بررسي كاهندگي شتاب بر اساس داده‌ هاي شتاب‌ نگاري استان مازندران

Study on Acceleration Attenuation Relationship Based on Seismic Data of Mazandaran Province

در اين مقاله ضرايب ثابت روابط كاهندگي شتاب موجود كه توسط محققين مختلف براي فلات ايران ارائه شده بود براي استان مازندران مورد ارزيابي مجدد قرار گرفت. تعداد شتاب نگاشت هاي مورد استفاده در اين مقاله 105 ركورد با بزرگاي گشتاوري بيشتر از 4 بين سال هاي 1975 تا 2016 بوده است كه از اين تعداد 71 ركورد متعلق به زمين هايي با بستر سنگي با سرعت موج برشي بيشتر از 375 متر بر ثانيه و 34 ركورد متعلق به زمين هايي با بستر خاكي با سرعت موج برشي 375 متر بر ثانيه و كمتر بوده است. شتاب نگاشت هايي كه نياز به اصلاح داشتند، توسط فيلتر چبيشف (Chebyshev Filter) اصلاح گرديدند. همچنين تصحيح خطاي خط مبنا بر روي اين شتاب نگاشت ها انجام گرفت. بررسي نتايج نشان مي دهد كه استفاده از داده هاي لرزه اي استان مازندران در تمامي مدل ها منجر به نتايج همگن مي گردد كه بيانگر دقت ضرايب ثابت به دست آمده در اين پژوهش مي باشد؛ اما با توجه به اهميت انحراف معيار در استفاده از روابط كاهندگي شتاب زمين در تحليل خطر زلزله، روابط ارائه شده نياز به انحراف معيار دارند كه در تحقيقات آتي به دست خواهند آمد. نتايج اين تحقيق اهميت مطالعه بيشتر بر روي كاهندگي مناطق مختلف فلات ايران را نشان مي دهد.

In order to determine the base design acceleration, we need the site special spectrum. Seismic hazard analysis is considered one of the most important methods to determine the base design acceleration. Attenuation relationship is one of the main factors in this method as the incorrect spectrum design will obtain in using an inappropriate attenuation relationship. This study aims to represent the constant coefficients of existing attenuation relationships in Iran plateau, which have been studied by other researchers previously, for Mazandaran province. Seismic data of Mazandaran have been used for this purpose. The total number of accelerogram records used with the moment magnitude more than 4 between 1975 and 2016 was 105 records in which 71 records had the shear wave velocities in the top 30 meters more than 375 m/s, which can be considered as rock. Moreover, 34 records with the shear wave velocity of less than 375m/s had been considered as soil. Baseline correction and frequency filtering have been done on records by using Seismosignal software to make them more precise. In this paper, four different attenuation models were selected to analyze by using SPSS software. Between the existing models the first ground motion model which has been studied, was presented by Nowroozi in 2005. The 178 reported data by Bard et al, was used in Nowroozi paper. The second one was from Ghodrati et al presented in 2007 for Iran plateau. They considered Iran into two main seismic zones of Zagros and Alborz and Central Iran according to tectonic conditions. All of the earthquake records in their databank had distances between 5 to 150 kilometers from site to source. They chose the shear wave velocity of 375 M/S for the boundary of soil and rock. The third attenuation relationship was Saffari et al work published in 2012. They used 351 records to present an attenuation ground motion model for Iran. They developed attenuation relationships for peak ground acceleration, peak ground velocity and acceleration response spectra with 5% damping. The last but not least attenuation relationship studied in this paper was the ground motion prediction equation by Soghrat and Ziyaeifar published in 2016. They used 325 three-component records of 55 earthquakes with magnitude ranging from 4.1 to 7.3 for estimation on the regression coefficients. In the presented attenuation relationships in this paper, the effects of earthquake magnitude, site to source distance and ground type on the peak ground acceleration have been investigated. According to this study, using the records of Mazandaran province leads to homogeneous results indicating the accuracy of extracting constant coefficient of this study. Standard deviation is such an important factor in attenuation relationships, which highly recommended to study. Furthermore, it has been found that mathematical models do not alter the final results significantly; probably, used data affects the attenuation relationship more. Accordingly, using the seismic data in each region is recommended in order to achieve the precise attenuation relationships.