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Source, site and propagation characteristics of multiply recorded micro and moderate sized earthquakes

Mikro ve orta büyüklükteki depremlerin kaynak, ortam ve zemin karakteristikleri

  1. Tez No: 109548
  2. Yazar: NİHAL AKYOL
  3. Danışmanlar: DOÇ. DR. AYBİGE AKINCI
  4. Tez Türü: Doktora
  5. Konular: Jeofizik Mühendisliği, Geophysics Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 2001
  8. Dil: İngilizce
  9. Üniversite: Dokuz Eylül Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 146

Özet

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Özet (Çeviri)

II ABSTRACT Micro and moderate-sized earthquake recordings (Ml= 1.8-4.1) of local events, in the distance range of 5-60 km, have been used to quantify attenuation, source and site effects in the vicinity of the Bursa city of Marmara region, Turkey. The data set consists of 120 three-component recorded accelograms from 69 earthquakes, recorded at six stations. Each stations is deployed on different geologic units, including Permian Limestone, slope deposit, Neogene and Quaternary young sediments, in the framework of the Marmara Poly-Project. Firstly, a non-parametric inversion method was applied to acceleration records from tectonically active Bursa region and results obtained using a two-step inversion. At the first step, attenuation functions were determined by analyzing the distance dependence of the spectral amplitudes and values of Qs = 46.59f 067 was retrieved. The attenuation functions including anelasticity of heterogeneous medium and geometrical spreading effect were used to correct the S-wave spectral records. At the second step, the corrected amplitudes were inverted to separate source and site response for 21 different frequencies selected between 0.5 and ~25 Hz. The near surface attenuation parameter was also estimated and found that it is smaller for stations located on rock, İğdır, (SIGD, Ko~0.004) compared to stations located on Neogene sediment, Hamitler, (SHMT, Ko~0.02). Çukurca, (SCKR) is unique station on Quaternary sediments having about 4.0 amplification value at 1.8 Hz. Demirtaş, (SDEM) amplifying the spectral amplitudes about 3.0 times at 2.0 Hz, SHMK about 3.5 times between 2.5 and 3.5 Hz and SHMT about up to 3.5 times between 1.5 and 4.0 Hz. However, stations located on the Uludağ Mountain Massive (SKAY and SIGD), which correspond to a deep Permian Limestone geological unit, have the smallest amplification values between 0.6 and 1.4.in Site amplification functions were also obtained using Standard Spectral Ratio, (SSR), and Horizontal to Vertical (H/V) Ratio Techniques in order to evaluate the differences between the methods. As a result of this evaluation, absolute level of site amplification does not correlate with amplifications obtained from Standard Spectral Ratio method. Large differences between SSR and H/V ratio methods were observed, amplification values are almost 3 times higher than those found from H/V ratio method. Station located within the Bursa basin (SCKR) has the largest amplification value obtained from three methods. However, stations located on the Uludağ Mountain Massive (SIGD and SKAY) which correspond to a deep Limestone geological unit, have the smallest amplification values between 1.0 and 1.6. Consequently, the all three methods used in this study are able to identify the resonant frequency, which are similar to the one obtained from other with different site amplification. The obtained site amplification functions show that the near surface characteristics are different from one site to another and should be taken into account for spectral and hazard studies in the area. The near-surface attenuation was also estimated and found that larger values of k for sediment sites characterized by the presence of loose sediments which is consistent with the literature. The amplifications due to near surface material and basin resonance effect are the main factors controlling the damage in urban areas from large earthquakes. Recently two large earthquakes of August 17, 1999, GöTcük-Kocaeli (Mw=7.4) and November 12, 1999, Düzce earthquakes (Mw=7.2) have demonstrated the dominant effect of alluvial sites on the damage altering the observed seismic motions. Since the city of Bursa is located at the southern zone of the western extension of the North Anatolian Fault (NAF) zone and the major part of the city is located on a sedimentary basin, the importance of the seismic risk in the region appeared to be progressively more crucial. Therefore, a stochastic model simulation used to estimate ground motions at different geological sites around the city of Bursa has been modified to include the effect of soil column thickness and different geological sites. Ground motions areIV simulated using the stochastic point source model, which matches the observations from micro and moderate events in the region. To estimate more realistic site effect, seismic shear wave velocity as a function of depth has been estimated from borehole data, refraction studies and geological stratigraphic data in the region. Then Quarter- wavelength approximation is used to calculate the site amplification. Using stochastic modelling, site-specific hazard distribution was obtained for the region by developing 5% damped Spectral Acceleration (SA) maps at 10 periods ranging from 0.02 to 4 sec. SA maps show strong dependence on the site conditions, suggesting that soil depth and near surface characteristics should be included in seismic hazard calculation at regional and local levels.

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