2:15 PM - 2:30 PM
[SCG54-09] Changes in seismic wave velocity during brittle deformation of gabbro and peridotite
Keywords:Peridotite, Gabbro, Seismic wave velocity, Brittle deformation, Dilatancy, Microcrack
We used gabbro collected from the Oman ophiolite, and peridotite collected from the Horoman Complex. Triaxial deformation experiments were performed with intra-vessel deformation and fluid flow apparatus at room temperature, a constant strain rate of ~10-6 s-1, and a constant confining pressure of 20 MPa. Compressional and shear waves traveling in a direction normal to the loading axis were measured during deformation, where shear waves were polarized normal and parallel to the loading axis. Seismic wave velocities were determined by a pulse transmission method, in which travel times of seismic waves through the sample were measured.
In gabbros, both Vp and Vs markedly decreased (Vp: ~40%, Vs: ~30%) as the samples approache failure. Crack density inverted from the velocities showed anisotropic crack orientation, where microcracks parallel to the loading axis (axial microcracks) were dominantly created. These results correspond to strain data and microstructural observation of recovered samples after deformation (Akamatsu et al., 2019). On the other hand, in peridotites, a small decrease in Vp and Vs (<20%) were observed up to the maximum stress, and inverted crack density showed no significant increase in axial microcracks. This indicates that microcracks developed in peridotites were mainly “mode II” shear microcracks, corresponding to strain data and microstructural observation. These contrasting evolutions of seismic wave velocity and crack density between gabbro and peridotite during brittle deformation can modify the seismic discontinuity and reflectivity at the Moho, and contribute to the variation of the Moho in the oceanic plate.