The 2012 Mw 8.6 Indian Ocean earthquake: Deep nucleation on a listric-like fault

Abstract

The 2012 Mw 8.6 Indian Ocean earthquake, the largest instrumentally recorded strike-slip event, has been described as a westward rupture on orthogonal faults that are part of the original oceanic fabric of the region. Reported estimates of its depth vary from 10 to 45 km, while the maximum depth of its inferred rupture extends to ~50–60 km. The complexity of the earthquake is undeniable both in terms of fault geometry and rupture propagation, with debates on the geometry of the causative faults. Results from recent seismic surveys enable a fresh look at the earthquake fault geometry, nucleation depths, and lithospheric rheology. In the present study, these new findings are incorporated into 22 forward models using a revised velocity structure and N-S geometries that flatten with depth. The rupture initiation is modeled in the uppermost mantle (25–30 km) as well as in the upper mantle at ~45 km, the latter of which is 15–20 km below the commonly accepted values of maximum depth for frictional failure. Our study shows that the westward directionality of wave propagation is achieved for strike-slip failure on a listric-like fault, in contrast to subvertical geometries as previously suggested. This propagation is persistent for a strike-slip failure only for events nucleating at upper mantle depths (~45 km) rather than for those set to originate at depths between 25 and 30 km. However, for a dip-slip failure the westward propagation is found to be consistent at both nucleation depths on a listric-like fault. This study highlights a deep nucleation within the lithospheric mantle for the 2012 Mw 8.6 earthquake that could be due to strain localization induced by fluid-rock interaction and/or mineral transformation/crystallization near the 680 °C isotherm at ~45 km depth (≈1.4 GPa), which correlates with deep serpentinization on faults.