نوع مقاله : مقاله تحقیقی (پژوهشی)
1 دانشجوی دکتری،گروه علوم زمین، دانشگاه علوم تحقیقات، دانشگاه آزاد اسلامی، تهران، ایران
2 استادیار، پژوهشگاه بین المللی زلزله شناسی و مهندسی زلزله، تهران، ایران
3 استادیار، گروه علوم زمین، دانشگاه علوم تحقیقات، دانشگاه آزاد اسلامی، تهران، ایران
عنوان مقاله [English]
The Kumamoto earthquake with magnitude Mj=7.3 was one of the destructive earthquakes in Japan which had more than 273 dead and 2809 injured. Investigation of temporal variation of coda wave attenuation before and after the earthquake, and also evaluating the lateral variation give important information about the crust and upper mantel beneath the studied area. Likewise, Attenuation of the seismic waves along with the velocity of wave propagation are among the important physical parameters that affect the propagation of the seismic waves. Careful investigation of these two parameters is required to determine the exact parameters of the earthquake source and also reduce the risk of earthquakes in the region. In the present study, the lateral changes of coda wave attenuation with time have been investigated. Here we checked the coda wave attenuations before and after the Kumamoto earthquake with the magnitude of 7.3 in the Kumamoto region of Japan. The estimation of Coda wave attenuation was performed using 474 local earthquakes recorded in 306 seismic stations by single back scattering method. This approach assumes that source and station are located at one point. The data used for estimation of attenuation have the epicentral distance between 1 and 200 km and the magnitude between 2.5 and 5.5. In order to considering the lateral variation of coda wave attenuation, the study area was divided into blocks with 0.3×0.3 degree in both dimension and the quality factor was calculated at the frequency of 1 Hz and lapse time of 30 seconds for the entire area. The results show the low quality factor Q0 and high frequency parameter n. So, the study area is tectonically active and has a high seismicity rate. Lateral changes in the quality factor indicate that the quality factor increases as it moves away from the epicenter of the main Kumamoto earthquake. The reason for this behavior can be attributed to the crushing of rocks due to multiple earthquakes, saturation and increased heterogeneity near the earthquake epicenter. Lateral attenuation changes before and after the main earthquake explain the fact that the quality factor decrease after the earthquake. After the main earthquake, the number of aftershocks occurred is greater than the number of pre-earthquakes. Increased earthquake occurrence in the region increases the fracture, crushes and saturation and as a result, reduces the quality factor. Comparison of the obtained quality factor in the region with other active regions in the world indicates the fact that the studied area is highly tectonically active and has also a high seismicity.