12:00 PM - 12:15 PM
[SCG60-05] The mantle anisotropy obtained from shear-wave splitting in the region of 1891 Nobi earthquake
Keywords:mantle, Splitting, Nobi earthquake
Shear-wave splitting is usually expressed by two parameters, the fast polarization azimuth φ (in degrees), and the time-lag τ (in seconds), which is the delay time between the fast and slow components of a shear wave. We use the following techniques to determine the shear-wave splitting. We pick S waves from individual seismograms. We calculated the cross-correlation of the two horizontal seismogram components over a grid -90o to +90o for φ and a window of 4 sec for τ with increments of 1o and 0.01 sec., respectively. The time-lag τ (in seconds) and the fast polarization azimuth φ (in degree) are defined to be the values that yielded the maximum correlation.
The results suggested very interesting and remarkable pattern of the polarization directions. The northeastern part of the research area showed polarization direction of NE-SW. The eastern part of the research area suggested that the polarization directions are almost E-W. However, the polarization direction at the 1891 Nobi earthquake area was NW-SE. Most of the averaged time lag values are larger than 0.5 sec. The value is much larger than that of the maximum time-lag value cause by crustal anisotropy in this area (Hiramatsu et al. submitted to EPS). The maximum crustal anisotropy was considered to be less than 0.1 sec. We consider that the observed shear-wave splitting was caused by the mantle anisotropy. The shear-wave splitting of the mantle anisotropy was studied by previous studies. The polarization direction with E-W at the eastern part of the research and the polarization direction with NE-SW are consistent with the results of previous studies (e.g., Ando et al. 1983, Iidaka et al., 2009). The polarization direction can be explained by the preferred orientation cause by the mantle flow relating to the subducting Pacific and Philippine Sea plates. However, the polarization direction with NW-SE is not consistent with both of the subduction directions of oceanic plates. The cause of anisotropy is considered based on the two models. One is the model that the shear-wave splitting is caused by the heterogeneous structure which is related to the fluid or magma in the mantle wedge. The other model is that the shear-wave splitting is cause by some irregular mantle flow. The cause of the polarization direction will be discussed based on other observations.