[S03-3] Imaging of heterogeneities in the Earth with seismic scattered waves and ambient noise III

[S03-3-04] Dominant source locations of secondary microseisms in Japan estimated by Hi-net data

^○Ryota Takagi¹, Kiwamu Nishida² (1.Graduate School of Science, Tohoku University, 2.Earthquake Research Institute, The University of Tokyo)

Microseisms are energetic ambient seismic wavefield generated by ocean swells, which are categorized into primary (10-14 s) and secondary microseisms (5-7 s). Although source locations are crucial for understanding the excitation mechanisms of microseisms, source locations of secondary microseisms still remain uncertain. Moreover, understanding microseism wavefield is important to correctly interpret results of seismic interferometry. In the present study, we estimate dominant source locations of Rayleigh waves in secondary microseisms observed in the Japan islands using Hi-net data. We first estimate back azimuths of Rayleigh waves in the period of 4-8 s based on polarization analysis. Since fundamental Rayleigh waves, dominating secondary microseisms, generally show retrograde particle motions, back azimuth of Rayleigh waves can be determined from three component records at single stations. We then search locations explaining the back azimuth distributions within subarrays in the Japan island. The estimated sources of Rayleigh waves distribute in specific regions. Dominant locations are 100-200 km off the coast of Fukushima and Miyagi in the Pacific, off Tottori in the Sea of Japan, and off the the Shonai Plain. The off Tottori sources show a clear seasonal variation, detected only in the winter season, which is consistent with ocean wave activity in the Japan Sea predicted by an ocean action model WAVEWATCH III. The off Tottori and off Fukushima-Miyagi sources are located at an ocean basin with the depth of 1000-2500 m and at shelf slope with the ocean depth of 2000-6000 m, respectively. The ocean depths are close to the resonance depth for the period of 4-8 s. The off Shonai sources are located in the regions in shallower ocean depth. We consider investigating frequency dependence of source locations may deepen our understanding of mechanism of microseisms.