Abstract

Increasing numbers of confirmed continental sub-Moho earthquakes beneath the Himalaya and Tibetan plateau have been taken to imply a brittle, seismogenic, uppermost mantle. However, differences between 'Moho' (a wavespeed/density boundary) and 'crust-mantle boundary' (a petrological distinction) complicate interpretation of these earthquakes. Applying the S-minus-P method to the Hi-CLIMB-2D array in south Tibet, we identify nine new earthquakes below the Moho as previously interpreted from spatially-smoothed receiver-function images at ~60 km, but find most lie above an ~80-km converter on our new CCP images. An ~100-km converter is below all these nine earthquakes and possibly beneath all sub-Moho earthquakes here. If the previously interpreted Moho represents the crust-mantle boundary, these sub-Moho earthquakes imply a brittle upper mantle. We suggest the 80-km converter is more appropriately interpreted as Moho, and as marking the (petrological) crust-mantle boundary, so that Himalayan sub-Moho earthquakes may be restricted to eclogitized Indian lower crust. Reversing recent understanding, beneath the Himalaya the deep crust may be stronger (more seismogenic) than ultramafic upper mantle.