10:45 〜 12:15
[SSS04-P05] Seafloor infragravity waves in northern Nankai Trough, resolved from DONET data
キーワード：Infragravity wave, Cross-correlation function, Cross-correlation beamforming, Ocean bottom seismometers
Infragravity waves (IGWs) are low-frequency ocean waves for 30 seconds or longer and can travel great distances without losing energy. They are typically generated by the non-linear interaction of ocean waves and modulated by water depth. IGWs play a role in ocean dynamics and significantly impact coastal hydro-sedimentary dynamics and ice-shelf collapse. In addition, IGWs can enhance the mixing of ocean water and redistribute nutrients and temperature through different water layers. Because IGWs will generate a detectable seafloor signal across the ocean, the seafloor seismometric observations can provide long-term monitoring of IGW energy.
We analyzed the IGWs recorded at the 45 ocean bottom seismometers of the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET). Deploying in the seismogenic area of the historical Tonankai and Nankai earthquakes, DONET consists of 22 seismometric observatories across the Kumano forearc basin (DONET1) and 29 observatories offshore the Kii peninsula (DONET2). The spectral analysis applied to the DONET seismograms shows that the main frequency band of the IGWs recorded at DONET stations is within 0.005 and 0.03 Hz. Furthermore, the strength of IGWs exhibits an annual variation. Through the DONET data, IGWs energy is relatively vital in winter and low in summer. The peak frequencies of IGWs show a systematical decrease with increasing water depth. Using the dispersion relation, we retrieve the phase velocities of the peak IGWs ranging from 81 to 153 m/sec for the sites within the forearc basin and along the continental slope.
We calculate the incoming direction and phase velocities of IGWs by employing the beamforming algorithm with cross-correlation functions (CCFs). Following Tonegawa et al. (2018), we use bathymetric relief to form wave dispersion and calculate the theoretical group velocity. The ray path is retrieved by solving the eikonal equation to each grid, and then we can locate the excitation of IGWs. The CCFs were from 55757 station pairs and divided into DONET1 (Kumano Basin) and DONET2 (Muroto Basin). The results of cross-correlation beamforming from DONET2 show the back-azimuth of incoming waves is about 121o at a phase velocity of 95.2 m/s and can alter to 91o during the boreal summer. The observations are very different from the data of DONET1, which show two significant incoming wave energy with back-azimuthal of 114o and 151o at a phase velocity of 124.6 m/s. The coastal reflection energy is observed only with DONET1. They are the opposite back-azimuths at 290o and 320o, respectively. Furthermore, the incoming wave directions can shift between 50o and 70o during the boreal summer but change to 210o during the boreal winter. We infer that the variance of the incoming wave direction may be associated with nearby bathymetry, surface pressure variation in season, and long-distance IGW energy.
With the DONET seismograms, we reproduced the evolution of daily excitation locations from 2017 to 2018. We found that IGW energy constantly transports within the Pacific Ocean with the secular atmospheric condition. For the northern Nankai trough location, the IGWs primarily originate from the western coast of South America. Minor energy of IGWs can also be converted from the coast of the Aleutian islands. Furthermore, tropical cyclones will generate local-to-regional event-type excitations that last for a limited time.
We analyzed the IGWs recorded at the 45 ocean bottom seismometers of the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET). Deploying in the seismogenic area of the historical Tonankai and Nankai earthquakes, DONET consists of 22 seismometric observatories across the Kumano forearc basin (DONET1) and 29 observatories offshore the Kii peninsula (DONET2). The spectral analysis applied to the DONET seismograms shows that the main frequency band of the IGWs recorded at DONET stations is within 0.005 and 0.03 Hz. Furthermore, the strength of IGWs exhibits an annual variation. Through the DONET data, IGWs energy is relatively vital in winter and low in summer. The peak frequencies of IGWs show a systematical decrease with increasing water depth. Using the dispersion relation, we retrieve the phase velocities of the peak IGWs ranging from 81 to 153 m/sec for the sites within the forearc basin and along the continental slope.
We calculate the incoming direction and phase velocities of IGWs by employing the beamforming algorithm with cross-correlation functions (CCFs). Following Tonegawa et al. (2018), we use bathymetric relief to form wave dispersion and calculate the theoretical group velocity. The ray path is retrieved by solving the eikonal equation to each grid, and then we can locate the excitation of IGWs. The CCFs were from 55757 station pairs and divided into DONET1 (Kumano Basin) and DONET2 (Muroto Basin). The results of cross-correlation beamforming from DONET2 show the back-azimuth of incoming waves is about 121o at a phase velocity of 95.2 m/s and can alter to 91o during the boreal summer. The observations are very different from the data of DONET1, which show two significant incoming wave energy with back-azimuthal of 114o and 151o at a phase velocity of 124.6 m/s. The coastal reflection energy is observed only with DONET1. They are the opposite back-azimuths at 290o and 320o, respectively. Furthermore, the incoming wave directions can shift between 50o and 70o during the boreal summer but change to 210o during the boreal winter. We infer that the variance of the incoming wave direction may be associated with nearby bathymetry, surface pressure variation in season, and long-distance IGW energy.
With the DONET seismograms, we reproduced the evolution of daily excitation locations from 2017 to 2018. We found that IGW energy constantly transports within the Pacific Ocean with the secular atmospheric condition. For the northern Nankai trough location, the IGWs primarily originate from the western coast of South America. Minor energy of IGWs can also be converted from the coast of the Aleutian islands. Furthermore, tropical cyclones will generate local-to-regional event-type excitations that last for a limited time.