Earthquake risk zoning and preparation of spectral acceleration maps for bedrock in the range of longitude 54-44 and latitude 40-26-5

Master thesis

Geophysics – Seismology

1389

Introduction

Spectral acceleration maps by probabilistic method for Iran in the range of  44-54 longitude and 40-26-5 latitude based on the analysis of earthquake risk and using historical data, devices and seismic sources. For this purpose, the active springs in the study area were identified and modeled as linear and regional springs, then the study area was divided into 17 tectonic earthquake provinces and seismicity parameters were calculated for each of the springs. Using reduction relations and combining relations using the logical tree method, spectral acceleration maps for the return periods of 75, 475, 975 and 2475 years were presented for the first time. These maps are intended to show the seismic risk of the studied area in the form of different risk levels provided by endo  They show the maximum acceleration of the earth for the return period of 75, 475, 975 and 2475 for different periods. The study area was gridded at intervals of 0.1 degrees in both latitude and longitude directions and the maximum acceleration of the earth was calculated in each of the nodal points and then spectral acceleration maps were obtained for the study area. tectonics, fault activity and seismic source models in Iran. These maps have been prepared to indicate the earthquake hazard of Iran in the form of iso-acceleration contour lines, and seismic hazard zoning, by using current probabilistic procedures. They display the probabilistic estimates of Peak Ground Acceleration (PGA) for the return of 75, 475, 975, 2475 years. The maps have been divided into intervals of 0.1 degrees in both latitude and longitude directions to calculate the peak ground acceleration values ??at each grid point and draw the seismic hazard curves. The results presented in this study will provide the basis for the preparation of seismic risk maps, the estimation of earthquake insurance premiums and the preliminary site evaluation of critical facilities

 Chapter One

General

Introduction

Despite the extensive efforts that have been made to determine earthquake risks, it is still possible to accurately predict the time, location and magnitude of future earthquakes. not written The Alpine-Himalayan seismic belt (one of the seismic regions of the world) passes through Iran. Also, the movement of the Arabian plate and the stability of the Eurasian plate have put Iran at a very high risk of seismicity. The occurrence of destructive earthquakes in the past has caused a lot of human and financial losses in this sector, and there is a possibility of these earthquakes occurring in the future as well. The basis and analysis of seismic risk is the analysis of seismicity or the occurrence of earthquakes in the field of time and space. A sound knowledge of seismicity is an important tool for understanding active tectonics. Determining seismicity parameters in earthquake risk analysis studies is of particular importance. In fact, these parameters represent the seismicity of a zone or a fault and describe the effects of an earthquake, which are expressed in numerical quantities.The most important parameters of strong earth movement, which are considered in the evaluation of earthquake risk and the behavior of various structures against them and have engineering applications, include the maximum values ??of acceleration, velocity, displacement and also the response spectrum of acceleration or velocity in different damping. In studies related to the estimation of seismicity parameters and earthquake risk analysis, it is usually assumed that the occurrence of earthquakes follows a Poisson process that is independent of each other in time and space. In order to obtain Poisson distribution, dependent events including foreshocks and aftershocks should be removed from the catalog. Various methods are used to estimate seismicity parameters, such as the Gutenberg-Richter method and Kiko method. The occurrence of earthquakes in tectonic springs is associated with the production of seismic waves. These are seismic waves that cause strong earthquakes in different parts of the area. Due to phenomena such as absorption and geometric expansion, seismic waves weaken with distance. Strong ground shaking can be represented by physical quantities such as acceleration, velocity or displacement. Reduction relations express the relationship between these parameters with magnitude and distance or other required parameters. By carrying out the above research in the form of earthquake risk analysis using the probabilistic method, earthquake risk maps are prepared for the desired return period. Most Zagros faults have an inverted structure and have a slope between 30 and 60 degrees. Earthquakes in this region often have a depth between 8 and 15 km. Two deeper earthquakes (approximately 28 km) occurred on the low-angle thrust in the north of the Zagros and on its northern edge, which may indicate the subduction of the Arabian plate under the central Iran in the narrowest part of the Zagros (Talebian and Jackson 2004). Faults in this area usually have a left-lateral and reverse-parallel strike-slip structure. Alborz earthquakes are usually shallow. The most important earthquake in recent years can be mentioned the 1369 Rudbar earthquake.

1-2-1 Historical earthquakes

Statistical estimation of historical earthquakes of earthquakes is obtained from the catalog of earthquakes prepared by Berberian (1374) and Ambersiz and Melville (1982). The figure below shows the center of historical earthquakes in the studied area. Magnitudes attributed to historical earthquakes have a high error compared to machine earthquakes. This error is up to 5/. ± For major earthquakes, historical earthquakes are considered (Figure 1-1). As it is clear from the figure below, earthquakes higher than 6.9 have occurred in Alborz. Most of the historical earthquakes of Zagros are between 9/4 and 9/5.

(Images can be seen in the main file)

1-2-2 Device earthquakes

The earthquakes that have been collected from different sources have been investigated. Machine earthquakes, like historical earthquakes, have large and spatial errors. This error has decreased over time with the increase of seismic devices. This error is obtained for different time intervals according to the accuracy of the data. This error and its values ??for different times will be explained in section 3-6. The figure below shows machine earthquakes (Figure 1-2).

1-3- Thesis structure

The first chapter of this thesis includes  The generalities of the thesis are the seismicity of the studied area, historical and mechanical earthquakes, and research objectives. The second chapter includes technical literature, general methods of earthquake risk analysis by probabilistic method, its steps which include identifying seismic sources, Gutenberg-Richter law and reduction relationships are presented. In this chapter, how to calculate the risk curves, the Poisson model and the need to use a logical tree to apply various uncertainties in seismic risk analysis are presented in a probabilistic way.  The tectonic earthquake of the studied area is described in the third chapter. Considering that the purpose of this research is to prepare spectral acceleration maps on a scale of 1:2,500,000, therefore, the faults in this scale in the studied area are given in the third chapter.