Mitrotemor ölçümleri ile depremlerin yerel geoteknik etkilerinin belirlenmesi
Determination of local geotechnical effects of earthquakes by microtremor measurements
- Tez No: 66456
- Danışmanlar: PROF. DR. ATİLLA ANSAL
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Geoteknik Bilim Dalı
- Sayfa Sayısı: 196
Özet
ÖZET Yapıların depreme dayanıklı olarak projelendirilmesinde, yapının oturtulacağı zemin tabakalarının depremler sırasında ortaya koyacakları dinamik davranış özelliklerinin belirlenmesi, depremlerin neden olacağı yapısal hasarın azaltılması bakımından büyük önem taşımaktadır. Depremler nedeniyle oluşacak yapısal hasan kontrol eden iki önemli zemin parametresi; herhangi bir kaynaktan yayılan dalgaların büyüklüğünde yerel zemin koşullarına bağlı olarak meydana gelen artış olarak tanımlanabilen zemin büyütmesi ve mevcut zemin yapısının özellikleriyle birlikte deprem kaynak ve dalga yayılma yolu özelliklerine bağlı olarak ortaya çıkan hakim peryot olmaktadır. Bu nedenle zemin tabakalarının dinamik etkiler altında sergileyeceği davranışı belirlemede zemin büyütmesi ve hakim peryot özelliklerinin araştırılması oldukça önemlidir. Yerel zemin tabakalarının dinamik davranış özelliklerinin belirlenmesine yönelik olarak bölgede her zaman mevcut olan ve mikrotremor olarak adlandırılan çok küçük genlikli titreşimlerin gözlenerek yorumlanması, arazi ve laboratuvar deneyleri yapılarak veya gerçek yer hareketi kayıtlarının analiz edilmesi ile yerel zemin koşullarının belirlenmesinin mümkün olmadığı yada yüksek maliyetler oluşturduğu durumlarda alternatif bir yöntem olmaktadır. Bu çalışmada, yapılması planlanan bir doğalgaz çevrim santrali sahasında, arazi deneylerinin uygulandığı sondajlar yakınında yapılan mikrotremor ölçümleri, eş zamanlı olarak alınan kayıtların spektral oranlarının elde edilmesi (Referans Yöntemi) ve bir noktada alınan kayıtların yatay hareket bileşenlerinin düşey hareket bileşenlerine göre spektral oranlarının belirlenmesi (Nakamura Yöntemi) mikrotremor analiz yöntemlerine göre değerlendirilmiş ve her iki yöntemden elde edilen sonuçlar arasında korelasyon bağıntıları geliştirilmiştir. Ayrıca mikrotremor ölçümlerinden belirlenen farklı sonuçlar üzerine mevcut zemin yapısının etkisi araştırılmıştır. Mikrotremor ölçümleri ile yerel zemin tabakalarının dinamik davranış özelliklerinin araştırıldığı bu çalışmanın ikinci bölümünde, mikrotremor kayıtlarının çeşitli yöntemler ile yapılan analizleri sonucu belirlenen hakim peryot ve büyütme oranlan, arazi deneyleri ve teorik zemin modelinden bulunan sonuçlarla karşılaştırmıştır. Mikrotremorların kararlı durum gösterdikleri ölçüm istasyonlarında alınan kayıtların analizinden belirlenen büyütme faktörleri ve hakim peryotlar ile arazi deneylerinden bulunan sonuçlar arasında korelasyon bağıntıları geliştirilmiştir. Mikrotremor ve kuvvetli yer hareketlerinin kaynak ve titreşim genliklerinde görülen farklılıklar, mikrotremorlardan faydalanarak depremler sırasında zeminlerin sergileyeceği dinamik davranışın modellenmesinde çeşitli tartışmalara neden olmaktadır. Buna karşın gerek kayıtların alınması sırasında ihtiyaç duyulan sürenin kısalığı, gerekse analizlerinin basit ve ekonomik oluşu, yerel zemin tabakalarının dinamik davranış özelliklerinin belirlenmesinde mikrotremorların kullanımını gün geçtikçe daha cazip bir yöntem haline getirmektedir. xvı
Özet (Çeviri)
SUMMARY The earthquake is one of the most important natural disasters, due to the fact that it can not be prevented and it is not possible to estimate the occurrence time and place. As long as the earthquakes can not be prevented, people have to work in order to mitigate the earthquake damages. Especially, in recent years, site response analysis during an earthquake have become important. The factors affecting the degree of earthquake damages are the quality of structures, earthquake characteristics and effect of local soil conditions respectively. The influence of earthquake characteristics composed of source and path effects are on macro level and were not sufficient to explain the local variations in structural damage observed in metropolitan areas in past earthquakes. However, the surveys on earthquake damages show that, considering the seismic intensity equal, the effects on structures vary considerably from structure to structure. Some are free from damage completely while others suffer heavy damage. This phenomenon occurs due to the seismic response characteristics of local soil layers and the characteristics of the structures. Local soil conditions can influence significantly the characteristics of earthquake ground motion, and hence the degree and extent of damage caused by an earthquake. Besides, the local geotechnical conditions that can be very different due to changes in thickness and properties of soil layers, depth of bedrock and water table may be one of the main reasons for the variations in the damage distribution. The effects of local soil conditions are of particular significance in seismic microzonation, in seismic design of important facilities as well as in seismic safety assessment of existing structures. Hence, investigation of predominant period and soil amplification characteristics of local soil layers are extremely important. The best procedure for determining the site response of a particular location is to observe the ground motion during an actual event. This can be done, using either strong or weak motion, by direct comparison of a sediment site to a reference site located on component ground. Unfortunately, in order to employ this method we must wait for earthquakes to occur. Therefore, to achieve site response surveys in a reasonable period of time, this approach is not practical. One such alternate approach involves determining the physical properties of the local xvusetting by conducting laboratory and in-situ tests (borehole and/or seismic profile studies). Measured parameters can then be used in theoretical models to predict the site response. Determining the dynamic characteristics of local soil layers by means of in-situ and laboratory tests is a widely used method. But, as the cost of dynamic in- situ tests such as down-hole, cross-hole, seismic refraction etc. and laboratory tests such as dynamic triaxial test, simple shear test etc. are too high and hence this method may not be considered as a convenient way for determining the dynamic characteristics of wide areas. Another alternate approach, involves the use of microtremors, or ambient seismic noise, to estimate the earthquake site response. Microtremors are low amplitude oscillations of the ground surface produced by natural sources such as wind, ocean waves, geothermal reactions and small magnitude earth tremors. Microtremors can also be produced by artificial sources such as cultural noise. The amplitude of microtremors is usually 1/1000 ~ 1/100 mm. Wave modes contributing to their mechanism have been inadequately defined. While some researchers accept them as surface waves, the others believe that they are body waves. In general, it appears that surface sources such as wind, ocean waves and cultural noise act as surface wave generators, whereas extremely small magnitude, naturally occurring earth tremors act as body wave microtremors. These body wave sources occur deep in the earth and contain vertically traveling components. Recently the microtremors attract geotechnical-earthquake engineers, because the dynamic characteristics of the ground or structures during earthquake motions can be estimated by analyzing the microtremors. In spite of the inadequately known wave characteristics, source features and propagation path, many researchers have been able to characterize local site response successfully using microtremor data. On the contrary, the differences in source, and vibration amplitude between a microtremor and earthquake ground motion necessitate further study on the application of microtremors to earthquake engineering. In spite of the problems related to their interpretation, microtremor measurements provide a very interesting approach to site effect evaluation by virtue of the low cost and rapidity of field operations and the simplicity of analysis. There are three different techniques commonly used to estimate site effects from microtremor measurements. These techniques are as follows; direct interpretation of Fourier amplitudes spectra or power spectral density, computation of spectral ratios relative to a firm site reference station (Reference Station Method), and, finally, computation of spectral ratios between horizontal components of motion relative to the vertical component obtained at same site (Nakamura's technique). Each technique implies an interpretation of the nature of microtremors and has proven to be useful in same cases. A common feature of these techniques is that all three assume that site effects are due to a single soft soil layer overlying an elastic half-space. Thus, site effect is defined by a resonant period and an amplification level given by the impedance ratio between the layer and the half-space. All 2D or 3D site effects are neglected (Lermo et al., 1994b). The spectral ratio technique has become a standard way to estimate site effects of soft soil relative to a firm site reference station. It requires a pair of instruments, one located at the site under investigation (generally on alluvium) and the other on a“reference site”, preferably a nearby rock site; both instruments must simultaneously xvmrecord the ground motion of a number of events. In many cases, it is difficult to find sites on bedrock that are close enough to alluvium ones. Apart from the standard spectral ratio technique, recently, another nonreference site-dependent technique was introduce by Nakamura (1989) for evaluation of site effects. The technique relies on the interpretation of microtremors as Rayleigh waves propagating in a single layer over a half-space. In the Fourier domain, there are then four amplitude spectra involved: horizontal and vertical components of motion at the surface and at the base of the soft layer. A further assumption is that microtremor motion is due to very local sources, such as traffic in the neighborhood of the seismometer, thus neglecting any contribution from deep sources. Besides, according to Nakamura' s technique, the vertical component of motion is not amplified by the soft soil layer. A procedure used more recently, and the one implemented in our study, is to take a point-by-point ratio of the microtremor spectra obtained at the site in question with respect to the microtremor spectra obtained simultaneously at a reference site (Reference Station Method). Computing spectral ratios has been shown to yield a good approximation of the response function of a given site when the two locations have the same source and path effects. The reliability of this method using microtremor signals will depend on the degree to which the assumption of similar source and path effects is satisfied. This condition will vary with time, location, atmospheric conditions, and frequency range of interest. As mentioned above, this assumption can be valid for longer-period microtremors since they typically originate from farther distances. For urban environments, where there is a high degree of local cultural noise, the validity of the assumption is dubious. However, if the two sites have at least similar sources, then some outstanding features, such as a fundamental resonance mode, may be identifiable. Caution must be exercised, however, not to assume that the resonances excited by local surficial noise sources will be the same as resonances excited by distant seismic sources. The assumption in computing a spectral ratio in terms of reference station is that source and path effects are similar for both sites. For urban environments with a high degree of local cultural disturbances, the assumption of similar source and path effects is not strictly valid. In that case, Nakamura Method has become more convenient method to assess microtremor data because of the fact that Nakamura Method effectively allows for compensation for source effects. The most difficult problem to assess spectral ratio of microtremor to explain the variability of the microtremor amplitude, especially when changes in meteorological conditions have taken place between 2 sets of readings. It was found that spectral ratio amplitude between two sites can vary depending on these conditions as well as with other intermittent localized sources. The effect of the intermittent localized sources on the spectral ratios was not as easy to eliminate. We investigated the feasibility of using microtremor recording to infer site response in the Bursa Natural Gas Combined Conversion Power Plant site. For this purpose, we measured microtremors at the vicinity of 14 boreholes. We used an instrument named TEAC for microtremor measuring. It had been provided by I.T.U Department of the Earthquake Engineering. It consist of two sensor units, an amplification unit, cables and a main body that contains a microcomputer. We implemented microtremor measurements the vicinity of 14 boreholes at Bursa Natural Gas Combined xixConversion Power Plant site. For each site microtremors were recorded at 100 samples per second while a simultaneously recording was made at the reference site (S21). At 4 observation points (S9, S 17, S21 and S26) in which in-situ seismic tests were made, microtremor measurements were repeated twice, at a point to secure highest quality of recording. Recordings were made in terms of velocity along 2.5~3 min. for about 16000 data at each point in the three directions, i.e. East- West, North- South and Up-Down directions. Stationary measurements were taken at S21 borehole while a simultaneously recording were made at the other mobile measurements. At all observation points, the measurements were taken in a close distance (max. 30 m) at two different points (named -a- and -b-) in which are located around the vicinity of boreholes. All microtremor measurements were taken from 10.00 a. m to 18.00 p. m at the same daytime. The purpose of stationary measurement was to check stability of microtremor both in amplitude and period distribution. The other purpose was to use as a reference site in computing spectral ratio, that is, the mobile measurements are normalized with respect to this site. The timely variation of amplitude due to changeable localized sources makes the use of absolute amplitude of microtremors difficult, in any case this variation is much smaller than the variation between different sites. Normalizing of the mobile measurements over reference site record simultaneously will reduce the effect of variation, if the microtremors at both places are expected to be of the same source. We used for analysis of microtremor records two different methods. Initially, Fourier amplitude spectra of the recorded data were obtained by a Fast Fourier Transform operation. An average spectrum in each directions at each point was determined by averaging all spectra in each direction. Later on, spectral ratios of average Fourier amplitude spectra were obtained in terms of each direction. After that, these spectral ratios evaluated in terms of Reference Station Method and Nakamura Method. In this study, we compare predominant frequency and site amplifications of microtremors, derived from both station-pair (Reference Station Method) horizontal spectral ratio method and Nakamura' s (horizontal/vertical spectral ratio) single station method. However, in order to compare the results that derived from Reference Station and Nakamura's Method we normalized the results of Nakamura Method in terms of reference station (S21). The soil stratification and soil properties were determined based on classification tests carried out in Istanbul Technical University and the dynamic properties of soil layers were estimated based on the in-situ SPT tests. Besides, in-situ seismic tests, namely, Cross-Hole and Suspension PS logging measurements were conducted to determine the shear wave velocity at 4 location (S26, S21, S 17, S9). At the other location in which the SPT in-situ tests were made the shear wave velocity profile was determined utilizing the relationship proposed by Iyisan (1994) and the amplification factor of soil layers was determined utilizing the relationship proposed by Midorikawa (1987). The objective of this study is to investigate the usefulness of microtremors to estimate dynamic soil properties like amplification factor and predominant period at local place. For this purpose, microtremor measurements were made in Bursa Natural Gas Combined Conversion Power Plant site at 14 points in which in-situ tests were made. xxBased on the results of microtremor and other previous studies (theoretical model and soil investigation studies), we conclude that easily obtained microtremor measurements can contribute valuable microzonation and site response information to a study of regional earthquake hazard. As a final conclusions, it is possible to say that, according to these results and in spite of some fluctuations in amplitude level (the evaluated amplifications factors varying, approximately, from 1.3 to 40 according to both microtremor analysis methods), the use of microtremor measurements seems to be very useful to estimate the site response, in terms of resonant frequency, contributing as a fast, simple and economic method especially for microzonation purpose. Besides, it might also be an useful tool for geotechnical survey, to get information on the thickness or softness of surficial deposits. xxi
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