S-wave anisotropy revealed by airgun seismic surveys using cabled seafloor seismometers in the Nankai Trough, Japan

Abstract:

In the Nankai Trough, Japan, a cabled real-time seismic, geodetic and tsunami observation system, comprising twenty DONET1 seafloor and one IODP C0002G borehole observatories, monitors seismic activity above the seismogenic zone, where mega-thrust earthquakes occur at 100-150 years interval. On Nov. 2013 through June 2015, we conducted three seismic airgun surveys above DONET1 and C0002G seismometers. In this study, we aimed to reveal spatial distribution of S-wave anisotropy, i.e., a proxy of stress state governing the fault dynamics in the subduction zone, from the dataset recorded by the cabled system during these surveys

In these surveys, airgun shootings at circular survey lines around each observatory with different radii of 3, 7 and 10 km were carried out. In the observed horizontal components for each shot from 3km radius lines (R3 lines), seismic waveforms in 3.0 to 5.0s after the shot are clearly visible to indicate the up-coming P-S converted waves from the bottom of the shallow sediment, although signal-to-noise ratio decreases with depth due to spherical divergence and to complicated structure such as dipping reflectors. We computed radial and transverse records for each shot-receiver azimuth, and then estimated S-wave anisotropy parameters, i.e., magnitude and azimuth, as a function of depth for R3 lines at thirteen observatories.

The obtained results confirm that the distributions of S-wave anisotropy, especially in shallow sediments, have localities: the magnitude of S-wave anisotropy becomes smaller in landward basin than in imbricated thrust zone. The axes of fast S-wave symmetry become closer to subduction direction for results from deep reflections. We plan to conduct more attentive analysis, including error evaluation using repeated shooting dataset to discuss the detectability of temporal change of S-wave anisotropy deepen the present discussion after integrating numerical analysis in complicated structure model with the current method.