A re-assessment of focal depth distributions in southern Iran, the Tien Shan and northern India: do earthquakes really occur in the continental mantle?

We investigate the depth distribution of earthquakes within the continental lithosphere of southern Iran, the Tien Shan and northern India by using sy nthetic seismograms to analyse P and SH body waveforms. In the Zagros mount ains of southern Iran, earthquakes are apparently restricted to the upper c rust (depths of <20 km), whereas in the Tien Shan and northern India they o ccur throughout the thickness of the continental crust, to depths of <simil ar to>40-45 km. We find no convincing evidence for earthquakes in the conti nental mantle of these regions, in spite of previous suggestions to the con trary, and question whether seismicity in the continental mantle is importa nt in any part of the world. In some regions, such as Iran, the Aegean, Tib et and California, seismicity is virtually restricted to the upper continen tal crust, whereas in others, including parts of East Africa, the Tien Shan and northern India, the lower crust is also seismically active, although u sually less so than the upper crust. Such variations cannot reliably be dem onstrated from published catalogue or bulletin locations, even from ones in which depth resolution is generally improved. In contrast to the oceanic m antle lithosphere, in which earthquakes certainly occur, the continental ma ntle lithosphere is, we suggest, virtually aseismic and may not be signific antly stronger than the lower continental crust. These variations in contin ental seismogenic thickness are broadly correlated with variations in effec tive elastic thickness, suggesting that the strength of the continental lit hosphere resides in the crust, and require some modification to prevalent v iews of lithosphere rheology.