The application of HVSR microtremor survey method in Yüksekova (Hakkari) region, Eastern Turkey
Introduction
Yüksekova (Hakkari) region, located in East Anatolian – Iranian plateau, has high seismicity risk because of its geological and tectonic characteristics and experienced many historical earthquakes. Yüksekova – Şemdinli Fault Zone (YSFZ) is one of the major fault zones in the study area because of the fact that it has a gap in the seismotectonic literature of Turkey (Demirtaş and Yılmaz, 1996). Earthquakes are known to be in the historical and instrumental period in the region. The study area and its surroundings are characterized by a strike-slip faulting that is dominant within neotectoinc regime and related structure (Koçyiğit, 2013). There are major structures in the region such as Yüksekova–Şemdinli Fault Zone (YSFZ), Başkale Fault (BF), Bitlis–Zagros Thrust Belt (BZTB) and Salmas (Iran) Fault Zone (SF) that can be specified as strike-slipe fault zones (Fig. 1). Most important earthquakes occured in the last century in this region are Başkale (M = 6.0; 1908), Salmas-Iranian (M = 7.2; 1930), Hakkari-Sütlüce (Mw = 5.9; January 25, 2005) and Yüksekova (M = 5.2; October 27, 2011) and Yüksekova settlement is adversely affected by these earthquakes. These earthquakes represent the seismic activity of the region and the related risks. Yüksekova town is settled on Quaternary deposits, mainly consisting of alluvial fan deposits including loosely-to-moderately cemented gravel, sand, silt and unconsolidated clay horizons. Besides, groundwater table is generally shallow-seated from place to place in the region. These facts contribute the earthquake sourced damage ratio in the region.
Parallel to the increasing population, the number of unplanned and uncontrolled buildings increase in Yüksekova town. This situation must necessitate suitable construction types. All these conditions such as poor construction quality and weak soil may increase the damage ratio in a porobable major earthquake occurence in Yüksekova. More severe earthquake damages are expected in the villages near to the faults because of the widespread settlement of unreinforced masonry and adobe buildings.
Within the scope of this study, the effect of soil condition was determined by conducting HVSR measurements in the study area. Data processing steps on the HVSR microtremor records taken from the region, HVSR peak frequencies/periods of the soil and HVSR peak amplitude values (amplification) were calculated. Because of the fact that the seismic activity of fault zones in Yüksekova region may trigger a major earthquake in the near future, the region may face with many casualities and economic losses due to local soil conditions of the urban area and low-quality construction practices in Yüksekova.
Section snippets
Geology of the region
The rock units in the study area are classified from Paleozoic to Quaternary units. The units in the region are a composition of the Hakkari nappes and the Bitlis–Pötürge–Malatya nappes of Cretaceous–Early Paleocene age and the Bitlis–Mordağ metamorphic units of Paleozoic age and the Kırkgecit Formation of Oligo-Miocene age and Quternary units (Fig. 2).
Hakkari nappes: A thick mass is well-observed at the southern part of the study area. The Hakkari nappes have a gray to brown, thin to medium
Active faults and seismicity
The study area is located in East Anatolian – Iranian plateau which is defined as East Anatolian tectonic block (EATB) (Koçyiğit et al., 2001, Koçyiğit, 2013). This block has a complex tectonics. For this reason, it is important to examine the historical and the instrumental period earthquakes on a regional scale as well the northwestern part of Iran incorporate to understand the seismicity and active tectonics of Yüksekova and surrounding area. There are many active faults and zones near
HVSR microtremor data analysis
Microtremors is commonly used to analyze site effects and characteristics of sediments due to earthquake ground motion (Nakamura, 1989, Lermo and Chavez-Garcia, 1993). Site characteristics, such as fundamental period and amplification are influenced by fault distance, geotechnical conditions, thickness of sedimentary units and soil type. Two techniques are commonly used for determining site effects: the standard spectral ratio (SR) and the HVSR methods (Nakamura, 1989, Lermo and Chavez-Garcia,
Results
HVSR method is applied to 40 microtremor measurements to provide information related to site response of Yüksekova urban area (Fig. 5). Table 2 summarizes the HVSR of 40 stations along with its geographical position and the corresponding HVSR peak period, frequency and HVSR peak amplitude values.
HVSR peak period values were found to be between 0.2 and 1.6 s and HVSR peak amplitude values between 1.5 and 6 in the study area. HVSR peak period values of 0.1 s and HVSR peak amplitude values of 1.5–2
Conclusion
Yüksekova (Hakkari) urban area and surrounding area are seismically active zones of eastern Anatolia. Yüksekova region comprise seismically-active fault systems with diverse magnitude and features. A strong earthquake might cause major effect to the region. Furthermore, Yüksekova region is mainly located on Quaternary aged terrestrial sediments composed of loose-soft sediments (conglomerate, sand and clay), which could amplify the earthquake ground motion to a considerable level whereas some
Acknowledgements
This study was financially supported by the Scientific Research Projects Office of Yüzüncü Yıl University (YYU-BAP, Project Number 2013-HIZ-MİM005). I also thank Dr. Ali Özvan, Alper Şengül and Dr. Mücip Tapan for their support during different stages of this study. The author is grateful to the anonymous reviewers for their valuable comments and suggestions.
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