3:00 PM - 4:00 PM
[S21-P-07] Effects of random heterogeneity in the upper mantle on apparent radial anisotropy
Recent progress in seismic tomography and numerical simulations of seismic wave propagation have revealed the nature of heterogeneity and anisotropy in the upper mantle on a variety of scales. Surface wave tomography at lower frequency (5-50 mHz) have shown strong radial anisotropy with faster SH wave speeds than SV in the asthenosphere, particularly in the fast drifting oceanic and continental plates (e.g., Nettles & Dziewonski, 2008; Yoshizawa, 2014). Also, the existence of rapid vertical change of anisotropic properties has been found in continental lithosphere (Yoshizawa & Kennett, 2015), whose depth coincides well with the enigmatic Mid-Lithosphere Discontinuity from receiver functions.
At higher frequency (-10Hz), recent observations of scattered body waves require the existence of horizontally elongated small-scale heterogeneity in the lithosphere (e.g., Kennett & Furumura, 2008, 2013). The effects of such small-scale heterogeneities (correlation distance of kilometer-scale) have generally been ignored in surface wave propagation, whose wavelength is about several hundred kilometers. Such small-scale heterogeneity can, however, cause non-negligible influence on surface-wave phase speeds by changing the effective rigidity of the media and generating “apparent" radial anisotropy (SH>SV).
In this study, we investigate the effects of small-scale heterogeneity on the long-period surface waves through numerical experiments with normal mode calculations as well as 2-D finite-difference method simulations incorporating finely quasi-laminated heterogeneity in the upper mantle. We calculated dispersion curves using a bunch of 1-D velocity profiles including random heterogeneity in the upper mantle. The results suggest that the fine-scale heterogeneity makes the phase speeds of Rayleigh-wave slower, while those of Love-wave faster to some extent, generating apparent radial anisotropy, even though no effect of anisotropy is imposed on the velocity profiles.
At higher frequency (-10Hz), recent observations of scattered body waves require the existence of horizontally elongated small-scale heterogeneity in the lithosphere (e.g., Kennett & Furumura, 2008, 2013). The effects of such small-scale heterogeneities (correlation distance of kilometer-scale) have generally been ignored in surface wave propagation, whose wavelength is about several hundred kilometers. Such small-scale heterogeneity can, however, cause non-negligible influence on surface-wave phase speeds by changing the effective rigidity of the media and generating “apparent" radial anisotropy (SH>SV).
In this study, we investigate the effects of small-scale heterogeneity on the long-period surface waves through numerical experiments with normal mode calculations as well as 2-D finite-difference method simulations incorporating finely quasi-laminated heterogeneity in the upper mantle. We calculated dispersion curves using a bunch of 1-D velocity profiles including random heterogeneity in the upper mantle. The results suggest that the fine-scale heterogeneity makes the phase speeds of Rayleigh-wave slower, while those of Love-wave faster to some extent, generating apparent radial anisotropy, even though no effect of anisotropy is imposed on the velocity profiles.