Seismic Hazard analysis of Hojedk city by probabilistic method

The country of Iran is part of the Alpine-Himalayan orogeny belt and is divided into different tectonic seismic states. Based on the classification of tectonic seismic states, Kerman province is located in the central Iran zone. The city of Hojedk is also located in the seismic state of central Iran, which is in a relatively high-risk zone in terms of seismicity and has experienced large earthquake events. In this research, for the first time, the city of Hojedk has been comprehensively investigated in terms of seismic conditions to facilitate the decision-making process for preventive measures, especially the improvement and strengthening of buildings against earthquakes. In this research, data from the catalog of earthquakes of the University of Tehran's Geophysics Institute were used to analyze the earthquake hazard, and geological maps of the Geological Organization were used in the geological surveys and faults in the area.

Earthquake hazard analysis in Hojedk City has been done in a probabilistic way. In this method, seismic springs have been identified and determined for the area. The information on the faults in the region and the earthquake catalog have been used for this research stage. At this stage, seismic springs are considered regionally. After determining the seismic springs, the seismic parameters were calculated and then the strong ground motion was estimated and finally, the earthquake hazard analysis was done by combining the uncertainties in the earthquake position and the magnitude of the strong ground motion parameters in the specific time period.

The city of Hojedk is placed in a very high seismic risk zone. Major earthquakes have occurred in the area of this city. Due to geology, this city is located on the Shemshak, Badamo, Hojedk and Shotori formations. There are many faults in the city of Hojedk, the closest fault to this city is the Deh Zana fault. The length of this fault is 18 km and its mechanism is thrust. The area of Hojedk City is an active area in terms of the occurrence of earthquakes. In this area, 6 historical earthquakes have occurred, the largest of which occurred with a magnitude of 6.1 in 1897. Historical earthquakes are in the large range and the smallest of them has a magnitude of 5.5. 4141 earthquakes occurred in the range from 1900 to 2023, of which 4 earthquakes occurred in this time range with a magnitude of 6 or more. 8 seismic springs of regional type were identified based on the trend of faulting and accumulation of earthquakes in Hojedk city. Research on aftershocks has shown that the number of events with a magnitude of about 3.4 to 5 is the highest, and the depth of most earthquake events is between 10 and 20 kilometers.

The seismic coefficients a, and b calculated in this range are 0.86 and 3.88, respectively. The seismicity rate in the studied area for magnitude 4 and greater is 1.47. The hazard analysis of the area for the return period of 475 years shows that the maximum acceleration of the bedrock is in the area of 0.45, which is in the area of Hojedk city is 0.25-0.35. The acceleration map shows that the highest accelerations are in the northwest-southeast trend, which is in good agreement with the trend of the faults. Therefore, in terms of construction laws, the studied area is of special importance, so by having soil boreholes in the area, seismic micro zoning can be done and its results can be used to improve construction and select and use materials.