Seismic evidence of a two-layer lithospheric deformation in the Indian Ocean.

Yanfang Qin,Satish C Singh

doi:10.1038/ncomms9298

Abstract

Intra-plate deformation and associated earthquakes are enigmatic features on the Earth. The Wharton Basin in the Indian Ocean is one of the most active intra-plate deformation zones, confirmed by the occurrence of the 2012 great earthquakes (Mw≥8.2). These earthquakes seem to have ruptured the whole lithosphere, but how this deformation is distributed at depth remains unknown. Here we present seismic reflection images that show faults down to 45 km depth. The amplitude of these reflections in the mantle first decreases with depth down to 25 km and then remains constant down to 45 km. The number of faults imaged along the profile and the number of earthquakes as a function of depth show a similar pattern, suggesting that the lithospheric mantle deformation can be divided into two layers: a highly fractured fluid-filled serpentinized upper layer and a pristine brittle lithospheric mantle where great earthquakes initiate and large stress drops occur.

Highlights

Intra-plate deformation and associated earthquakes are enigmatic features on the Earth
This was recently confirmed by the occurrence of the Mw 1⁄4 8.6 earthquake on April 11, 2012 in the Wharton Basin (WB), the largest strike-slip earthquake ever observed on the Earth
A similar behaviour is observed for a number of earthquakes originating in the mantle. These results suggest that the deformation in the mantle consists of two layers, an upper serpentinite layer (SL) where a large number of earthquakes occur and a pristine layer (PL) where great earthquakes initiate and large stress drops occur

Summary

Introduction

Intra-plate deformation and associated earthquakes are enigmatic features on the Earth. The Wharton Basin in the Indian Ocean is one of the most active intra-plate deformation zones, confirmed by the occurrence of the 2012 great earthquakes (MwZ8.2) These earthquakes seem to have ruptured the whole lithosphere, but how this deformation is distributed at depth remains unknown. To the east of the NER in the WB, the direction of the maximum stress is NW-SE, and the deformation is accommodated along N5°E-trending re-activated fracture zones with left-lateral strike-slip movements[2,5,10,11] This was recently confirmed by the occurrence of the Mw 1⁄4 8.6 earthquake on April 11, 2012 in the WB, the largest strike-slip earthquake ever observed on the Earth. We describe the shallow deformation and their link with deep mantle faults, and shed light upon the nature of the lithospheric deformation in the WB

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Conclusion