Centroid-moment-tensor solutions for deep earthquakes predating the digital era: Discussion and inferences

We present a discussion of 139 new centroid-moment-tensor solutions for deep earthquakes covering the WWSSN (1962-1976) and earlier years, obtained by applying the standard Harvard inversion algorithm to data sets of seismograms digitized from analog records. Our solutions fill several gaps in seismicity reflecting undersampling by the Harvard catalogue. While most focal mechanisms previously published for WWSSN-era earthquakes are in good agreement with our inverted solutions, focal geometries for older events appear unreliable. We confirm that the region of most intense moment release is around 650 km, and that a minimum exists at 500 km. However, the local maximum around 450 km may be less marked than previously suggested. The Bonin-Marianas subduction zone exhibits a different pattern with an overall maximum of seismic activity at 420 km. All other subduction zones demonstrate the potential for seismicity at least at the 10 ²⁷ dyn-cm level. Finally, while non-double-couple components are found systematically in our enhanced data set, they correlate neither with depth nor with earthquake size, suggesting that their presence does not necessarily relate to the exiguity of the space available for rupture at the tip of subduction zones.