Spatially dependent seismic anisotropy in the Tonga subduction zone: A possible contributor to the complexity of deep earthquakes

The deep part of the Tonga subduction zone consists of two differently oriented slab segments: the northern segment within latitudes 17-19°S, and the southern segment within latitudes 19.5-27°S. The orientation of the slab is (strike/dip): 110°/57° in its northern part and 210°/46° in its southern part. Both segments are seismically active at depths from 500 to 700 km. The mechanisms of deep-focus earthquakes reported in the Harvard moment tensor catalogue contain compensated linear vector dipole (CLVD) components that behave differently in both segments. The mean value of the CLVD is 3% for the northern segment but −10% for the southern segment. The mean absolute value of the CLVD is 12% for the northern segment and 16% for the southern segment. The complex behaviour of the CLVD is explained by spatially dependent seismic anisotropy in the slab. The inversion for anisotropy from the non-double-couple components of moment tensors points to orthorhombic anisotropy in the both slab segments. The anisotropy seems to have a uniform strength of 5-7% for P-waves and 9-12% for S-waves, and is oriented according to the orientation of each segment and the stress acting in it. The spatial variation of velocities is roughly similar in both segments. The retrieved anisotropy might have several possible origins. It can be: (1) intrinsic, caused by preferentially aligned anisotropic minerals such as wadsleyite, ringwoodite, ilmenite or others, (2) effective, caused, for example, by intra-slab layering, or (3) partly apparent, produced by systematic errors in the moment tensors due to neglecting 3D slab geometry and the slab/mantle velocity contrast when calculating the Green functions in the moment tensor inversion.