[S-SS04] Seismological advances in the ocean

[SSS04-04] Seismic imaging with teleseismic waves and ambient noises observed by a submarine cable in the Nankai subduction zone with distributed acoustic sensing

*Takashi Tonegawa¹, Eiichiro Araki¹ (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:DAS, Submarine cable, Seismological structure, Surface wave, Receiver function

Using ambient noise records obtained by distributed acoustic sensing (DAS) techniques, fine-scaled seismic structures, particularly for S-wave velocity (Vs), have been estimated by measuring phase velocities of surface waves at multiple segments of cables. However, because wavelengths of surface waves at low frequencies are typically larger than lengths of the cable segments, it is difficult to constrain the velocity structure at deeper depths. Here, we measured the phase velocity of surface waves at frequencies of 0.02–0.5 Hz, and examined the travel time tomography along the submarine cable. To measure the phase velocity at frequencies of 0.1–0.5 Hz, we examined the method introduced by Liu (2020). We applied ambient noise correlations for three stations along a cable where one is the virtual source and other two are receivers, used different narrow frequency bands to the obtained cross-correlation functions, and measured the frequency-dependent differential travel times of surface waves propagating between the two receivers. For low frequency components at 0.02–0.1 Hz, we measured the differential travel times of surface waves from teleseismic events between two stations along the cable. This technique is equivalent to the one often used in global seismology. The obtained results show low velocity anomalies at shallow depths in the regions where the water depths are relatively deep.
Moreover, we tried to detect P-to-s (Ps) converted waves by cross-correlating P waves in the vertical component at a land Hi-net station with Ps waves emerged in the horizontal component at every channel along the cable. Using the velocity model from the surface wave analysis, we converted the time-domain cross-correlation functions to the depth-domain ones. As a result, we imaged dipping positive amplitudes corresponding to the top of the oceanic crust within the subducting Philippine Sea Plate.

Acknowledgement
This work was supported by JSPS KAKENHI Grant No. JP21H05202, JP21H05204 in Scientific Research on Transformative Research Areas “Science of Slow-to-Fast earthquakes”. We used records from Hi-net (DOI:10.17598/NIED.0003) which is operated by National Research Institute for Earth Science and Disaster Resilience (NIED).