Güney Marmara Bölgesi Sismik Gürültü Kaynaklarının Konum Ve Zaman Özelliklerinin İncelenmesi

Abstract

Bu tez çalışmasının amacı, endüstriyel gelişmişlik bakımından ülkenin önde gelen, yüksek nüfuslu, sismik olarak aktif ve Kuzey Anadolu Fayı’nın Güney Kolu (KAFGK)’nun geçtiği Güney Marmara Bölgesi’nin sismik gürültü niteliğinin zamana, coğrafyaya, jeolojiye ve kültürel yapılara bağlı olarak araştırılması ve değerlendirilmesidir. Çalışmada kullanılan 2011 yılına ait sürekli sismik kayıtlar, Boğaziçi Üniversitesi Kandilli Rasathanesi Deprem Araştırma Enstitüsü (KRDAE) ve T.C Başbakanlık Afet ve Acil Durum Yönetimi Başkanlığı Deprem Dairesi Başkanlığı (AFAD-DAD)’ na ait, Güney Marmara bulunan 15 adet geniş bantlı hız kayıtçıları ile toplanmıştır. Çalışmada, gürültü değerlendirilmesinde kullanılan yöntemin ayrıntıları verilmiştir. Problemli verilerden örnekler gösterilmiştir. Gürültü analizi yönteminde, düşey bileşenli verilerin kullanılarak güç yoğunluğu spektrumlarına (GYS) ait, olasılıksal yoğunluk fonksiyonları (OYF), PQLX (Passcal Quick Look eXtented) programı kullanılarak hesaplatılıp çizdirilmiştir. Bölgedeki gürültü dağılımının zamana ve konuma bağlı değişiminin incelenmesi için GMT (Generic Mapping Tools) programı kullanılarak hazırlanan haritalarda, kültürel gürültülerin (trafik, insan, makine vb.) baskın olarak gözlendiği 0.1-1 s bandı ve mikrotitreşimlerin (derin ve sığ su etkileşimleri) gözlendiği 4-8 s bantları arasındaki ‘İkincil Yüksek Frekans’ (İYF) ve 10-20 s ‘Birincil Yüksek Frekans’ (BYF) bantları arasındaki gürültü verileri, 2011 yılı, altı aylık, aylık ve gece-gündüz saatleri için haritalanmıştır. Gürültü haritaları aylık olarak değerlendirildiğinde, yıl boyunca bölgenin kuzeybatı kesiminde yüksek gürültü seviyelerinin hakim olduğu görülmüştür. Türkiye’nin son 30 yıldaki ortalama rüzgar hızı haritası incelenmiş ve çalışma alanının kuzeybatısındaki bölgenin Türkiye’nin en yüksek rüzgar potansiyeline sahip bölgelerinden biri olduğu görülmüştür. Bölgedeki aylık ortalama rüzgar hızı verileri incelenmiştir. Çalışma bölgesinin sınırları içerisinde kalan ve bölgeye yakın lokasyonlarda kurulu Harita Genel Komutanlığı (HGK)’ nın, Türkiye Ulusal Deniz İzleme Sistemi (TUDES) kapsamında kıyı bölgelerine kurulmuş mareograf istasyonlarından 2011 yılında elde edilen, 15 dk örneklemeli, deniz seviyesi (m), rüzgar hızı (m/s), hava basıncı (mBar) verileri incelenmiştir. Elde edilen sonuçlara göre, kültürel gürültü periyotlarında gözlenen yüksek gürültülerin, rüzgar hızı ile ilişkili olduğu tespit edilmiştir. Çalışma alanına yakın bölgede meydana gelen 5.8 büyüklüğündeki bir depremin ve artçılarının 10-20 s bandında ortamdaki gürültü seviyesini arttırdığı, İYF bandında ise belirgin bir etkisinin olmadığı gözlenmiştir. Çalışmada kullanılan düşey bileşen kayıtlar için periyoda bağlı elde edilen en yüksek ve en düşük mod değerleri grafiklenerek Güney Marmara bölgesine ait yüksek ve düşük gürültü modeli oluşturulmuştur. Elde edilen en yüksek ve en düşük gürültü modeli ve gürültü haritaları ileriki zamanlarda bölgeye kurulacak istasyonlar için referans model olarak kullanılabilir. Gürültü seviyesi düşük bölgeler daha düşük manyitüdlü depremlerin kayıtlarına olanak sağlayabilirken, yüksek gürültülü bölgelerde manyitüdü daha büyük depremler kayıt edilebilecektir. Bu çalışma ile bölge istasyonlarının çalışma performansı geçmişe yönelik izlenmiş ve toplanan veri kalitesi hakkında bilgi elde edilmiştir. Kullanılan yöntem ile, aktif istasyonların sürekli takibinin yanısıra ilgilenilen bölgelerin anlık gürültü haritalarının elde edilmesine olanak verir. Bu çalışmada elde edilen bilgiler yine gürültü verilerinin kullanılacağı dizilim işleme, artalan sismik gürültü tomografisi, yatay/düşey spektral oranlama, gibi çalışmalara öncül olarak kullanılabilir.

Recently, the usage of ambient seismic noise become prominent tool in global seismology and engineering seismology studies. Morever, it is a efficiently used tool in petroleum industry. Also, the number of studies is increased in the retrieval of surface waves Green’s functions that is used to obtain the velocity distributon in the Earth’s crust and mantle. In this study the characteristics of background seismic noise levels in the Güney Marmara Region are investigated which are useful to monitor and quantify the state of health of broadband stations. The Bursa basin, where placed in Sakarya Zone, is a pull-apart basin that showing North – South opening regime. The strike slip movement of The North Anatolian Fault (NAF) have effect on the place of this pull-apart regime. There are a number of grabens with the opening regime and together they come into the area of long and thin horsts. These horsts normally appear as long ridges and mountains separated from each other by normal long-standing and oval. The aim of this study is to characterize the seismic noise and the quality of the continuous noise data in the region which will contribute to the recent and future seismic studies. For this purpose, the vertical component continuous data recorded during 2011 by fifteen broadband velocity sensors with sampling rates 50 and 100 sps are used. The national seismic networks in the investigated sites are operated by KOERI (Kandilli Observatory and Earthquake Research Institute) and ERD (Earthquake Research Department of Disaster and Emergency Management Presidency of Turkey). A continuous seismic record can contain system transients and instrumental glitches (data gaps, clipping, spikes, mass recenters, calibration pulses). To successfully characterize the ambient noise, these kind of instrumental distortions need to be eliminated from raw data. In a recent method explained in Chapter 2, the calculation of the probability density functions (PDF) allow to discriminate these distortions asthey have low probabilities, when compared to stationary background noise. In Chapter 3, PDF graph of Power Spectral Density (PSD)s are investigated for each station. The power PSD curves and spectrograms of a local and teleseismic earthquake are examined. In Chapter 4, investigation of spatial and temporal noise properties in the Güney Marmara region are studied. At last, final comments and results are discussed (Chapter 5). In this study, an open source software named PQLX (Pascal-Iris Quick Look eXtended) is used to process the continuous data. First the continuous data are partitioned to 1-hr segments with a %50 overlap.. Each 1-hr time segment is divided into 13 segments with an overlap of %75, to increase the signal to noise ratio in the power spectral density estimates. In order to minimize long period contamination and to prevent the data from large distortions in spectral processing, mean and trend are removed from data. The Fourier Transform is taken after applying %10 cosine taper to data. The PSD of each time series segment is calculted and the results are, first converted to ground acceleration and then to dB unit to compare the PSDs with New High Noise Model (NHNM) and New Low Noise Model (NLNM) These processes are repeated for all available data in database. After calculation of PDF of PSDs, the graphical representation is obtained for each station. In PDF graphics, PSD curves are generally observed between Peterson’s NHNM and NLNM. There are some local anomalies detected in PSD curves for short periods at some stations. The main source of this noise might be due to marble cutting plants and marble querries around the stations. The impact of local earthquake (ML=5.8) and teleseismic earthquake (ML=9.0) on seismic ambient noise is investigated. The obtained spectrograms showed that, while the local earthquake increased the ambient noise around the period 8-10 s, the teleseismic earthquake increased the ambient noise at very long periods. In order to investigate the noise characteristics in the region, PSD mode values are obtaioned and used for the cultural noise and microseism periods. Noise distribution maps are prepared for annually, semiannualy, monthly and diurnal variations by using GMT (Generic Mapping Tools). Cultural noise (T=0.1-1s, noise source: traffic, machinery) and microseisms that are classified as Double Frequency Peak (DFP, T=4-8s, noise source: ocean waves) and Single Frequency Peak (SFP, T=10-20s, noise source: shallow coastal waters) are defined as investigation period bands. To observe the geographic variations on noise levels, PSD mode values are used to prepare noise maps for these 3 different period band for the year of 2011. Results showed that, in cultural noise band, higher noise levels are observed when compared to DFP, SFP bands. Higher noise levels are observed along coastline which are in correlation with the wind speed of the region. To observe the semiannual noise changes, the data from first six month (Janurary 2011 – June 2011) and last six month (July 2011 – December 2011) are processed. In first six month, the high noise is only dominant at the shoreline region for cultural noise band. In last six month, the high noise is homogeneously distributed almost all over the region except eastern part for the cultural noise band. At DFP band, there is no significant variation between those two time period. The higher noise levels around -130 dB starts from northwest part and decreases in the southwest direction for whole year. Similar situation is observed for SFP band too for both time periods. To observe the daily noise changes, the day (06:00 - 20:00) and the night (20:00 - 06:00) times data are mapped. In day time, higher noise levels are observed due to the level of cultural activity. The noise level difference between day and night is about 20 dB. For the DFP band, there is no significant difference between day and night times. The noise level are changing between –135 dB and –125 dB for this band. For both DFP and SFP band, the noise levels are relatively higher in Bursa Basin for both the day and night times. In addition to three specific period band that is defined above, another period band (1 – 3 s) is examined by considering the station locations. For the 1 – 3 s band, the noise levels are decreasing from coastline to inland. To track changes in noise levels monthly, twelve seperate noise maps are prepared for the year 2011. Observations showed that, the noise levels are higher in February in all periods in the region. The examinations have shown that these high noise levels are correlated with the wind speed of the region obtained from the wind speed map of Turkey. Another supporting data set (average wind speed, sea level and air pressure) is obtained from mareograph stations which are placed at coastal locations. Mareographic stations established within the scope of Turkish National Sea Level Monitoring System (TUSELS) project and operated by GCM (General Command of Mapping). These observation are in agreement with the high noise levels of the region in the cultural band periods. The effect of a local earthquake and its aftershocks is investaged on the maps. Observations showed that the local earthquake increases the level of seismic noise about 40 - 45 dB for all studied periods. The aftershock events increase the noise level for SFP period, whileit has no significant effect on DFP and cultural noise period bands. With this study the quality of the stations in the Southern Marmara region is investigated. The noise maps are prepared for cultural, DFP and SFP periods by considering different time intervals. The low and high noise mode values are obtained for the region and compared with Peterson curves, Adana and Ireland seismic noise models. In general the results are in agreement with the conisdered seismic noise models. The characterization of the quality of noise data and the seismic noise levels of the region may contribute to future station installations and seismic studies based on noise such as ambient noise tomography, array processing and H/V spectral ratio studies.