Dissertation for Master's degree
in the field of Civil Engineering - Earthquake Engineering
Abstract
The city of Birjand, as the capital of South Khorasan province, is located in the structural zone of Sistan, which due to its proximity to seismic and fundamental faults (the Nine East and West faults in the south, the Ardekol fault in the northeast, the Nawad fault in the east, and the Dasht Beyaz fault in the north) that are prone to earthquakes Major and destructive have been associated in the last century, it has a special sensitivity. The investigation of earthquakes that occurred in the area of ??the project shows a high level of seismicity, and in historical periods and after 1900, earthquakes with a magnitude of around 6 and even more appear to be normal. Therefore, the current study was conducted under the title of earthquake risk analysis and preparation of a uniform risk spectrum for Birjand city, in which the maximum acceleration on bedrock (PGA) and a uniform risk spectrum with two levels of risk were prepared for Birjand city. A set of seismogenic sources and historical and instrumental seismicity data with a time coverage from the 8th century AD to today up to a radius of 200 kilometers has been used. The Gutenberg-Richter method and the final value fitting method have been used to estimate the seismicity parameters, but due to the lack of appropriate seismicity data and the low accuracy of the available information, as well as the uncertainty in large quantities at different times, the Kijko [2000] method has been used. In order to determine the maximum acceleration on bedrock (PGA), the five reduction relations of Akkar & Bommer (2010), Ambraseys et al (1996), Campbell & Bozorgnia (2000), Ghodrati et al (2007), Campbell & Bozorgnia (2008) have been used. These five relationships were combined using the logical tree method with weights of 0.20, 0.30, 0.15, 0.20, and 0.15, respectively, and using ? and ? obtained from Keiko's method, and the final result was obtained. Berge-Thierry (2004), Ghodrati et al (2010) has been used to determine acceleration spectra due to its spectral nature and greater suitability to regional conditions using the logical tree method with weights of 0.20, 0.30, 0.35 and 0.15 respectively. Probability analysis of Birjand earthquake risk has been done using SEISRISK III program. The results of this analysis are presented by spectral acceleration maps with 2% and 10% event probability in 50 years for the city of Birjand. Also, at the end of the uniform risk spectrum as well as the constant shape spectrum with 2% and 10% event probability in 50 years for the city of Birjand.
Key words:
Uniform risk spectrum, seismic risk analysis, maximum horizontal ground motion acceleration (PGA), fixed shape spectrum, attenuation relationships, Birjand city.
Chapter 1
Generalities and concepts Basic
Introduction
Despite the extensive efforts made by scientists around the world to determine the risks of earthquakes, it is not possible to accurately predict the time, place and magnitude of future earthquakes and the resulting tremors on the earth's surface. An exact result is not predicted in the near future. The most important reason for this issue is the existence of many complications in the earthquake mechanism and the prevailing conditions that cause it, as well as the passage of waves through different layers of the earth with completely different characteristics. Of course, the expression of the above concept does not mean that it is not possible to predict the risks of an earthquake to an acceptable level and to secure structures against it. Experience and scientific findings have well shown that based on available information and using statistical and probabilistic methods, the safety of structures against earthquakes can be estimated to the optimum level.
In feasibility, analysis, construction and maintenance of structures against earthquakes, two basic factors of construction safety are examined. The safety of the construction depends on the geotechnical and geological hazards that can occur in the construction, such as landslides, liquefaction and intensification of ground movements due to the effect of construction conditions, past earthquakes show that the effect of construction conditions plays a very fundamental and important role in the amount and type of destruction of structures.
The adverse effects of earthquakes that cause damage to structures and facilities are related to the following two phenomena:
Earth tremors due to the passage of seismic waves
Shear displacement caused by the shear movement of faults
The passage of seismic waves causes vibration that can directly cause the destruction of structures, or as a result of phenomena such as subsidence, liquefaction and sliding The foundation of the structure should be destroyed. The intensity of the earthquake depends on the magnitude of the earthquake, the focal distance, the damping characteristics of the building, the type and thickness of the sedimentary deposits, and the topographical conditions.
The purpose of earthquake risk estimation is to rationally evaluate the ground movement parameters (maximum acceleration, maximum speed, maximum displacement, etc.) in the construction in question, due to an earthquake event in potential seismic springs in a certain period of time, which is usually the useful life of the structure.
Actually, earthquake risk analysis is the calculation of the probability of occurrence of certain levels of ground shaking per unit of time, due to the occurrence of an earthquake. This analysis is often summarized by an earthquake hazard curve, which indicates the probability of annual exceedance against the amplitude of the earthquake oscillation. In fact, earthquake risk analysis is the basis for entering into the decision-making process to reduce earthquake damage. The ultimate goal in risk analysis and earthquake engineering is to determine the intensity of the earthquake, which leads to the quantification of the effect of the earthquake on the structures. This process is reflected in the response spectrum. Seismic Hazard is synonymous with seismic risk by many engineers and designers. This shift brings a lot of risk, because during the analysis, these two quantities have different meanings. (Trakshund A. 1388 (
Seismic risk means the possible occurrence of a severe movement by a seismic event in the future. While the seismic risk is the possible consequences of that severe movement. Consequences such as life, financial, economic losses, which are defined as loss and damage function based on the needs of the plan. Therefore, risk can be defined by using the seismic risk and considering the vulnerability function. In any seismic zone, it will crack. 1388 (
The vulnerability of a structure or a set of similar structures is indicative of the degree of damage or loss caused by the strong movement of the earth. (movements that are preferably expressed by some physical parameters such as acceleration and intensity). The time of occurrence of a possible risk and possible risk is not known in the future, and therefore it is not surprising that the principles of probability are the basis of the analysis of seismic risk and risk. 1388 (
The concept of earthquakes:
Earthquakes are basically vibrations of the earth's crust, which are caused by the sudden release of energy accumulated in the earth's crust, and in the case of high intensity, it causes a lot of damage in human centers, so that every year a large number of people in different parts of the world are involved in its undesirable effects. The passage of seismic waves causes vibrations that It can directly cause the destruction of structures or due to phenomena such as subsidence, liquefaction and landslide, the intensity of earthquakes depends on the magnitude of the earthquake, the focal distance, the type and thickness of the sedimentary deposits and the topographic conditions.
Earthquake waves: The impact of energy and disturbances in the environment causes the generation of waves that propagate from the energy source (for example, fault rupture) with a certain speed and cause the vibration of the particles of the environment. The speed of propagation depends on the properties of the environment and the speed of vibration depends on the amount of energy. (Zare, 1379)
: Internal or body (volumetric) waves:
Those waves that move inside the earth and propagate in all directions and move at a higher speed than surface waves and are divided into two groups of shear and pressure waves (P and S). (Zare, 1379)
1-5-1-1: Pressure or longitudinal waves (P):
These waves cause successive tensions and contractions along the movement of the wave. The propagation speed of these waves is higher than other waves and they are the first wave that reaches the seismograph station. Compression waves pass through all environments that can withstand pressure, including gases, solids and liquids. The particles that are affected by the P wave oscillate forward or backward in the direction of wave propagation.