Journal of Geodynamics 67 (2013) 21–29 Contents lists available at SciVerse ScienceDirect Journal of Geodynamics j ourna l ho me page: http://www.elsevier.com/locate/jog Seismicity and strain accumulation around Karliova Triple Junction (Turkey) Bahadir Aktug a,∗ , Unal Dikmen b,c , Asli Dogru a , Haluk Ozener a a Bogazici University, Kandilli Observatory and Earthquake Research Institute, Geodesy Department, Cengelkoy, Istanbul, Turkey b Ankara University, Engineering Faculty, Department of Geophysical Engineering, Ankara, Turkey c Ankara University Earthquake Research Centre (ADAUM), Ankara, Turkey a r t i c l e i n f o Article history: Received 21 June 2011 Received in revised form 15 April 2012 Accepted 15 April 2012 Available online 21 April 2012 Keywords: East Anatolian Fault System Seismicity GPS Block modeling b-Value a b s t r a c t GPS studies in Turkey date back to the early 1990s, but were mostly focused on the seismically active North Anatolian Fault System (NAFS), or on the more populated Western Anatolia. Relatively few studies were made of the seismically less-active East Anatolian Fault System (EAFS), although it has the potential to produce large earthquakes. In this study, we present the results of a combination of geodetic and seismological data around the Karliova Triple Junction (KTJ), which lies at the intersection of the North- and East Anatolian Fault Systems. In particular, the geodetic slip rates obtained through block modeling of GPS velocities were compared with b-values to assess seismicity in the region. Yedisu segment, one of the best-known seismic gaps in Turkey, was specifically analyzed. The relatively low b-values across Yedisu segment verify the accumulation of seismic energy in this segment, and the GPS-derived geodetic slip rates suggest that it has the potential to produce an earthquake of Mw 7.5 across an 80-km rupture zone. Additionally, analysis of earthquake data reveals that the study area has a ductile or rigid–ductile behavior with respect to its surroundings, characterized by varying b-values. Although, seismic events of moderate- to high magnitudes are confined along the major fault zones, there are also low-seismicity zones along the eastern part of the Bitlis Suture Zone and around Yedisu. Since the high seismicity areas within the region may not accumulate sufficient stress for a large earthquake to occur, it is considered that the deformation in such areas occurs in a ductile manner. On the other hand, the areas characterized by low b-values may have the capacity of stress accumulation, which could lead to brittle deformation. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Although the East Anatolian Fault System (EAFS) is seismi- cally less active than the North Anatolian Fault System (NAFS), historical evidence suggests that it is, nevertheless, capable of producing earthquakes with magnitudes up to Mw 7.0. More- over, poor quality buildings and engineering structures within the region also increase the potential impact and losses asso- ciated with earthquakes, as demonstrated by recent seismicity. The Karliova Triple Junction (KJT) (Fig. 1) is considered the easternmost boundary of the westward transport of Anatolia. The ongoing deformation in the region was previously investi- gated by Reilinger et al. (2006) and, more recently, by Ozener et al. (2010) using Global Positioning System (GPS) obser- vations. The latter study provided an unprecedented spatial coverage of velocity field, through a project (Ozener, 2010) ∗ Corresponding author. Tel.: +90 312 5952255; fax: +90 312 3201495. E-mail addresses: bahadir.aktug@boun.edu.tr (B. Aktug), dikmen@eng.ankara.edu.tr (U. Dikmen), asli.dogru@boun.edu.tr (A. Dogru), ozener@boun.edu.tr (H. Ozener). carried out by the Geodesy Department of Kandilli Observatory and Earthquake Research Institute (KOERI) of Bo˘ gazici University. Both studies provide invaluable data to further the understand- ing of deformation in this region. However, a detailed study of the region, specifically the current slip rates of the main faults, is still needed. This study combines both earlier GPS work and seismic data to better quantify the ongoing deforma- tion. The study area lies between 37.5–42.5 ◦ E and 37.5–40 ◦ N, in the Eastern Anatolia region of Turkey (Fig. 1). In this study, the micro- geodetic network of the Geodesy Department of KOERI and the velocity field given in Reilinger et al. (2006) were homogenously combined by estimating a relative rotation rate, and the densest GPS network was utilized in the region to allow for better inter- pretation and modeling. The triple junction and the surrounding tectonic blocks were discretized into four blocks, corresponding to the main tectonic framework of the region, and the block rotations and slip rates on the boundaries were simultaneously estimated by minimizing the misfit between the observed and modeled veloci- ties. The current state of seismicity and the ongoing accumulation of strain energy in the region were further examined via analysis of b-values. 0264-3707/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jog.2012.04.008