Browsing by Author "Alkan, Hamdi"

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6 Şubat 2023 Kahramanmaraş Merkezli Depremler Sonrasında Muş İli ve Çevresinin Deprem Tehlikesine Yönelik Bir Değerlendirme
(2023) Dölek, İskender; Ekinci, Rezzan; Alkan, Hamdi; Büyüksaraç, Aydın; Ekinci, Yunus Levent
Türkiye’nin doğusunda bulunan Muş havzası birçok aktif fay ve fay zonuna sahiptir. Ayrıca bölge tarihsel ve aletsel dönemde yıkıcı etkiye sahip orta ve büyük birçok depreme maruz kalmıştır. Bu çalışmada, Muş havzasında 2010-2023 yılları arasında meydana gelen ve büyüklükleri Mw≥4.0 olan depremler kullanılarak bölgenin tektonik gerilme durumu Coulomb gerilme analizi ile incelenmiştir. Derinlik ile birlikte statik gerilme değişimlerini modellemek için farklı derinlik seviyeleri içim Coulomb gerilme değişimi haritaları oluşturulmuştur. Özellikle, Muş havzasının kuzey batısında bulunan Karlıova üçlü birleşimi civarında bulunan Kuzey Anadolu Fay Zonu ile ilişkili Kargapazarı ve Elmalı segmentleri ile Varto Fay Zonu boyunca sığ derinliklerde pozitif Coulomb gerilme değerleri göze çarpmaktadır. Bunun aksine, Muş şehir merkezinin doğusunda bulunan Bulanık Fayı, Haçlıgölü Fayı ve Malazgirt Fayı civarında ise gerilme değişiminin daha durağan olduğu gözlenmiştir. Sonuç olarak, Muş şehir merkezinin özellikle kuzey doğusunda sığ derinliklerde sismik aktivite ihtimali diğer bölgelere göre daha yüksektir.
Analyzing the October 16, 2024 Mw 5.9 Kale (Malatya) Earthquake in Relation To the February 2023 Kahramanmaraş Earthquake Sequence and Local Tectonic Dynamics
(Springer int Publ Ag, 2025) Senkaya, Mustafa; Alkan, Hamdi; Ozturk, Serkan; Buyuksarac, Aydin
The 6 February 2023 Mw 7.7-7.6 Kahramanmara & scedil; earthquake sequence has significantly impacted the East Anatolian Fault zone, including the city of Malatya and its immediate surroundings. In addition to the aftershocks of February 2023, the Mw 5.9 Kale earthquake that occurred on October 16, 2024, further underscores the ongoing seismic activity in the region. This study analyzes the distribution of b-values derived from a comprehensive dataset comprising 14,549 earthquakes and the Coulomb stress variations associated with the aftershocks of the February 2023 sequence and the Kale earthquake. The primary objective of this analysis is to enhance the understanding of the tectonic setting that contributed to the occurrence of the Kale earthquake. The findings indicate that the low b-values and stress transfer through the P & uuml;t & uuml;rge segment toward the unnamed fault near Kale are significant contributing factors to the occurrence of the Kale earthquake. Additionally, positive stress variations from Do & gbreve;an & scedil;ehir to Malatya's city center suggest a potential fault oriented toward the city center that may increase the current earthquake hazard. Furthermore, the distribution of seismic events around Malatya suggests a possible barrier supported by previous magnetic data analysis between Malatya and Kale. Lastly, the observed stress variations for the Kale earthquake indicate the likelihood of upcoming seismic events in both the northeast and southwest directions of the Kale.
Benzetilmiş Tavlama Yöntemi: Karadeniz Havzası için Bir Sismolojik Uygulama
(2020) Çınar, Hakan; Alkan, Hamdi
Bu çalışmada Karadeniz Havza’nın ve civarının güncel kabuk ve üst manto yapısı çalışılmıştır. Bu amaç doğrultusunda, Karadeniz’in çevresinden seçilen geniş-bantlı deprem istasyonlarının telesismik kayıtları kullanılmıştır. Her bir istasyon altı için P-dalgası hızı, S-dalgası hızı ve Vp/Vs oranı (300 km derinliğe kadar) elde edilmiş ve güncel tektonik durum yorumlanmıştır. Hızların hesaplanması için P- ve S-dalgası Alıcı Fonksiyonları Benzetilmiş Tavlama yöntemi vasıtasıyla birleşik ters çözüm işlemine tabi tutulmuştur. Veri seti yaklaşık olarak 400 adet depremi kapsamaktadır. Elde edilen hız histogramlarına göre, Doğu Karadeniz Havzası’nda kuzeyden güneye doğru Litosfer-Astenosfer geçişi kubbe gibi bir eğim ile artmaktayken, Batı Karadeniz Havzası’nda ise aynı doğrultuda daha yassı bir artış gözlemlenmiştir. Ortalama P-dalgası ve S-dalgası hızları Karadeniz Havzası’nın çevresinde kıtasal özellikli bir litosferin varlığını işaret etmektedir. Tüm bu sonuçlara göre, tektonik olarak güneye eğimli bir yitimin var olduğu söylenebilir.
Çaldıran Fay Zonu ve Cıvarındakı Coulomb Stress Transferı ve Deprem Tehlıkesı Analızı
(2023) Alkan, Hamdi; Öztürk, Serkan; Akkaya, İsmail
Çaldıran fay zonu (CFZ) bir çok yıkıcı deprem üreten ve depremsellik açısından önemli bir aktif zondur. Bu fay sisteminde, 1976 tarihinde bu yıkıcı depremlerin en sonuncusu (Ms = 7.3) meydana gelmiştir. Dolayısıyla bu fay zonunda sismotektonik b-değeri, deprem olasılığı, tekrarlama zamanı ve Coulomb gerilme değişimi parametrelerine bağlı olarak gelecek deprem potansiyeli ve pozitif/negatif gerilme değişimleri araştırılmıştır. Büyüklüğü 3.6'dan büyük olan depremler, sismotektonik parametrelerle ilgili olarak gerilme transfer yönünü araştırmak için kullanılmıştır. Çalışmanın sonuçlarına göre, CFZ’nin güneydoğusu boyunca küçük b değerleri elde edilmişken, büyük b değerleri Van Gölünün doğusu civarında hesaplanmıştır. Diğer taraftan, CFZ’nin kuzeydoğusu mevcut stress birikimini temsil etmektedir. Benzer şekilde, 1976 depreminin Coulomb gerilme değişimi bu fay sisteminin kuzeybatısında gerilmelerin farklı derinliklerde biriktiğini göstermektedir. Ayrıca, daha önceki çalışmalardaki uzun dönem kayma oranları ve yatay öteleme değerleri dikkate alındığında, 6.0'dan büyük bir deprem için tekrarlama zamanı yaklaşık 302 yıldır. Sonuç olarak, bu karşılaştırmalı analizler bölgede sismik tehlike değerlendirmesinin etkilerini ve olası bir sonraki deprem oluşumunun tahminini açıklamaktadır.
Comparison of Natural Vibration Periods of Medium-Height Reinforced Concrete Buildings by Microtremor Method and Modal Analysis: a Case From the City Center of Van
(2022) Okay, Diyar; Alkan, Hamdi
Doğu Anadolu bölgesi depremsellik açısından Alp-Himalaya Orojenik Kuşağı'nda bulunan oldukça aktif bölgelerden bir tanesidir. Bölgede, Doğu Anadolu Fay Zonu, Kuzey Anadolu Fay Zonu ve Bitlis-Zagros Bindirme Kuşağı gibi oldukça önemli tektonik yapılar bulunmaktadır. Bölgenin aktif tektonizması ile ilişkili olarak, Van ili ve civarı da birçok fay ve fay zonuna sahiptir. Dolayısıyla, bu aktif tektonizma belirli zamanlarda yıkıcı depremlere sebep olmuştur. Son olarak, 2011 Van depremi (Mw=7.1) Van ili sınırları içerisinde oldukça hasar verici ve can kaybına sebep olacak etkilere neden olmuştur. Mühendislik yapılarının inşa edildiği zemin koşullarının statik yükler altındaki davranışı yanında, sismik dalgaların etkileri gibi dinamik yükler altında nasıl davranış göstereceği oldukça önemlidir. Bu tez çalışması kapsamında, Van ili şehir merkezinin farklı lokasyonlarında seçilen ve 2011 Van depreminden sonra inşa edilmiş olan orta-yükseklikteki 4 adet betonarme bina incelenmiştir. Seçilen yapılarda toplam 15 adet ve ortalama 30 dakika uzunluğunda mikrotremor ölçüsü alınmıştır. Bu kayıtlara Nakamura (H/V) spektral oranlar yöntemi uygulanmıştır ve hesaplanan hâkim titreşim periyotları onaylı statik hesap raporlarındaki yapı periyotları ile kıyaslanmıştır. Diğer taraftan, ilgili binaların Zemin ve Temel Etüd Veri raporlarında verilen Vs30, sondaj logu ve zemin sınıfı bilgileri de irdelenmiştir. Elde edilen sonuçlara göre, H/V spektral oranlar yönteminden ve modal analiz yönteminden elde edilen hâkim titreşim periyotları arasında önemli farklılıklar yoktur.
Comparison of Seismic and Structural Parameters of Settlements in the East Anatolian Fault Zone in Light of the 6 February Kahramanmaraş Earthquakes
(Mdpi, 2024) Isik, Ercan; Hadzima Nyarko, Marijana; Avcil, Fatih; Buyuksarac, Aydin; Arkan, Enes; Alkan, Hamdi; Harirchian, Ehsan
On 6 February 2023, two very large destructive earthquakes occurred in the East Anatolian Fault Zone (EAFZ), one of T & uuml;rkiye's primary tectonic members. The fact that these earthquakes occurred on the same day and in the same region increased the extent of the destruction. Within the scope of this study, twenty different settlements affected by earthquakes and located directly on the EAFZ were taken into consideration. Significant destruction and structural failure at different levels were induced in reinforced concrete (RC) structures, the dominant urban building stock in these regions. To determine whether the earthquake hazard is adequately represented, the PGA values predicted in the last two earthquake hazard maps used in T & uuml;rkiye for these settlements were compared with the measured PGAs from actual earthquakes. Subsequently, the damage to reinforced concrete structures in these settlements was evaluated within the scope of construction and earthquake engineering. In the final part of the study, static pushover analyses were performed on a selected example of a reinforced concrete building model, and target displacement values for different performance levels were determined separately for each earthquake. For the 20 different settlements considered, the displacements were also derived based on the values predicted in the last two earthquake hazard maps, and comparisons were made. While the target displacements were exceeded in some settlements, there was no exceedance in the other settlements. The realistic presentation of earthquake hazards will enable the mentioned displacements predicted for different performance levels of structures to be determined in a much more realistic manner. As a result, the performance grades predicted for the structures will be estimated more accurately.
Coulomb Stress Analysis İn Nemrut Caldera (East Anatolia, Türkiye)
(2024) Alkan, Hamdi; Bektaş, Özcan; Büyüksaraç, Aydın
In volcanic areas, seismic events with low energy occur before seismic activity or due to the movement observed in the magma. These earthquakes, which are caused by the expansion-contraction movement that has been revealed in different studies and is mostly observed in the magma chamber, can be recorded with continuous observations. On the other hand, it is not easy to distinguish between tectonic and volcanic origins of earthquakes occurring in volcanic areas. In this study, Coulomb stress analysis was carried out using earthquakes in the Nemrut Stratovolcano, which is located in the eastern Türkiye and is at the westernmost end of a volcano arc, and it was concluded that the stress is related to the Nemrut Caldera, therefore the positive stress caused by the expansion of the magma chamber of the Nemrut Volcano creates tremors.
Coulomb Stress Changes and Magnitude-Frequency Distribution for Lake Van Region
(Maden Tetkik ve Arama Genel Mudurlugu-mta, 2022) Alkan, Hamdi; Bayrak, Erdem
The tectonic structure of Turkiye is under the influence of Arabian, Eurasian, African, and Anatolian plates. Lake Van region, located in eastern Turkiye, has been exposed to many devastating earthquakes in historical and instrumental periods. In this paper, using regional earthquakes, the tectonic stress variation of Lake Van region was investigated using the Coulomb stress change and the b-value distribution. 83 earthquakes that occurred between 2000 and 2020 are used to calculate the Coulomb stress change, while 17815 earthquakes that occurred between 1903 and 2021 are used to calculate the b-value distribution. Coulomb stress change gives an idea about the transfer of energy to nearby faults. Coulomb stress change and b-value distribution maps were created at different depths to model the variation of stress. The low b-values and positive Coulomb stress values were especially observed around the Van and Yenikosk faults. On the contrary, no significant variation in stress change was observed around Suphan and Nemrut volcanoes, and high b-values were calculated in this region. Coulomb stress change and earthquake epicentral distribution are compatible and most events occurred in positive stress regions. In conclusion, stress change and b-value distribution were interpreted together and positive stress regions were revealed in the region.
Crustal S-Wave Structure Around the Lake Van Region (Eastern Turkey) From Interstation Rayleigh Wave Phase Velocity Analyses
(Tubitak Scientific & Technological Research Council Turkey, 2017) Cinar, Hakan; Alkan, Hamdi
This study focuses on the average crustal and the upper mantle structure throughout the Lake Van region of eastern Turkey. The study aimed to investigate the structure with the fundamental mode interstation Rayleigh wave phase velocities from the local and the regional earthquakes recorded by Kandilli Observatory and Earthquake Research Institute stations. Considering back azimuth differences of each source and station path, six different broadband station pairs and 27 earthquakes were selected to determine the 1-D shear-wave velocity structures throughout the region by using an interstation method (slant stacking technique). The linearized least squares algorithm was used to obtain the 1-D shear-velocity model that best fit the observed phase velocity dispersion curve. The normalized statistical resolution matrix was calculated to measure the reliability of the solution. Inversion results revealed that the solution quality of the upper crust is weak due to the high resolution lengths. The average shear-wave velocities in the lower crust scale down to approximately 3.5 km/s. It was inferred that this low-velocity zone shown in the lower crust may be associated with widespread volcanism. Final 2-D S-wave velocity models obtained from the inversion revealed that the crust-mantle boundary is similar to 42 km, and shear velocities vary from 3.6 to 4.2 km/s. Furthermore, the upper mantle (similar to 45-70 km) velocities are slower than globally suggested models (e. g., IASP91), and this is possibly related to shallow hot asthenospheric material.
Crustal S-Wave Structure Beneath Eastern Black Sea Region Revealed by Rayleigh-Wave Group Velocities
(Pergamon-elsevier Science Ltd, 2016) Cinar, Hakan; Alkan, Hamdi
In this study, the crustal S-wave structure beneath the Eastern Black Sea Region (including the Eastern Black Sea Basin (EBSB) and Eastern Pontides (EP)) has been revealed using inversion of single-station, fundamental-mode Rayleigh-wave group velocities in the period range of 4-40 seconds. We used digital broadband recordings of 13 regional earthquakes that recently occurred in the easternmost EBSB recorded at stations of the Kandilli Observatory and Earthquake Research Institute (KOERI). The average group-velocity-dispersion curves were generated from 26 paths for the EBSB, and 16 paths for the EP, and they were inverted to determine the average 1-D shear-wave structure of the region. We have created a pseudo-section, roughly depicting the crustal structure of the region based on the group velocity inversion results of all station-earthquake paths. The thickness of the sedimentary layer reaches 12 km in the center of EBSB (V-S= 2.5-3.1 km/s) and decreases 4 km in the EP. There is a thin sedimentary layer in the EP (V-S = 2.7 km/s). A consolidated thin crust that exists in the EBSB possesses a high seismic velocity (V-S = 3.8 km/s). While a thin (similar to 26 km) and transitional crust exists beneath the EBSB, a thick (about 42 km) continental crust exists beneath the EP where the Conrad is clearly seen at about a 24 km depth. Thick continental crust in the EP region is clearly distinguished from a gradational velocity change (V-S = 3.4-3.8 km/s). The Moho dips approximately southwards, and the V-S velocity (4.25-4.15 km/s) beneath the Moho discontinuity decreases from the EBSB to the EP in the N-S direction. This may be an indication of a southward subduction. (c) 2015 Elsevier Ltd. All rights reserved.
Crustal Structure in and Around the East Anatolian Volcanic Belt by Using Receiver Functions Stacking
(Pergamon-elsevier Science Ltd, 2022) Alkan, Hamdi
This study aims to explore the crustal seismic properties across the East Anatolian volcanic belt. Teleseismic receiver functions obtained through deconvolution in the time domain are used to estimate crustal discontinuities and Moho interface. Further, slant stacked method, employed for the first time in the study region, is applied to identify the arrival times of P to S conversion and reverberations (Ps and PpPms) that give Vp/Vs ratio beneath each station. For slant stacking method, radial receiver functions are calculated using deconvolution based on frequency domain. Inversion results indicate that Moho depth varies from at most ~& nbsp;40 km near the Bitlis-Zagros thrust zone to 45 km in the northeastward of Quaternary volcanic centers. The southwest of study region is characterized overall by a low average crustal Vp/Vs ratio (less than 1.71), implying a felsic crustal composition. A high average crustal Vp/Vs ratio of 1.82-1.87 is found near volcanic centers, characterized by predominantly a mafic crustal composition. The prominent crustal low-velocity zones especially around the Nemrut and Ararat volcanoes are identified below ~20 km depth and likely imply the presence of partial melt zones or active magma chambers in the region. As a result, the findings of this study provide useful new constraints to the crustal thickness and composition in the study region.
Deprem Kayıtlarından Elde Edilen Yatay/düşey Spektral Oranların Ters Çözümüyle Kayma Dalga Hız Yapısının Belirlenmesi: Van Gölü Doğusu Örneği
(2022) Akkaya, İsmail; Alkan, Hamdi
Van Gölü doğusu farklı özellikteki aktif fayların varlığı sebebiyle deprem üretme potansiyeli yüksek olan bir bölgedir. Depremlere bağlı oluşan hasarların değerlendirilmesinde yeraltının fiziksel özelliklerinin ve deprem sırasındaki davranışının iyi bilinmesi gerekmektedir. Kayma dalgası hız değişimi ve anakaya derinliğinin belirlenmesi bu açıdan son derece önemlidir. Bu çalışmada 2011-2021 yılları arasında Van Gölü doğusunda meydana gelen ve farklı tipteki faylarda oluşmuş dokuz deprem verisinden yararlanılmıştır. 6 istasyonda kaydedilen depremler yatay-düşey spektral oran yöntemi ve Monte-Carlo ters çözüm algoritması ile analiz edilerek, kayma dalgası hız yapısı ve anakaya derinlikleri belirlenmiştir. İstasyonlar altında alüvyon birimlerinin kalınlığına bağlı olarak nispeten düşük frekans değerleri elde edilmişken, farklı kaya birimlerin varlığı baskın frekans değerlerini yükseltmiştir. Spektral oran eğrilerindeki farklı frekanslardaki pikler, jeolojik yapının özelliklerine bağlı olarak değişkenlik göstermiştir. İstasyon altı anakaya derinliği 10-350 m arasındadır. Artan anakaya derinlik seviyeleri yıkıcı depremlerin hasar oranını arttıran bir faktördür.
Determination of the Shear Wave Velocity Structure of Substations by the Hvsr Inversion Method Using Broadband and Strong Ground Motion Earthquake Data in the Lake Van Region, Eastern Türkiye. Baltica
(inst Geology & Geography, 2024) Akkaya, Ismail; Alkan, Hamdi
In this research, the horizontal -to -vertical spectral ratio analysis and the Monte -Carlo inversion algorithm are applied to calculate the substations shear wave velocity structure beneath the Lake Van region. To discover the variation of shear wave velocity, the local earthquakes recorded on 15 strong ground motions and 14 broadband stations, located on dissimilar geological units, are used. The most important aim of this investigation is to reveal the geological structure and bedrock depths beneath each station. For this purpose, two-dimensional cross -sections are generated from velocity models in different directions. The average bedrock depth variation is determined at 10 m and 250 m as a result of the horizontal -to -vertical spectral ratio inversions at the earthquake stations in the region. The deepest bedrock depths are calculated at the earthquake stations close to the settlements in the study area. These depth variations are interpreted as the local site effects of possible destructive and big earthquakes that may be a factor increasing the damage ratio in these regions.
An Evaluation of the Earthquake Potential With Seismic and Tectonic Variables for the West Anatolian Region of Turkiye
(inst Geology & Geography, 2024) Ozturk, Serkan; Alkan, Hamdi
In the present study, an evaluation of the region-time-magnitude behaviours of the earthquake occurrences in the West Anatolian Region (WAR), Turkiye, is carried out using the statistical and seismotectonic parameters such as the b-value of Gutenberg-Richter relation, occurrence probabilities, and return periods of earthquakes. We also have mapped the Coulomb stress changes to observe the current and future earthquake hazard. In recent years, several large earthquakes such as the 1919 Soma (Mw = 6.7) and the 2022 and 2024 Aegean Sea (Mw = 5.3 and Mw = 5.1) revealed earthquake potential in the WAR. Coulomb stress analyses of 41 local events with mostly normal fault mechanisms have shown that positive lobes (> 0.0 in bars) are mainly confined in the crust and uppermost mantle depths around Samos, Kos, and south of Lesvos. The smaller b-values (< 1.0) are observed in the same regions. On the contrary, we have observed a higher b-value from the offshore to onshore, south to north-trending direction, and negative scattered stress lobes (< 0.0 in bars) in slightly NW-SE oriented. The relationship between an increased b-value and negative stress change may indicate a similar seismicity for the region. In addition, we have analyzed the occurrence probabilities and return periods of the earthquakes, which showed us that Mw = 6.0 may occur at 75% in the intermediate term with an estimation of similar to 7 years. Our results reflect that these types of multiple-parameter assessments are important to define regional seismicity, seismic, tectonic, and statistical behaviours. Consequently, the areas with reductions in b-values and increments in stress imply the possible seismic hazard in the intermediate/long term.
Fault Modeling Around Southern Anatolia Using the Aftershock Sequence of the Kahramanmaras Earthquakes (Mw=7.7 and Mw=7.6) and an Interpretation of Potential Field Data
(Springer int Publ Ag, 2024) Buyuksarac, Aydin; Bektas, Ozcan; Alkan, Hamdi
On February 6, 2023, southeastern Turkiye experienced devastating doublet earthquakes (Mw = 7.7 and Mw = 7.6) with a series of aftershocks along the East Anatolian Fault Zone. The mainshocks were followed by similar to 15,000 aftershocks mainly distributed in the NNE-SSW direction, including similar to 400 events with an Mw >= 4.0 in the following 30 days. Although many moderate to large earthquakes have occurred in the historical and instrumental periods of this region, these double earthquakes and their aftershocks majorly impacted lives and released great seismic energy. In this study, we interpret the gravity-magnetic data and the epicenter and hypocenter distributions of the aftershocks to correlate the tectonic structures and the active fault zones. The results of potential field anomalies reveal that the rotational anomalies in the southwestward direction are associated with the tectonic structure of Anatolia. Results show that shallow aftershocks are associated with high-gravity anomalies, whereas deeper aftershocks are associated with low-gravity anomalies and they become shallower in places where gravity values increase. After the derivative transformations are applied to the magnetic anomalies, it is seen that the faults and regions of magnetic discontinuity are in good agreement. Consequently, the findings on gravity, magnetic anomalies and aftershock sequences demonstrate that the first mainshock occurred in the unbroken segment of the East Anatolian Fault Zone.
Hakkari ve Civarının (Güneydoğu Anadolu, Türkiye) Güncel Deprem Potansiyeli: Bölge-zaman-magnitüd Analizleri
(2024) Öztürk, Serkan; Alkan, Hamdi
Hakkari (Türkiye), depremsellik ve tektonizma olarak Güneydoğu Anadolu Bölgesinin en aktif illerinden bir tanesidir. Aletsel ve tarihsel dönemde, Hakkari ili ve civarında bulunan bu fay ve fay zonları birçok yıkıcı/hasar verici deprem üretmiştir. Son yıllarda Aralık 2023 (Mw=4.7) ve Şubat 2024’de (Mw=4.3) meydana gelen depremler, bu bölgedeki güncel depremselliğin önemli göstergelerindendir. Analizlerden elde edilen bulgulara göre, bölgede ortalama b-değeri 0.880.09 olup en düşük b-değerleri Yüksekova-Şemdinli Fay Zonu civarında gözlenmiştir. Buna ek olarak, pozitif Coulomb gerilme değişimleri Hakkari ili ve civarında KB-GD ve KD-GB yönlü olarak ortaya çıkmıştır. Diğer taraftan, seçilen bölge için Mw=5.0 büyüklüğündeki bir depremin tekrarlama zamanı ~16 yıldır. Ayrıca, Mw=5.0 büyüklüğündeki bir depremin 10, 16 ve 20 yılda oluşma olasılıkları ise sırasıyla ~%45, ~%62 ve ~%69 olarak hesaplanmıştır. Sonuç olarak, bu çalışmada hesaplanan sismotektonik parametrelerin birlikte değerlendirilmesi ile Hakkari ili ve civarının güncel sismik tehlike potansiyeli ortaya çıkarılmıştır.
Investigation of Earthquake Sequence and Stress Transfer in the Eastern Anatolia Fault Zone by Coulomb Stress Analysis
(Tubitak Scientific & Technological Research Council Turkey, 2024) Alkan, Hamdi; Buyuksarac, Aydin; Bektas, Ozcan
A devastating earthquake with a magnitude of (M-w = 7.7) occurred on February 06, 2023, in the Pazarcik segment of the Eastern Anatolian Fault Zone, which has not shown major earthquake activity for a long time. On the same day, another earthquake with a magnitude of (M-w = 7.6) occurred in Ekinozu-Elbistan (Kahramanmaras) in the northwest. Three more earthquakes with magnitudes of M-w = 6.6, M-w = 5.9, and ML = 5.7 occurred on the same day, and significant damage, loss of life, and property occurred in 11 provinces and districts. A sixth earthquake occurred with M-w = 6.4 magnitude in Defne-Hatay on February 20, 2023. In addition, more than 32,000 aftershocks were recorded while this study was in progress. The interconnection of these earthquakes was investigated by Coulomb stress analysis. It showed compatibility with the distribution of both these six earthquakes and small aftershocks, in which earthquakes transfer stress to each other during their formation phases. By calculating Coulomb stress changes, mainshock ruptures have played an important role in transferring stresses between Elazig and Malatya in the northeast, Kahramanmaras-Goksun in the west, and Hatay and Syria in the south. Also, our calculations showed that shallow depths in and around source zones received positive stress changes (similar to 1.0 bar) due to the focal depths of mainshocks with their aftershocks. Finally, it has been better understood with these earthquakes that the stress transfer time of high-energy earthquakes can be shortened considerably.
Investigation of the Crustal and Upper-Mantle Structure of the Eastern Pontides Orogenic Belt (Ne, Turkey): a Receiver-Function Study
(Springer, 2019) Alkan, Hamdi; Cinar, Hakan; Oreshin, Sergey; Vinnik, Lev
In this study, we use teleseismic P and S receiver functions (i.e., S-to-P and P-to-S converted signals) and their joint inversions to determine seismic discontinuities in the crust and upper mantle. Eight permanent broadband stations from the KOERI (Kandilli Observatory and Earthquake Research Institute) that are distributed along the eastern Pontides orogenic belt (EPOB), NE, Turkey, comprise our database. Inversion is performed by using a simulated annealing technique with and without travel time residuals. Our inversion results reveal the Moho depth, a high S velocity lid, a low-velocity zone, and the underlying upper mantle layer. The studied area is divided into two regions based on the station locations: (a) a northern region and (b) a southern region. The inversion results from the northern area produce crustal models that indicate that the uppermost crust is represented by a low P and S wave velocity (Vp=5.0km/s and Vs=2.8km/s). These velocities are clear evidence of (1) the sedimentary and volcanic rocks that widely crop out in the region and (2) a thinner uppermost crust, whereas the velocities of the southern region (Vp=6.0km/s and Vs=3.1km/s) indicate a thicker uppermost crust (7km). Our calculated Vp/Vs velocity ratio in the lower crust is approximately 1.90 and 1.80 in the northern and southern regions, respectively. These ratios are generally attributed to mafic rocks. Beneath the northern stations, the crustal thicknesses are 30, 33, 37, and 40km from east to west, while the depths of the Moho are 46, 42, 39, and 44km beneath the southern stations. Some velocity histograms show a transition from the high S velocity mantle lid to the low-velocity zone, which is known as the lithosphere-asthenosphere boundary. A representative value of the boundary's depth is around 83km in the north and 88km in the south. The hypothesis of some researchers regarding the existence of southward subduction beneath the eastern Pontides orogenic belt during the Late Mesozoic-Cenozoic is supported by our 2-D and 3-D velocity-depth models.
Joint Inversion of Rayleigh Wave Phase Velocities and P-Receiver Functions Along the East Anatolian Fault Zone
(Springer, 2022) Alkan, Hamdi
The detailed crustal and uppermost mantle structure along the East Anatolian fault zone is investigated from the joint inversion data set recorded in 30 broadband seismic stations. Inter-station Rayleigh wave phase velocities are retrieved 5-50 s periods for all stations, whereas radial receiver functions are calculated beneath 17 stations. Firstly, the H-k stacking analysis of receiver functions is applied to calculate the average Moho depth and V-p/V-s ratio, used in the initial model parameters for joint inversion. Then, the joint inversion is performed to develop a new 3-D S-wave velocity model. The findings obtained from these different algorithms show that the Moho discontinuity reaches similar to 42 km and the Conrad discontinuity appears in the depth of similar to 17 km. The low-velocity zones with a minimum S velocity of similar to 2.5 km/s are identified in the upper crust beneath many of the seismic stations and correlate well with the upper crust-lower crust boundary. Combined with previous studies, the high V-p/V-s ratio of the crust and low shear wave velocities interpreted the presence of partial melting and high heat flow. On the other hand, the current seismicity in the region presents that the earthquakes occurred in the depth of 10-20-km depths and major earthquakes (Mw >= 5.0) intensified to the northwest, corresponding exactly to the brittle-ductile transition zone, which may contribute to the occurrence of the earthquakes in shallow depths towards the southwest especially. In conclusion, the new model produced in this study can serve as a reference model for Earth crust studies in the near future.
Lake Van (Southeastern Turkey) Experiment: Receiver Function Analyses of Lithospheric Structure From Teleseismic Observations
(Springer Basel Ag, 2020) Alkan, Hamdi; Cinar, Hakan; Oreshin, Sergey
The P and S wave receiver functions and their joint inversions are used to study the lithosphere beneath the Lake Van region using approximately 600 teleseismic earthquake data (M-w >= 5.8) at different azimuths collected from 10 permanent broadband stations operated by Kandilli Observatory and Earthquake Research Institute and Disaster and Emergency Management Authority in the region. The dataset is taken from the European Integrated Data Archive. The simulated annealing method is used for the joint inversion of P-wave and S-wave receiver functions, since the P410s phase cannot be detected reliably in the depth stacks. This may be due to the low olivine content and high basalt content at this depth. The inversion process is therefore performed without travel time residuals. The crustal thickness is observed at nearly 45 km in all the velocity models obtained from the inversion. The relatively low-velocity layer (V-s = 3.4 km/s) at depths of the middle crust may be associated with volcanic centers near Nemrut, in the west of the region. Additionally, another low-velocity layer (V-s = similar to 3.0 km/s) is observed in the upper crust around the Suphan Volcano. Also, V-p/V-s and Poisson's ratios are calculated for the study area. Their high values (V-p/V-s >= 1.85, and sigma >= 0.285) correspond to the partial melting of the lower crust in the region. According to velocity models obtained from the inversion results, the most important outcome is that the average variance of the Moho discontinuity (similar to 45 km) and lithosphere-asthenosphere boundary (similar to 90 km) under the seismograph stations can be interpreted as a likely part of the Arabian oceanic plate in relation to the collision zone.