Aoto KUBOTA

Analysis of frequency characteristics of seismic waves observed in inland crustal earthquakes around Niigata Prefecture

Takaaki IKEDA

Given the large number of damaging earthquakes that occur in Japan every year, reducing the damage from these earthquakes is considered to be an important issue. In particular, since seismic waves generated by earthquakes have a great impact on buildings, understanding the frequency characteristics contained in observation records is necessary for examining the possibility of damage caused by major earthquakes that are expected to occur in the future, and understanding the frequency characteristics will lead to countermeasures for buildings not only after the occurrence of major earthquakes but also before the occurrence of earthquakes.
In this research, we analyzed the frequency characteristics of the records observed in recent large earthquakes of magnitude 6.
The running spectrum was used as a method to analyze the frequency characteristics of the observation records.
For the calculation of running spectra, we used parameters downloaded from the strong-motion observation network (K-NET, KiK-net) of records observed at the KNET Nagaoka site (NIG017), the target site for analysis, for five earthquakes: the Niigata-ken Chuetsu earthquake, the Niigata-ken Chuetsu-oki earthquake, the earthquake in northern Nagano prefecture, the earthquake in northern Tochigi prefecture, and the earthquake off Yamagata prefecture. The results of the analysis are divided into three groups: Group 1 for earthquakes in northern Nagano Prefecture, Niigata Prefecture, and northern Tochigi Prefecture, which are located to the south of the observation point (NIG017); Group 2 for earthquakes off Yamagata Prefecture and Niigata Prefecture, which are located to the north.
From the results of the analysis, it is considered that the components of the high frequency band were dominant in Group 1 (the Niigata-Chuetsu earthquake, the earthquake in northern Nagano Prefecture, and the earthquake in northern Tochigi Prefecture), and that the NIG017 was relatively susceptible to strong shaking. The way the seismic wave propagated suggests that it passed through a hard ground.
In Group 2 (earthquakes off the Niigata-Chuetsu-Oki coast and off the Yamagata coast), the high frequency component is not as dominant as in Group 1, and strong shaking is not felt as easily. In addition, the way the seismic waves were transmitted suggests that they passed through soft ground.
In this research, we analyzed the frequency characteristics of the records observed in recent large earthquakes of magnitude 6. In this study, we focused on Nagaoka point (NIG017) in Niigata Prefecture, and analyzed five earthquakes recorded there. In order to analyze the frequency characteristics in more detail in the future, it is important to increase the number of earthquakes with different arrival directions and sizes.