10:45 AM - 12:15 PM
[SSS07-P09] Excitation mechanisms of backgraound Rayleigh waves by Ocean infragravity waves
In a frequency range from 5 to 20 mHz, the solid Earth oscillates persistently even on seismically quiet days. This phenomenon is also known as Earth's background free oscillation and is explained by the persistent excitation of Love and Rayleigh waves. Ocean infragravity wave (IG) is the most probable excitation source because of its dominant frequency range corresponding to that mentioned above and its characteristics as a long wave that can put water pressure fluctuation on the sea bottom. Although we consider this intuition doesn't lead to the excitation of Love waves, the observations show the equipartition of energy between Love and Rayleigh waves. A probable excitation mechanism for both Love and Rayleigh waves could be the topographic coupling between seismic surface waves and IG waves (Nishida et al., 2008; Fukao et al., 2010). This study aims to understand the excitation mechanism based on the recent seismic observations quantitatively.
We analyzed vertical components of broadband seismometers from 2006 to 2019 at F-net stations of the National Research Institute for Earth Science and Disaster Prevention (NIED). First, we calculate cross-correlation functions (CCF) between an island station and the other stations. The CCFs show significant later phases after the Rayleigh wave arrivals. Because broadband seismometer records both seismic waves and deformations caused by a load of IG waves (Nishida et al., 2019), the CCFs could also record conversion from an IG wave to a Rayleigh wave. In this study, we assume that IG waves are excited at point sources and propagate as cylindrical waves outward. We select three possible locations on the Pacific Ocean, near the Aleutian Islands, off the West Coast, and off the coast of Chile (Tonegawa et al., 2018) as the point sources. Based on Fukao et al. 2010, the net horizontal force, which averages over pressure fluctuations acting on the undulated sea bottom, excited Rayleigh waves. The net horizontal forces were located at grid points with uniform spacing according to the slope of the seafloor. The Rayleigh wave amplitudes were calculated by the sum of the excitations using the single force Green's function. We compare the observed data and the synthetics and discuss the excitation mechanism.
We analyzed vertical components of broadband seismometers from 2006 to 2019 at F-net stations of the National Research Institute for Earth Science and Disaster Prevention (NIED). First, we calculate cross-correlation functions (CCF) between an island station and the other stations. The CCFs show significant later phases after the Rayleigh wave arrivals. Because broadband seismometer records both seismic waves and deformations caused by a load of IG waves (Nishida et al., 2019), the CCFs could also record conversion from an IG wave to a Rayleigh wave. In this study, we assume that IG waves are excited at point sources and propagate as cylindrical waves outward. We select three possible locations on the Pacific Ocean, near the Aleutian Islands, off the West Coast, and off the coast of Chile (Tonegawa et al., 2018) as the point sources. Based on Fukao et al. 2010, the net horizontal force, which averages over pressure fluctuations acting on the undulated sea bottom, excited Rayleigh waves. The net horizontal forces were located at grid points with uniform spacing according to the slope of the seafloor. The Rayleigh wave amplitudes were calculated by the sum of the excitations using the single force Green's function. We compare the observed data and the synthetics and discuss the excitation mechanism.