Use of Land Gravity Data in Small Areas to Support Structural Geology, a Case Study in Eskişehir Basin, Turkey

Various researchers have contributed to the literature on the locations and lengths of existing faults in the Eskişehir Basin, Turkey. However, the majority of the literature on the subject bases its results on fault indications observed on the surface, for example, surface ruptures. In addition, studies using geophysical methods in order to reveal buried faults have also fallen short regarding depth compared to gravity. In order to have a better understanding, the gravity method was applied with a total of 448 gravity measurements on five parallel lines in the north–south direction of the study area, which also includes the urban area of the Eskişehir Basin. Considering the neotectonics of the Eskişehir basin, the measurement lines were chosen to perpendicularly cut the east–west extending faults of the Eskişehir fault zone. For the first time in the literature, a detailed Bouguer gravity anomaly map has been obtained for the Eskişehir Basin using land gravity measurements. The edge detection Horizontal Gradient Magnitude (HGM) and Euler Deconvolution (ED) methods were applied to obtained Bouguer anomaly data. Both of these use spatial analysis of Bouguer gravity anomalies. An HGM map shows the presence of maximum amplitude areas in the south and north of the study, and these areas were found to be compatible with the known faults in the literature. ED solutions also support HGM maximums. The relationship between the lineaments obtained from the edge detections and the seismicity of the region were examined. It can be seen that the results obtained from both the HGM and ED edge detection methods are highly compatible with each other, and highly related to the structural geology of the region. Although great agreement with the faults in the literature was determined by both methods, only the ED method showed a number of newly found faults in the area. In addition, the locations of the known faults in the region were supported by the geo-physical gravity method for the first time

Comparison of the bedrock depth from array measurements of Rayleigh waves associated with microtremor and seismic profile obtained the Seismic Reflection Data, Eskisehir Basin, Turkey

Ground motion estimation for future earthquakes is one of the most challenging problems in seismology and earthquake engineering. The bedrock depth has a considerable seismic risk for the urban area of Eski¸sehir. In this study, multiple station microtremor measurement methods which are more practical, non-distructive, fast and economical compared to seismic reflection method were implemented. These method using microtremor recordings have become a very useful data for microzonation studies because of their simple acquisition and analysis. Extensive ambient noise measurements were performed in the basin of Eskisehir from June 2010 to spring 2012. We use data recorded by a broadband seismometer and digitizer CMG-6TD, Guralp seismometer. Some of the measurement locations, the CMG-6TD sensor was located into 30 cm-deep holes in the ground to avoid strongly wind-generated, long-period noise. Dominant frequency (f), bed-rock depth (h) and shear-wave velocity (Vs) were determined from Spatial Autocorrelation (SPAC) methods. With the SPAC Method, it is possible to constrain the velocity structure underlying the site using microtremor array measurements. The results obtained were compared to the 96-channel seismic reflection data with explosive energy source. Several seismic reflection surveys with P-Gun seismic source have been performed on the same place with array measurements. We used two types of seismic sources: 36 cartridge Gun. Shot interval was 10 meters, group interval (one geophone per group, 48 geophones in total) was 10 meters, near offset was 10 meters, far offset was 480 meters, CDP interval was 5 meters. We adapted the “Off-End Spread” technique while using the Gun. Reflection images within the sedimentary section correlate well with the velocity structure obtained from SPAC

Assessment of the Fundamental Resonance Frequency of the Sedimentary Cover in the Eskisehir Basin (Turkey) Using Noise Measuremeets

Geological observations indicate that the Eskisehir graben is related with Eskisehir Fault Zone (EFZ), one of the major active structures within the Anatolian plate. The fault zone has a considerable seismic risk for the urban area of Eski¸sehir. Local site conditions substantially affect the characteristics of seismic waves and generally have a direct effect on the potential of the earthquake damage. Site effects are very complex factors. The importance of site response is well known, and few would question the assertion that the motion on soil is usually greater than on rock, when all other things were being held equal. In general, site response estimation can be obtained from instrumental recordings. The purpose of this study was to estimate if there would be a relationship between the structural geology in the Eskisehir Basin and the fundamental resonance frequency. Extensive ambient noise measurements were performed in the basin of Eskisehir from June 2010 to spring 2012. In this work, we conduct microtremor surveys to investigate the 3-D basin structure of Eskisehir Basin and determine the fundamental resonance frequency of the sedimentary cover in the Eskisehir Valley area. The measurements of single-station microtremor were carried out at 318 sites in the Eski¸sehir Basin. We use data recorded by a broadband seismometer and digitizer CMG-6TD, Guralp seismometer. Some of the measurement locations, the CMG-6TD sensor was located into 30 cm-deep holes in the ground to avoid strongly wind-generated, long-period noise. Finally, a map showing the fundamental resonance frequency distribution in the studied area was drawn using the results obtained from applying the HVSR technique. A fundamental resonance frequency map of the Eskisehir Basin was produced from main peak in the horizontal-to-vertical component (H/V) spectral rati

Farklı Jeofizik Çalışmaların Birlikte Kullanımı ile Büyükçekmece Körfezinin Deprem-Zemin Etkileşiminin Kestirilmesi

Depremler nedeniyle meydana gelen hasarın birçok farklı nedeni vardır. Ülkemiz özelinde meydana gelen depremlerde bu nedenlerin en önemlileri arasında yerel zemin etkisi, sıvılaşma, sıvılaşmaya bağlı oturma ve heyelanlar ilksel nedenler olarak öncelikle ortaya çıkmaktadır. Farklı zemin problemlerinin çözümünde ise farklı mühendislik çalışmalarının yapılması gerekmektedir. Farklı Jeofizik çalışmalar ile elde edilen farklı parametreler ise bu problemlerin çözümünde kolaylıklar sağlamaktadır. 17 Ağustos 1999 İzmit depremi sırasında İstanbul-Avcılar ve Büyükçekmece ilçelerinde can kayıpları ve yapısal hasarlar meydana gelmiştir. Büyükçekmece ilçesi farklı jeolojik özellik ve zemin problemleri ile birçok mühendislik problemi içinde barından bir yerleşim alanı konumundadır. Büyükçekmece Körfezinde doğusunda bulunan yapılar daha çok heyelan sorunu ile karşı karşıya iken, ilçenin merkezinde sıvılaşma ve buna bağlı oturmalar yapılan hesaplamalarda dikkati çekmektedir. Ayrıca körfezin kuzeyinde bulunan Büyükçekmece Gölünün her iki yakasında farklı jeolojik birimler (İstanbul Zonu-Istranca Masifi) yüzeylenmektedir. Bu bilgide bize bölgede bu birimleri kesebilecek fay zonlarının mevcut olabileceği ve körfezin her iki yakasında farklı sediman kalınlıklarının olabileceğini gösterebilmektedir. Sediman kalınlık bilgisi ile birlikte bölgedeki anakaya derinliğine bağlı basen yapılarının olabileceğini düşündürmektedir. Bu bilgi ise “site effect” olarak bilinen yerel zemin etkisi ve bu etkiye bağlı zemin büyütmesi/rezonans durumunun oluşabileceğini düşündürmektedir. Tüm bu problemlerin varlığının belirlenmesi amacıyla çalışma alanında bütünleşik jeofizik çalışmalar (mikrotremor, sismik kırılma, yüzey dalgaları analizi (MASW-MAM), sismik yansıma, düşey elektrik sondajı, yer radarı ölçümler) yapılmıştır. Elde edilen jeofiziksel parametreler/bilgiler kullanılarak çalışma alanında yukarı sözü edilen problemlerin varlığı araştırılmış ve çözülmüştü

Exploration of S-wave velocity profiles at strong motion stations in Eskisehir, Turkey, using microtremor phase velocity and S-wave amplification

We have explored 1D S-wave velocity profiles of shallow and deep soil layers over a basement at strong motion stations in Eskisehir Province, Turkey. Microtremor array explorations were conducted at eight strong motion stations in the area to know shallow 1D S-wave velocity models. Rayleigh wave phase velocity at a frequency range from 3 to 30 Hz was estimated with the spatial autocorrelation analysis of array records of vertical microtremors at each station. Individual phase velocity was inverted to a shallow S-wave velocity profile. Low-velocity layers were identified at the stations in the basin. Site amplification factors from S-wave parts of earthquake records that had been estimated at the strong motion stations by Yamanaka et al. (2017) were inverted to the S-wave velocities and Q-values of the sedimentary layers. The depths to the basement with an S-wave velocity of 2.2 km/s are about 1 km in the central part of the basin, while the basement becomes shallow as 0.3 km in the marginal part of the basin. We finally discussed the effects of the shallow and deep sedimentary layers on the 1D S-wave amplification characteristics using the revealed profiles. It is found that the shallow soil layers have no significant effects in the amplification at a frequency range lower than 3 Hz in the area

A missing-link in the tectonic configuration of the AlmacA +/- k Block along the North Anatolian Fault Zone (NW Turkey): Active faulting in the Bolu plain based on seismic reflection studies

The North Anatolian Fault Zone (NAFZ) starts to branch off in the western Bolu plain. The branches of the NAFZ in this location create the AlmacA +/- k block which is surrounded by the latest surface ruptures of significant earthquakes that occurred between 1944 and 1999, but its northeastern part remains unruptured. The most recently formed rupture, that was a result of the 1999 November 12 Duzce earthquake, ended to the northwest of the Bakacak Fault. The connection between the Bakacak Fault and the main branch of the NAFZ via the Bolu plain has until now remained unknown. This paper establishes that the route of the missing link runs through the Dagkent, Kasaplar and Burnuk faults, a finding achieved with the help of seismic reflection studies. The paper also argues that the cross cutting nature of these newly determined faults and a stress analysis based on focal mechanism solutions of recent earthquakes demonstrate the termination of the suggested pull-apart nature of the Bolu plain