Flow fields within the Tonga Slab determined from the moment tensors of deep earthquakes

The seismic strain rates and instantaneous velocity fields within the plane of the Tonga slab are investigated using 450 Harvard centroid moment tensor (CMT) solutions from the period 1977–1992. Down‐dip seismic strain rates at the bottom of the slab are around 1 × 10 −15 s −1 . Spline functions are fitted to the distribution of seismic strain rates to recover the velocity gradient tensor within the plane of the slab. Solutions show a northward motion of the 200 km depth contour in the Tonga slab at rates of 8±2 mm/yr relative to the 670 km depth contour of the slab. Relative to the deep, southern portion of the Tonga slab, down‐dip velocities increase from near zero in southern Tonga to 10±2.5 mm/yr in northern Tonga, consistent with the increased subduction rate from south to north described by the Pacific‐Australia pole of rotation. Moreover, patterns of strain and velocity obtained from all events show a remarkable similarity with the patterns of strain and velocity obtained using only events with scalar moment values less than 1 × 10 25 dyn cm. Along‐strike extensional strain rates in the deep zone of northern Tonga are approximately 0.5 × 10 −15 s −1 . This is of the same order of magnitude as the extensional strain rates associated with changes in curvature produced by the subduction of a spherical plate, suggesting that the seismic strain rates within Tonga represent a large fraction of the total strain rate budget within the seismogenic zone. An estimated maximum of 60% of the relative motion between the surface and 670 km is absorbed as strain within the slab, indicating considerable slab thickening and penetration of the slab into the lower mantle.