Doctoral Dissertation
Earthquake Geophysics Science
September 1392
Abstract
In this treatise, several combined simulation methods are used in order to produce artificial maps of strong earth movement. On this basis, two methods that were previously used in some other parts of the world were used as advanced methods for simulating two earthquakes that occurred in the country, as well as a simulation method that improves the quality of the synthetic data output is presented as an innovation. It was used to simulate the earthquake of October 17, 2009 in Tehran, which in recent years was the first earthquake close to the city of Tehran from which strong movement data was recorded. This earthquake has been modeled in two dimensions along the east-west and southeast-northwest profiles and its results have been compared with the recorded real data. Despite the lack of basic data to carry out the exact simulation process, it has been shown that the use and application of advanced methods that include source physics, path and site effects in their process can achieve acceptable estimations. Using this method, the estimated amount of resonance in Tehran due to local sediments has been calculated at different frequencies. The waveform of acceleration maps, speed maps and observed and synthetic response spectra have been compared and described.
Next, a new three-dimensional method that was previously used up to the low frequency range (1 Hz) was used to simulate the Bam earthquake. This method has been used for the first time in Iran and has been extended to the high frequency range (6 Hz) for the first time, the results of which clearly show the improvement of the quality of synthetic maps in 3D mode. Here, using the "mode sum method and ray tracing" Along with the WKBJ estimation, a seismic wave is transmitted from the source to the receiver, and along the way, the desired mapping is updated and completed based on the modes of the seismic waves that are produced based on the three-dimensional models. Several one- and two-dimensional methods were used to simulate the Bam earthquake, and finally the results were compared with the aforementioned three-dimensional method, and the process of quality improvement, in the time and frequency domain, was clearly shown. Finally, a new method is proposed in this thesis by combining the three methods "mode sum, finite difference, and Green's function". We have used it to overcome the limitations of the two primary methods presented. Based on this, an aftershock from a moderate-magnitude Bam earthquake is estimated by the combined method of "sum of mode-finite difference". Simulation and the generated artificial mapping was used as Green's function to produce the main event of the Bam earthquake. The proposed method has the great advantage of upgrading the point spring used in the previous two methods to a wide spring in order to significantly improve the quality of the simulation results, especially in the frequency domain. This method has been evaluated in three stations, Abaraq, Mohammad Abad and Jiroft Dam in order to compare observational and synthetic data.
Key words
Simulation of strong earth movement, numerical simulation, analytical simulation, hybrid simulation, mode sum, finite difference, Green's function, WKBJ estimation, response spectrum, Tehran earthquake, Bam earthquake.
Chapter one:
Introduction and Generalities
-1- Introduction
The phenomenon of seismicity in Iran and its importance is not a matter that needs a lot of explanation. The existence of active seismic springs, historical and systematic destructive earthquakes, young tectonics and shortening rate due to regional and global tectonics are known to a large extent and make Iran a significant region from the point of view of seismicity and seismic hazards. During consecutive years and especially in recent years, various studies have been carried out in all fields of seismology on Iran. Over time, these studies have become more accurate with the increase of device data and help to better understand the quantitative characteristics of this phenomenon in Iran.
Extensive studies to know the underlying structure of Iran, to great depths, are being implemented today, or some similar studies have reached results that can be referred to and used.Also, many engineering studies, which are a more practical aspect of seismological studies, have grown well, and of course, there is still a lot of work to be done in this field. The occurrence of earthquakes with magnitudes of up to 7 and the resulting destruction and deaths always gives a warning that special measures should be taken for earthquake engineering aspects in Iran. The main work in this field is summarized in the two areas of knowledge of active seismic springs, their behavior and maximum possible risks on the one hand and compliance with the principles of structural engineering on the other hand. The large-scale map of the global seismic risk shows that we are facing a high seismic risk in almost all regions of Iran, which is a clear expression of this issue in Figure (1-1). Based on this map, it is quite clear that Iran is located in a very dangerous seismic belt and is placed in the ranks of Southeast Asian countries, part of Africa and America, which themselves have a high probability of seismic risk. Perhaps the review of the number of deaths in the last 50 years in Iran due to earthquakes is simply indicative of this issue. Earthquakes in Tabas, Boyin Zahra, Qir, Silakhor, Rudbar Manjil, Chengoureh-Auj, Bam, Zarand, Ardabil and . As some of the most famous and destructive earthquakes are always in front of our eyes, so that based on the bitter experiences gained from them both in terms of rescue and engineering, and in terms of engineering and expertise, we can face this important phenomenon logically.
In addition to the global earthquake risk distribution map, domestic maps, on a national scale, also confirm this issue and they also show a high seismic risk. Iran's seismic risk maps (Figure 1-2) and relative earthquake risk zoning in Iran (Figure 1-3) both illustrate this well and show that almost all parts of the country have the potential for significant earthquakes and therefore high destruction. 2- The purpose of this thesis This dissertation is based on the perspective of engineering seismology and has been completed with the aim of investigating some combined simulation methods. Since the recorded acceleration data of an earthquake is one of the most important information needed for engineering seismological analyzes and interpretations, therefore, having a data bank of this important quantity is one of the things that is always needed. On this basis, the accelerometer network (Figure 4-1) has grown and developed in Iran for many years until today we are recording acceleration data with about 1,300 accelerometer stations in the country. As we know, acceleration data and accelerography equipment do not occur and are not recorded every day, like seismic data (accelerography) due to the inherent characteristics of the earth's acceleration. On the other hand, since earthquake acceleration is strongly dependent on the local characteristics of an area, we need to have local data of the same area for each area to provide the real function of accelerography and engineering seismology. Also, due to the fact that large earthquakes do not occur every day and in short periods of time, therefore, in many areas that have not experienced a large earthquake after the installation and commissioning of the accelerometer network, we are deprived of important accelerometer data, and the sum of these factors and some other issues, such as determining the parameters of the seismic spring and the spring that causes earthquakes and using it for future purposes, lead seismologists to earthquake modeling and simulation methods.
In addition to the mentioned issues, it should be kept in mind that the existing estimates of seismic vulnerability, both on a global and a national scale, face shortcomings that must be carefully re-evaluated during practical investigations for important structures and studies requiring engineering seismological analyses. For example, Panza et al. (2012) [4] have shown well (Table 2-1) how in some of the important earthquakes of the last two decades, the global map of seismic hazard (Figure 1-1) has an error and this error sometimes reaches four levels of earthquake intensity, which must be taken into account and the errors removed. The main solution to this problem is the realistic study of the powerful movement of the earth and providing new methods of seismic risk analysis.