Are there earthquakes in the continental mantle?
What is it about?
In this paper, we reviewed and recalculated the depths of earthquakes in three continental regions: southern Iran, the Tien Shan (a mountain range in central Asia), and northern India. These areas had been studied over 15 years earlier to infer which parts of the continental crust and mantle were brittle enough to cause earthquakes. We found an unexpected result: instead of confirming the prevailing theory – that earthquakes should occur in the upper crust and the shallow continental mantle but not in the lower crust – the depths of earthquakes in these regions contradicted it, as they lay in both the upper and lower crust and not at all in the mantle.
Why is it important?
This paper sparked a decade-long scientific debate on the relative strengths of the upper crust, lower crust, and upper mantle in the continents. Two end-member models struggled for supremacy: the "jelly-sandwich," featuring a brittle upper crust and upper mantle and a ductile lower crust; and the "crème brulée," featuring a brittle upper (and sometimes lower) crust and a ductile upper mantle. Neither survived the ensuing war unscathed, but the debate did focus attention how Earth structure influences its dynamic behavior. The earthquake depth observation itself stood its ground despite many subsequent studies.