Probabilistic seismic hazard analysis (PSHA) of the Ardabil city using Ez-Frisk

The city of Ardabil is surrounded by faults, including the large seismic fault of Astara, which throughout seismic history of the region, several earthquakes have occurred along that. The focal depth of most earthquakes is low and close to the surface (5 to 30 km) at Ardabil. In this research, 7 linear sources and 23 regional sources have been identified. After aggregating historical and instrumental earthquake information, the reliability and completeness of the catalog should be evaluated. Due to the lack of reports of small earthquakes in historical data and the new periods of instrumental data, there is always an incomplete seismic catalog for a region. Because the existing catalog covers a long period of time, it contains temporal and spatial heterogeneities. The Stepp method is used to calculate the completeness of the catalog. Most earthquakes in this region have a depth of less than 40 and according to the cumulative distribution diagram, also most earthquakes have a focal depth of less than 20 km.

To carry out this research, the recorded earthquake data of the region taken from 4 sites IIEES, ISC, NEIC and University Geophysics Tehran, and Matlab, Zmap, Kijko and Ez-frisk software were used. Seismic device data used in the region includes details related to the earthquake spring, such as: date, time, latitude and longitude, magnitude and related seismic information, which are used to determine the seismic source of the faults. Data analysis for earthquake risk has been done using Ez-frsik software. In this study, the earthquake catalog for northwestern Iran has been examined in three-time series:A: historical earthquakes (period before 1900 AD); B: Systematic earthquakes of the first period (1900 to 1963) and C: Systematic earthquakes of the second period (1963 to 2020).

Kijko and Sellevoll method was used to determine the seismicity parameters of Ardabil region. This method has useful capabilities in using the list of heterogeneous earthquakes which is consistent with the characteristics of seismic data of Iran. The functions used in the Kijko program include the distribution of final values for earthquakes before the 20th century, which are often large but with high error, and the Gutenberg-Richter function for recorded earthquakes and using the statistical method of maximum likelihood estimation. In this method, it is also possible to simultaneously use historical and recorded earthquakes by making appropriate classifications by considering the magnitude error, threshold magnitude, and maximum magnitude differently for each category. It is also possible to somehow include the effect of seismic absences or lack of information in the calculations. The estimated seismicity parameters, including the maximum magnitude of the earthquake (Mmax), the coefficient β and the annual volume λ in the 100 km area using the Kijko method, are:Mmax=7.7, β=1.86, λ=0.9The probability of the event or return period of earthquakes is another parameter that are calculated by this software. The return period increases greatly with the increase and the probability of an earthquake event decreases in a certain period of time. For example, in a 100-year period, the probability of an earthquake with a magnitude of 6 is 90%.

Graphs show an increase in the return period of earthquakes, so the probability of an earthquake event in a certain period of time decreases. Earthquake risk map for PGA was performed on bedrock with 1% damping for a return period of 475 years in a set of networked points at 0.05 ° × 0.05 intervals. According to the acceleration set for the city of Ardabil, this area is divided into 4 micro-zones. The results show that the acceleration values ​​in this range for PGA vary from 0.19 in the north to 0.21 in the southwest. It should be noted that the acceleration values ​​are the same throughout each designated area, so the city of Ardabil can be introduced as one of the cities with moderate risk in seismic activity.