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

Through a detailed analysis of seismicity at the base of the transition zone, we obtain an updated value of the maximum reliable depth of confirmed seismicity, we investigate regional variation in the maximum depth of seismicity among those Wadati-Benioff zones which reach the bottom of the transition zone, and we attempt to quantify the maximum possible rate of seismic release in the lower mantle compatible with the failure to detect even a single event since the advent of modern seismological networks. We classify deep 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 (and 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 ! pv +mw transition responsible for the ‘660-km’ seismic discontinuity, which is deflected to greater depths in cold slabs than in warmer ones. We note that this transition marks the depth below which thermal perturbation of phase transitions no longer generates buoyancy anomalies and their large attendant down-dip compressive stresses and below 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 transition zone. Introduction and background The purpose of this paper is to provide enhanced constraints on the cessation of seismicity around 690 km near the base of the transition zone. Our goal is severalfold: (i) to obtain an updated value of the maximum depth of reliably located global seismicity; (ii) to investigate the regional variation in the maximum depth of seismicity among those Benioff zones which do reach the bottom of the transition zone; and (iii) to attempt a quantification of the maximum possible rate of seismic release in the lower mantle which would be compatible with the failure to detect even a single event since the advent of the present seismological networks. We are motivated in this endeavor by the fact that any theory for the origin of deep earthquakes must in particular explain their cessation at the bottom of the transition zone. In this respect, it is important to give as precise a figure as possible for the maximum depth of confirmed seismicity, and to study its variation among slabs. In addition, recent tomographic models generally indicate that at least some slabs do penetrate into the lower mantle (e.g., Van der Hilst et al., 1991, 1997), thus refuting the earlier argument that earthquakes are absent from the lower mantle simply because subduction itself stops at the bottom of the transition zone. The unusual suggestion that earthquakes may actually occur in the lower mantle deserves some discussion. The idea of a global limit to the depth-extent of deep seismicity may, for example, be compared to apparent regional limits on the depth-extent of seismicity. In many areas of the world, seismicity stops at intermediate depths, even though we know from tomographic studies that the slab actually penetrates deeper than the maximum extent of seismicity. However, in several instances, a number of isolated, rare earthquakes have been documented beyond doubt at depths greater than the generally recognized limit of seismic activity in the relevant subduction zone. The most striking example is New Zealand where Adams (1963) and Adams and Ferris (1976) have identified the repeated occurrence of small events around 585 km depth, more than 270 km below the rest of the WadatiBenioff Zone (hereafter WBZ), with a few more such : PIPS Nr.: 163239 BIO2KAP jose66.tex; 18/06/1998; 15:18; v.7; p.1