[S-SS27_29PO1] Seismic wave propagation: Theory and Application

[SSS27-P05] Ocean acoustic Rayleigh wave persistently excited by earthquake signals

*Takashi TONEGAWA¹, Yoshio FUKAO¹, Tsutomu TAKAHASHI¹, Koichiro OBANA¹, Shuichi KODAIRA¹, Yoshiyuki KANEDA¹ (1.JAMSTEC)

Keywords:acoustic Rayleigh wave, ambient noise, correlation analysis

In the interferometry, the wavefield propagating between two positions can be retrieved by correlating ambient noise recorded on the two positions. This approach is useful for applying to various kinds of wavefield, such as ultrasonic, acoustic (ocean acoustic), and also seismology. Off the Kii Peninsula, Japan, more than 150 short period (4.5 Hz) seismometers, in which hydrophone is also cosited, had been deployed for 2 months on 2012 by Japan Agency for Marine-Earth Science and Technology (JAMSTEC) as a part of “Research concerning Interaction Between the Tokai, Tonankai and Nankai Earthquakes” funded by Ministry of Education, Culture, Sports, Science and Technology, Japan. In this study, correlating ambient noise recorded on the hydrophones, we attempt to investigate characteristics of wavefield observed at seafloor. The observation period is from Sep. 2012 to Dec. 2012. Station spacing is around 5 km. For 5 lines off the Kii Peninsula, the 30‑40 seismometers are distributed at each line. Sampling interval is 200 Hz for both seismometer and hydrophone. The instruments are located at 100‑4800 m in water depth. In the processing for the both records, we applied a bandpass filter of 1‑3 Hz, replaced the amplitude to zero if it exceeds a value that was set in this study. We calculated cross correlation function (CCF) by using continuous records with a time length of 600 s, stacked the CCFs over the whole observation period. We first aligned only CCFs using two stations with a separation distance of 5 km along lines off Kii Peninsula. As a result, we could detect strong signals in the CCFs that clearly show travel time variation as a function of water depth. The group velocity of the signal gradually changes from 1.3 km/s to 0.7 km/s at water depths from 2000 to 4000 m. In addition to the wave, a relatively weak signal with a group velocity of 1.4‑1.5 km/s can be seen in the region at water depth of 4,000 m. We investigated the wavefield by using a numerical simulation with finite difference technique. As a result, all of these signals can be explained by acoustic Rayleigh wave, which has the energy within not only the ocean but also sediment. A case in which vertical forces are located at subseafloor generated the acoustic Rayleigh wave well, and the CCFs using synthetic waveforms match well with the observed ones. However, another one in which vertical forces are located at sea surface failed to describe the observation. This means that the observed acoustic Rayleigh wave in background wavefield would be generated by earthquake signal, not signals due to microseisms. Moreover, we will show that the amplitude of the signals possibly correlates with seismicity distribution, which also supports that the signals are excited by earthquake signals.