2:30 PM - 2:45 PM
[SSS09-14] Study on Microseisms excited by Typhoons around Japan
Keywords:microseisms, typhoons, seismographs, ocean waves, storms
Seismograms include not only intermittent, high-amplitude oscillations due to earthquakes, but also continuous, low-amplitude ones due to atmospheric and oceanic disturbances. The oscillations at 0.05-0.5 Hz are generally called microseisms and have relatively large amplitudes. Microseisms are classified into weak primary microseism at 0.05-0.1 Hz and strong secondary microseism at 0.1-0.5 Hz. The former is thought to be excited by the ocean waves themselves, and the latter by the interaction of ocean waves. Microseisms are mainly composed of Rayleigh and Love waves, so that the energy of microseism efficiently propagates in the solid earth. Studies linking typhoons that excite large ocean waves and microseisms have been conducted so far, but the source and excitation mechanism of microseisms have not yet been fully understood. Locating the source area of microseism is important not only for understanding the interaction between atmosphere, ocean and solid earth, but also for estimating the Earth's internal structure using microseisms. In this study, we estimated the source areas of the primary microseism excited by typhoons approaching Japan.
We used seismic records from the F-net broadband seismograph network as the seismic wave data. We obtained 1st percentile power spectra for all the 72 F-net stations to select quiet 20 stations with the consideration of the station distribution. For typhoon information, we referred to the database of the Japan Meteorological Agency. In this study, we analyzed the seismic records in the periods of six large typhoons approaching Japan with the maximum wind speed of 33 m/s or more in 2019 and 2020, FRANCISCO (typhoon number 1908), KROSA (1910), FAXAI (1915), HAGIBIS (1919), HAISHEN (2010) and CHAN-HOM (2014).
Source areas were estimated using Rayleigh waves with a similar method to Park & Hong (2020) for locating the source areas of microseisms caused by the typhoon 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 vertical component whose phase is shifted by π/2 in the negative direction of the time axis is maximum, the source is in that direction. In this study, we obtained the station averages of the cross-correlation coefficients computed for source grids with an interval of 0.1 degrees on the surface to estimate the source distribution of the primary microseism during the approach period of typhoon. We defined the area with high correlations as the source area of microseism.
The result has shown that the seismic energy of the primary microseism at 0.08-0.1 Hz is greatly excited with the approach of the typhoons, and the main source areas are almost the same as the typhoon storm areas. However, in the four typhoons HAGIBIS, FAXAI, HAISHEN and CHAN-HOM, it has been shown that the source areas stagnate after the passage of typhoon. There was a strong correlation between ocean-wave and microseism amplitudes during the stagnation. The stagnant source areas might be due to the seafloor sandwiched by topography highs, which efficiently excited the microseism even after the passage of typhoon.
We used seismic records from the F-net broadband seismograph network as the seismic wave data. We obtained 1st percentile power spectra for all the 72 F-net stations to select quiet 20 stations with the consideration of the station distribution. For typhoon information, we referred to the database of the Japan Meteorological Agency. In this study, we analyzed the seismic records in the periods of six large typhoons approaching Japan with the maximum wind speed of 33 m/s or more in 2019 and 2020, FRANCISCO (typhoon number 1908), KROSA (1910), FAXAI (1915), HAGIBIS (1919), HAISHEN (2010) and CHAN-HOM (2014).
Source areas were estimated using Rayleigh waves with a similar method to Park & Hong (2020) for locating the source areas of microseisms caused by the typhoon 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 vertical component whose phase is shifted by π/2 in the negative direction of the time axis is maximum, the source is in that direction. In this study, we obtained the station averages of the cross-correlation coefficients computed for source grids with an interval of 0.1 degrees on the surface to estimate the source distribution of the primary microseism during the approach period of typhoon. We defined the area with high correlations as the source area of microseism.
The result has shown that the seismic energy of the primary microseism at 0.08-0.1 Hz is greatly excited with the approach of the typhoons, and the main source areas are almost the same as the typhoon storm areas. However, in the four typhoons HAGIBIS, FAXAI, HAISHEN and CHAN-HOM, it has been shown that the source areas stagnate after the passage of typhoon. There was a strong correlation between ocean-wave and microseism amplitudes during the stagnation. The stagnant source areas might be due to the seafloor sandwiched by topography highs, which efficiently excited the microseism even after the passage of typhoon.