Kazuma KITAZONO

Analysis of frequency characteristics of seismic waves observed during the Fukushima and Miyagi earthquakes in 2021 and 2022

Takaaki IKEDA

Since many damaging earthquakes occur in Japan every year, it is considered to be an important issue to reduce the damage from them. In particular, since seismic waves generated by earthquakes have a great impact on buildings, it is necessary to understand the frequency characteristics of seismic waves in the observation records in order to consider the possibility of damage from a large earthquake that is expected to occur in the future, Understanding the frequency characteristics of a large earthquake will lead to building countermeasures not only after the earthquake, but also before the earthquake.
In this study, we analyzed the frequency characteristics of the records of the magnitude 7 class earthquakes that occurred in 2021 and 2022 and caused damage to buildings.
The running spectrum was used as a method for analyzing the frequency characteristics of the observation records.
The running spectra were calculated for the four major earthquakes: the 2022 and 2021 off the coast of Fukushima Prefecture, the March 2021 off the coast of Miyagi Prefecture, and the May 2021 off the coast of Miyagi Prefecture. The downloaded records are used as parameters downloaded from the strong-motion observation networks (K-NET and KiK-net). The downloaded records were analyzed using the strong-motion tool SMDA2. The results of the analysis are used to examine how the different directions of rupture propagation and other factors affect the frequency characteristics of the in-slab earthquakes (off the coast of Fukushima Prefecture) and the plate boundary earthquakes (off the coast of Miyagi Prefecture), which have different source characteristics.
The analysis results show that the running spectra of the in-slab earthquakes (Fukushima Prefecture offshore earthquake) have similar trends even if the direction of rupture propagation is different. This indicates that the difference in the direction of rupture propagation has little effect on the frequency characteristics.
In this study, we focused on the observation points at Sendai (MYG013), Shinjuku (TKY007), and Nagaoka (NIG017), and analyzed the four earthquakes recorded there. In order to analyze more detailed frequency characteristics in the future, it is considered important to analyze a larger number of earthquakes with different arrival directions and different magnitudes.