Seismic risk analysis of Qom state

Dissertation

Master's degree

Field: Structural Engineering

Abstract

The increase in vulnerability caused by earthquakes and the large number of earthquake casualties caused by structures and houses that are weak in terms of construction forces mankind to conduct studies to reduce these risks and damages. Studies such as severe earth movement and seismic hazards related to it play an important role for the development of societies in earthquake prone areas. By using risk analysis, it is possible to make probability estimates about the financial and life damages caused by earthquakes and the vulnerability of structures, facilities, and as a result, before building the structures, choose the right place or design the structure in terms of resistance according to the place. Observing and recording the strong movement of the earth, the sequence of aftershocks, along with the investigation of the destruction caused by these earthquakes, the sciences of seismology and earthquake engineering, along with valuable and instructive data, establish important experiences. The land of Iran, as a part of the active tectonic zone of the Alps-Himalayas, is a complex plan of a collection of crusts, tectonic fragments and different tectonic zones, which has special characteristics from the point of view of neotectonics and seismicity. Seismic hazard analysis can be done by determining seismic springs and strong ground motion parameters for different return periods. In this research, using two deterministic and probabilistic methods, the seismic hazard zoning in Qom province, which is located in the central states of Iran, and in an area of ??about 14,000 square kilometers in a range of longitude 49 degrees 40 minutes to 52 degrees 40 minutes east and latitude 33 degrees 40 minutes to 35 degrees 50 minutes north, and the risk analysis curve of the region was calculated by Hazard Risk Analysis program and We obtained the risk analysis map of the area by the surfer program, the results show that the seismic risk is high in the area, and as a result, it reveals the need for such studies before the start of any construction. The risk analysis curve obtained from the probabilistic method was calculated for 475-year return periods for 10% probability of increase in 50 years and for 50-year return periods for 63% probability of increase in 50 years, and a seismic risk zoning map was presented. The present study indicates that the acceleration value increases with the increase of the return period. According to the results, for the 475-year return period, the maximum horizontal stress is calculated as g0.35 and for the 50-year return period, g0.18.

Keywords: seismicity, seismic risk zoning, strong earth movement, return period, probabilistic method, deterministic method, Qom

Chapter 1: Earth structure

1-1- Effective factors in strong earth movement:

In general, the effective factors in the strong movement of the earth due to the earthquake event can be examined in two parts, these two parts include the characteristics of the seismic spring, and the geotechnical conditions of the construction of the structures. Therefore, each of the parts and their role in the strong movement of the earth should be investigated in order to estimate the characteristics of the strong movement of the earth in the foundation of the structures and to optimize the design criteria of the structures against the earthquake and be used in the calculations. :

The occurrence of earthquakes in the part of the earth's crust is caused by tectonic forces based on the theory of plate tectonics proposed since 1960. In this theory, it is stated that the earth is made up of a large number of blocks in the form of sheets, and these sheets are moving relative to each other.  The border of these blocks always faces the event of large earthquakes. The most reliable explanation for the cause of plate movement is based on the thermodynamic balance of the Earth's constituents. The upper part of the mantle is in contact with the cold crust, while the lower part is in contact with the hot core of the Earth. It is obvious that a temperature gradient must exist in the mantle. Figure 1: Side view of the Earth The theory of plate tectonics simply describes and determines the relative movement of plates according to three types of boundaries (subduction plates, lateral extension and translational extension). In other cases, due to expansion, the edges of the sheets may break and cause the formation of a smaller sheet or a fragment of a sheet enclosed between larger sheets. The movement between two parts of the crust causes a new discontinuity or the advancement of existing fault lines in the geological structure of the crust, which is faulted.The movement between two parts of the crust causes a new discontinuity or the advancement of fracture lines in the geological structure of the crust, which is called a fault. Faults are classified into three main groups depending on their movement type, which are: dip-slip faults, strike-slip faults, or a combination of them.

Figure 2: type of fault movement

The theory of elastic return states that the occurrence of earthquakes causes the release of stress along a part of the fault, and as long as the stresses have enough time to be stored again, the next rupture, or in other words, the next earthquake, will occur. fell Since earthquakes cause the release of energy accumulated on the fault, their occurrence is more likely in areas where seismic activity has not occurred for a short period of time or at all. By identifying the movements of the fault during the past earthquake and along it, it is possible to realize the absence of seismic activity in some of its places.

With seismic studies, it is possible to learn about the fracture of geological structures and their geometry around the construction of the structures, and finally, a seismic model can be prepared from them or at the depth of the seismic section, and the possible risks of ground faulting or a possible earthquake event on them for Estimating the event of future earthquakes and predicting their characteristics.

The geometry of faults, fractured zone, their type and mechanism can help us in estimating the maximum possible earthquake potential on them, and this is done in tectonic studies and tectonic earthquakes. The age of faults is one of the important factors in the earthquake event on them, so young faults are more important and new tectonic studies can be of great help in classifying faults from the point of view of seismic activity. Usually, there are experimental relationships in connection with the geometry of the fault, the maximum seismic power and the maximum amount of displacement on it, which can be effective to some extent in estimating future earthquake events in the region. The magnitude of the earthquake has a direct relationship with the energy released by the earthquake.

One of the features of the seismic source is the focal depth of the earthquakes. Many studies have been done on the focal depth of earthquakes.  In general, the depth of earthquakes caused by subduction is relatively deep, which has been reported up to a depth of 800 km on the earth's surface, but the focal depth of earthquakes observed in connection with lateral oceanic extension is shallow and their depth is less than 20 km, and the focal depth of earthquakes observed with translational extension in the continental crust has a depth less than the earth's crust, i.e. less From 60 km. The occurrence of earthquakes in Iran has been very shallow, and except for the Makran region, it has been estimated to be less than 20 km in almost all of Iran, and for this reason, the seismic layer in the plateau of Iran can be considered between 10 and 20 km deep. It has a significant effect on all the important parameters of strong ground motion such as amplitude, frequency content and duration of shaking. The degree of impact is a function of the geometry, the properties of the materials of the subsurface layers, the topography of the building, and the characteristics of the seismic waves that are generated from the seismic source and pass through different rock layers to enter the foundation stone of the building. He explained the strength of the ground surface in constructions with different conditions from the construction in question.

The geometrical effects of the bedrock have an effect on the strong movement of the ground. Although the topographical irregularities of the bedrock scatter the earthquake waves and create complex patterns of amplification or reduction of the powerful earth movement, but anyway, the powerful earth movement at high altitudes is usually strengthened and intensified. They make up the 70 to 150 km outer layer of the earth, which is known as the crust or lithotrip. In addition, we know that these plates move slowly and their driving force is not fully known.