Seismicity and Seismic Hazard Analysis of Shahid Dam Site

To perform seismic design or seismic diagnosis, it is very important to evaluate the earthquake hazard predicted for a dam site in order to predict earthquake damage and propose disaster prevention measures. The researches were conducted for dams, exploited 10-25 years, necessary to reconstruct. To secure seismic resistance, in design calculations of dams the initial seismic intensity, disregarding of local ground conditions under dams, was accepted in the fundamentals. So it was necessary overestimate seismic hazard degree according to a modern scientific-technological level. This paper presents seismic hazard assessment and seismic zoning of Shahid dam site and its vicinity based on the probabilistic approach. Due to the significance of the dam site, seismicity studies and active faults identity is essential in this area.  In this study, seismotectonics framework and earthquake hazard in 1:100,000 scale map of Yasuj were examined. The dam site is located in 51˚ 44' 33'' E and 30 ˚48' 49'' N. The study consists of broadly two parts, the first part basically gives a detail overview of the seismicity of the region and identification of various faults existing within the Dam site with all their details and the second part includes the detemininstic seismic hazard analysis for the same. An attempt was made to compile the occurrence of past and recent seismic activities within 26 km radius, around the Shahid Dam site. The study area is located in the High Zagros zone.  Using satellite images, historical and instrumental earthquake in formation, geological and tectonic maps and aeromagnetic maps, the fundamental faults of the area and active seismic area were evaluated. An updated catalogue, containing both historical and instrumental events, is used. Seismic source regions are modeled and relationships between earthquake magnitude and earthquake frequency is established. An attenuation relation for Zagros is used. Seismic hazard assessment was carried out for the area. Seismic hazard maps of the studied area based on magnitude, intensity and depth of earthquake are shown. Earthquakes with magnitude of 1.96 or more, and earthquakes with epicentral depth interval from 1 up to 15.5 km and then gradually increased up to 35 km. The main benchmark and indicator involved in carrying out the hazard analysis is the correctness and completeness of the data which needs to be attained. Finally the results are furnished which can be used directly by engineers as fundamental considerations, for generating earthquake-resistant design of structures in and around Shahid Dam site. According to the available information, the seismotectonics map of the area was prepared and six seismic zones were rated. The dam site has been built up at the region with high seismicity. Seismic activity of a region, is usually characterized in terms of the Gutenberg–Richter frequency–magnitude recurrence relationship log10 (N) = a – b*Mw, where N stands for the number of earthquakes greater than or equal to a particular magnitude Mw. Parameters (a, b) characterize the seismicity of the region. The simplest way to obtain (a, b) is through least square regression, but due to the incompleteness of the database, such an approach may lead to erroneous results. Earthquake data file is using from 1995 to 2015 August 31. The bounded Gutenberg-Richter recurrence law for M0 is 4.0 and Mmax is 8.0. Seismic risk analysis in the study area using the Gutenberg-Richter law predicted the probability for an earthquake with a magnitude of 5.5 up to 6 on the Richter scale in the area in the next 100 years, 100 percent.  The risk analysis for Earthquakes with a magnitude of 7.5 on the Richter scale was 61.76 percent and the risk analysis for earthquakes with a magnitude of 8 on the Richter scale was 33.94 percent.