pattern and consists 2.5-km offset on the Orhaneli River. In this study, palaeoseismological findings related to the Holocene activity and active tectonic properties of the MF are presented. The trenches exposed mismatched stratigraphy, demonstrating evidence of events across the fault. We identified three events (before BC 2190, later AD 1425 and 1850) that have occurred during the past 4000 years. We suggest a long non-characteristic recurrence interval and ~0.7 mm/y slip-rate for MF, based on trench data and offset of the Late Pliocene drainage of Orhaneli River.