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.
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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.
Perspectives
When my thesis advisor gave me my first "simple" task of reviewing and recalculating the depths of earthquakes in these particular regions, I had no inkling of its ramifications. At first, when my depth distributions didn't match the established picture, I questioned my results and went back to find the bug. When, after checking and double-checking everything, the results still held true, I finally allowed myself to wonder "Wow, maybe the established picture is wrong..." It was exhilarating. I can still feel the excitement of sitting on the floor surrounded by the outputs of all the earthquake depth inversions and coming to the conclusion that I might hold the key to something meaningful. It's not something I've often experienced since then, but it hooked me on research and led to my choosing an academic career.
Dr Alessia Maggi
University of Strasbourg
Read the Original
This page is a summary of: A re-assessment of focal depth distributions in southern Iran, the Tien Shan and northern India: do earthquakes really occur in the continental mantle?, Geophysical Journal International, December 2000, Oxford University Press (OUP),
DOI: 10.1046/j.1365-246x.2000.00254.x.
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