13 Mart 1992 Erzincan depremi artsarsıntılarının kaynak zaman fonksiyonları
Başlık çevirisi mevcut değil.
- Tez No: 39211
- Danışmanlar: PROF.DR. NEZİHİ CANITEZ
- Tez Türü: Yüksek Lisans
- Konular: Jeofizik Mühendisliği, Geophysics Engineering
- Anahtar Kelimeler: Deprem, Erzincan, Earthquake, Erzincan
- Yıl: 1993
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Bir deprem sismogramındaki dalga biçimleri, deprem odağındaki hareketin mekanizması, ortamın katmanlı yapısı ve deprem kaynağında yırtılmanın zaman içindeki değişimi ile ilgili bir kaynak zaman fonksiyonu tarafından belirlenir. Dalga biçiminin ters çözümünü yapabilmek için ya deprem odak mekanizmasını yada ortamın katmanlı yapısını bilmek gerekir. Bu problem sismogram analizlerinde en büyük güçlüklerden birisidir. Bu ve bunun gibi güçlükler karşısında, kaynak zaman fonksiyonunun belirlenmesinde izlenenebilecek ve deprem odak mekanizması ile ortamın etkisini ortadan kaldırabilecek alternatif bir yol sismolojinin önemli problemlerinden birini oluşturmaktadır. Bu amaçla, küçük artsarsıntıların sismogramlarını Green fonksiyonu olarak kabul eden ve bunu büyük (karmaşık) artsarsıntı sismogramından ayırarak kaynak zaman fonksiyonunu belirleyen“Dekonvolüsyon Tekniği ”geliştirilmiştir. Bu tez çalışmasında, bahsedilen bu yöntem, 13 Mart 1992, Erzincan depremi artsarsıntıları üzerine uygulanmış ve bu artsarsıntıların kaynak zaman fonksiyonları elde edimiştir. Elde edilen kaynak zaman fonksiyonlarından, yükselme zamanları ile duraylılık zamanları belirlenmiş ve bu manyitüd aralığındaki artsarsıntılar için, çoğul kaynak bulunmadığı sonucuna varılmıştır. vıı
Özet (Çeviri)
SUMMARY THE SOURCE TIME FUNCTIONS OF THE 13 MARCH 1992, ERZİNCAN EARTHQUAKE AFTERSHOCKS We can classify the parameters from which the waveform on the seismograms are affected in three groups. These are: 1) Source. Source time function. Seismic moment. Source dimension. Focal mechanism 2) Path. Structure. Density and velocity distribution. Attenuation effects 3) Instrumentation. Frequency band. Transfer function of the recording system Study of the physical processes associated with earthquake source in detail by using seismic body waves is one of the recent interests of seismology. From the time domain analysis of the source time functions, someof the source properties, such as complexity and directivity and some of the source parameters such as pulse width, rise time and source dimension are determined. Furthermore, with a good azimutal distribution of stations, the direction and magnitude of rupture velocity can be estimated. When we use the observed seismogram to determine the characteristics of an earthquake source, one problem we face is how to remove the effects of the path, the recording site and instrument response. One way to resolve this problem is to determine Green's function between the source and the receiver to deconvolve it from the seismogram. One technique often used to study the mechanism of earthquakes is to compute the synthetic seismograms and compare them with the data. There are two major problems which must be dealt with in the calculation of synthetic seismograms ; 1 ) The description of the source 2 ) The calculation of the earth response. These are all difficult tasks. Often the details of focal mechanism are not known or poorly determined. For a realistic model of a large earthquake the finiteness of the source should be taken into account. One approximation which has proved useful is superposition of point sources, phase delayed to account for rupture propagation. The calculation of the earth response is typically done by assuming a model composed of homogeneous layers. With this simple model, standard method, such as The Reflectivity Method or Generalized Ray Method, can be applied. However, if the earth sturucture is not known, these computational tools cannot be used effectively. The same difficulties are valid for introducing of the source time functions. DCDeconvolution technique requires aftershock recordings from source on the fault piane of the main event. These aftershocks should be well distributed over the fauit plane and should be well approximated as point sources. The ground motion due to the aftershock must be recorded at the same station that recorded the main event. The fault plane of the main event is represented by distribution of point sources. The ground motion for the aftershock ( Uj ), which is closes to a given point source, is used as the point source response. The ground motion for the main event ( U ) is then approximated by weighted and phase delayed summation over Uj. u(t) = £ [«$.<?$] *(*-*} i=1 where «iW = S$ * Pt(t) * R$ * It(t) The index i in these equations runs over the n- point surce on the fault plane. In the equation given above, Q i : is a generalized scaling factor H : is the unit step function Tj : is a phase delay term, which includes both the delay due to the rupture propagation and the delay due to the travel time from source to the receiver. S j : is the source function P I : is the earth response R j : is the receiver function.The effects of Sj, Pj and Rj are all included in the aftershock ground motion. As stated above, the Uj are all recorded at the same station that recorded ground motion for the main event, therefore Rj is constant. There is an assumption for the first equation that aftershocks have the same focal mechanism as the main event. There is no guarantee that this will be true. The strike and dip of the fault plane may vary along its length for a long rupture, reflecting variations in tectonic stress or geology. However, by demanding good coverage of aftershocks over the fault plane, variations in focal mechanism can be approximated. It is also important to consider the magnitude of the aftershock used in the summation, The aftershock should be small enough to approximately satisfy our point source criterion, but large enough to excite the earth response sufficiently above the noise level. It is hard to quantify this size criterion since it depends on particular earth structure and earthquake. The delay times, Tj are the function of the rupture velocity and direction as well as the point of initiation of rupture and travel time to the receiver. With this approach, one need not know the actual focal mechanism. Also, the numerical and analytical difficulties of computing the earth response for an approximate earth model are avoided and at the same time, the effects of the true earth structure are included. On the other hand, the method may not work if the large earthquake source has significant directivity and locally recorded seismogram contains significant reflected energy. In this study, this method is used to model the aftershocks of 13 April 1992 Erzincan earthquake. Aftershocks are treated as approximate point dislocations. The effects of the true earth structure are included in the modelling process by using the aftersock responses. Assuming that a small earthquake has a simple source, its seismogram can be treated as an empirical Green's function and deconvolved from the seismogram of a large earthquake by spectral division. XIStudies are done with records from 9 telemetric station established after 13 Mart 1992, Erzincan eathquake in association with İ.T.Ü - I.P.G The studies done could be put in sequence like given below: - After reading of P wave seismograms, they are taperred with Hamming Window and the effect of the change of the window length on source time function is studied. - P wave seismogram of small aftershock recorded just after the complex event is taken as Ampirical Green's function. - Then P wave seismogram of complex event which is windowed is deconvolved with ampirical Green function. - After that, it is filtered with low pass Butterworth filter to clear the effects of high frequency effects formed after deconvolution. - Finally, the instantaneous amplitude values are calculated for seeing the chancing of the energy of the source time function with time. After these processes, source time functions are found (in the shape of triangle) all the stations. The general characters of these founctions are nearly the same at the magnitude interval considered and the rise times of the source time functions are found between 0.03 - 0.08 sec. By looking this result, we can see that there are not multiple sources for these events. Since the magnitude range is quite narrow (2.3 - 2.9) we have not tried to look for the relation between the magnitude and the source time function. The results obtained are summarized in Table 1. By using these source time function, studies could be done to predict the seismogram of the comlex earthquake. The Earth crust structure in not known in detail in our country. Because of this reason, the computation of the Green's functions and the determination of the exact wave shape can not be possible. From this point of view, it is possible to produce velocity and acceleration seismograms with using procedure had been mentioned above. It is indisputabla the benefits of this process from the view of earthquake engineering. Furthermore, the application xiiand the comparison of this method to different eathquake regions will give important information. Especially, comparison the results of this study with the source time function of the earthquake which are over West Anatolian Graben systems will be interesting. XIIITABLE 1 -The locations, magnitudes, rise times and stability times of the aftershocks that we interprate. XIV
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