Through a detailed analysis of seismicity at the base of the transition zon
e, we obtain an updated value of the maximum reliable depth of confirmed se
ismicity, we investigate regional variation in the maximum depth of seismic
ity among those Wadati-Benioff zones which reach the bottom of the transiti
on zone, and we attempt to quantify the maximum possible rate of seismic re
lease in the lower mantle compatible with the failure to detect even a sing
le event since the advent of modern seismological networks. We classify dee
p subduction zones into three groups: those whose seismicity does not reach
beyond 620 km, those whose seismicity appears to terminate around 650-660
km, and Tonga-Kermadec land the 'Vityaz' cluster) whose seismicity extends
to 685-690 km. We suggest that the depth extent of seismicity is controlled
by the depth of the gamma --> pv + mw transition responsible for the '660-
km' seismic discontinuity, which is deflected to greater depths in cold sla
bs than in warmer ones. We note that this transition marks the depth below
which thermal perturbation of phase transitions no longer generates buoyanc
y anomalies and their large attendant down-dip compressive stresses and bel
ow which strain energy generated by other mechanisms may not accumulate to
seismogenic levels due to superplastic weakness in fine-grained materials.
We find that the maximum level of seismic activity in the lower mantle must
be at least three orders of magnitude less than that observed in the trans
ition zone.