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

Abstract

SUMMARY We investigate the depth distribution of earthquakes within the continental lithosphere of southern Iran, the Tien Shan and northern India by using synthetic seismograms to analyse P and SH body waveforms. In the Zagros mountains of southern Iran, earthquakes are apparently restricted to the upper crust (depths of <20 km), whereas in the Tien Shan and northern India they occur throughout the thickness of the continental crust, to depths of ∼40–45 km. We find no convincing evidence for earthquakes in the continental mantle of these regions, in spite of previous suggestions to the contrary, and question whether seismicity in the continental mantle is important in any part of the world. In some regions, such as Iran, the Aegean, Tibet and California, seismicity is virtually restricted to the upper continental crust, whereas in others, including parts of East Africa, the Tien Shan and northern India, the lower crust is also seismically active, although usually less so than the upper crust. Such variations cannot reliably be demonstrated from published catalogue or bulletin locations, even from ones in which depth resolution is generally improved. In contrast to the oceanic mantle lithosphere, in which earthquakes certainly occur, the continental mantle lithosphere is, we suggest, virtually aseismic and may not be significantly stronger than the lower continental crust. These variations in continental seismogenic thickness are broadly correlated with variations in effective elastic thickness, suggesting that the strength of the continental lithosphere resides in the crust, and require some modification to prevalent views of lithosphere rheology.

Intermediate-Depth Earthquakes in a Region of Continental Convergence: South Island, New Zealand

“…Since only small earthquakes have occurred, they cannot be observed in teleseismic data. Thus, any conclusions regarding the depth distribution of seismicity based only on teleseismically observed intermediate and large earthquakes may need to be re-evaluated (Maggi et al, 2000). The Gutenberg-Richter relation that predicts greater magnitudes may be truncated at a cut-off magnitude corresponding to the maximum dimension of the brittle zone Richter, 1941, 1944 Figure 6.…”

Why is Africa rifting?

“…In contrast, oceanic lithosphere is stronger because it lacks a thick quartz-rich crust, which can explain why rifting is more common in the continental lithosphere (Vink et al 1984). This model is not universally accepted and arguments have been made that the strength of the lithosphere is controlled by a single seismogenic crustal layer (Maggi et al 2000;Jackson 2002). The strength envelopes will be strongly affected by the amount of water in the lower crust and upper mantle.…”

Crustal-scale cross-sections across the NW Zagros belt: implications for the Arabian margin reconstruction

“…The involvement of basement in the Zagros deformation is largely proved by seismicity occurring along reverse faults (e.g. Maggi et al 2000;Talebian & Jackson, 2004;Tatar, Hatzfeld & GhaforiAshtiany, 2004; (Fig. 1).…”