On the cessation of seismicity at the base of the transition zone

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.