Seismicity assessment of Fukushima region, fault kinematics and calculation of PGA value for Idosawa fault in Hamadori area, Japan

In depth assessment of seismicity distribution for a particular region like Fukushima, prefecture is key towards reliable seismic hazard analysis for the area. Assessment of seismicity becomes more challenging for those areas lying close to mega subduction zone like in this study. Development of accurate input data like seismicity and faults data can reduce bias in the results and can improve quality of analysis. To assess the influence of seismicity on hazard analysis, two approaches known as Gutenberg–Richter law and the Gumbel’s extreme asymptotic distributions are employed. It is found that b value calculated through Gutenberg–Richter law has shown better b value of 1.0 according to tectonics of the Fukushima area. However, Gumbel’s approach which uses maximum earthquake of each years have shown b value of 0.6 and used for calculating future earthquake potential. Comparison of earthquake distribution in both cases has shown that in areas where we have rich seismicity data Gutenberg–Richter law is found much applicable as compared to Gumbel’s approach. Fault kinematics is done through software called FaultKin6 developed by (Rick Allmendinger) which resulted that Idosawa fault is a normal fault based on the fact that P axis is greater than 45° showing extension in N–S direction for Idosawa fault. Resultantly, it is tried to highlight the need to take normal faults in seismic hazard analysis probabilistically in design when they exist within stress field of big subduction zone. Peak ground acceleration is calculated by using PEER NGA West-2 models for Idosawa fault, and 0.22 g is found as PGA value.