9:15 AM - 9:30 AM
[SSS13-02] Estimation of the sources of the primary microseisms excited by typhoons around Japan
Keywords:microseisms, seismic noise, typhoons, seismic waves
Among the background noise of the solid earth, oscillations in the frequency band of about 0.05-0.10 Hz are called primary microseisms. The primary microseisms are mainly composed of seismic surface waves, and are thought to be generated by the direct coupling between ocean waves and seafloor topography. Since storms such as typhoons excite ocean waves with strong atmospheric energy, studies have been conducted to estimate the source of the primary microseisms excited by storms so far. However, no systematic studies have been conducted for the source distribution of the primary microseisms excited by typhoons approaching Japan. Locating the source of microseisms is important not only for understanding the interaction between atmosphere, ocean, and solid earth, but also for applied research such as elucidating the earth's internal structure and predicting ocean waves using the microseisms. In this study, we estimated the sources of the primary microseisms excited by typhoons that approached Japan during the decade of 2010-2020.
We used seismic records from the F-net broadband seismograph network. We obtained hourly averaged power spectra for all stations, and selected the stations with clear spectral peak of the primary microseisms. For typhoon data, we referred to the database of the Japan Meteorological Agency. In JPGU 2021, we focused only typhoons in 2019 and 2020. In this study, we focused on the strong typhoons with maximum wind speeds of 33 m/s or higher that approached Japan during the decade of 2010-2020, and analyzed the records for the period when the typhoons approached.
In this study, source regions were estimated using Rayleigh waves with the similar method to Park & Hong (2020), which identified the sources of microseisms caused by the typhoons that passed through the South China Sea. This method utilizes the Rayleigh wave characteristic that the horizontal component is phase-shifted by π/2 in the negative direction of the time axis with respect to the vertical component. When the cross-correlation coefficient between the horizontal component rotated to a certain direction and the phase-shifted vertical component is maximum, the source is expected to be in that direction. To estimate the sources of the primary microseisms, we created virtual source grids on the surface with an interval of 0.5 degrees in the range of 10°-60°N, 100°-160°E, and computed the station averages of cross-correlation coefficients for virtual source grids.
As a result, The Tokai and Kanto coastal regions were found to be places where the primary microseisms were particularly excited. Since primary microseisms are usually generated in shallow coastal areas by the direct coupling between ocean waves and seafloor topography, the sources move with typhoon propagations. However, it was observed for several typhoons that excitation sources were estimated at the Tokai and Kanto coastal regions even after the passage of typhoons. This suggests that the Tokai and Kanto coastal regions are particularly suitable regions for the excitation of the primary microseisms around Japan.
We used seismic records from the F-net broadband seismograph network. We obtained hourly averaged power spectra for all stations, and selected the stations with clear spectral peak of the primary microseisms. For typhoon data, we referred to the database of the Japan Meteorological Agency. In JPGU 2021, we focused only typhoons in 2019 and 2020. In this study, we focused on the strong typhoons with maximum wind speeds of 33 m/s or higher that approached Japan during the decade of 2010-2020, and analyzed the records for the period when the typhoons approached.
In this study, source regions were estimated using Rayleigh waves with the similar method to Park & Hong (2020), which identified the sources of microseisms caused by the typhoons that passed through the South China Sea. This method utilizes the Rayleigh wave characteristic that the horizontal component is phase-shifted by π/2 in the negative direction of the time axis with respect to the vertical component. When the cross-correlation coefficient between the horizontal component rotated to a certain direction and the phase-shifted vertical component is maximum, the source is expected to be in that direction. To estimate the sources of the primary microseisms, we created virtual source grids on the surface with an interval of 0.5 degrees in the range of 10°-60°N, 100°-160°E, and computed the station averages of cross-correlation coefficients for virtual source grids.
As a result, The Tokai and Kanto coastal regions were found to be places where the primary microseisms were particularly excited. Since primary microseisms are usually generated in shallow coastal areas by the direct coupling between ocean waves and seafloor topography, the sources move with typhoon propagations. However, it was observed for several typhoons that excitation sources were estimated at the Tokai and Kanto coastal regions even after the passage of typhoons. This suggests that the Tokai and Kanto coastal regions are particularly suitable regions for the excitation of the primary microseisms around Japan.