An analysis of split shear waves observed above crustal and uppermost mantle earthquakes beneath Shikoku, Japan: Implications in effective depth extent of seismic anisotropy


  • Satoshi Kaneshima,

  • Masataka Ando


Shear wave splitting has been detected in three-component seismograms from a number of small earthquakes taking place either within the upper crust or the uppermost mantle beneath the Shikoku area, Japan. The faster shear waves are polarized nearly in E-W at the stations in the central part of Shikoku, and in NEN-SWS at the southern stations. The arrivals of slower shear waves polarized orthogonally to the faster ones are also clearly recognized in many seismograms at one of the stations. The faster shear wave polarizations at the stations in the central part of Shikoku and the pattern of travel time differences between split shear waves are successfully explained in terms of crustal anisotropy generated by vertical alignment of stress-induced microcracks. The difference in polarization directions suggests that the tectonic stress state beneath the southern Shikoku region is significantly different from that beneath the central part of Shikoku. A quantitative analysis of travel time differences between split shear waves suggests that the presence of seismic anisotropy which causes the observed shear wave splitting is limited to the upper 10 or 15 km of the crust. Nevertheless, the possibility that anisotropy is concentrated in a much thinner surface layer cannot be ruled out. The average aspect ratio of crack might be greater than of the order of 10−2, or the cracks may not be purely vertical but have some fluctuations in the dip angle.