Direct marine geophysical constraints on the rupture of the 2012 Mw 8.6 Wharton basin earthquake

Current understanding of earthquake rupture and earthquake cycles is largely derived from continental fault systems, implicitly assuming their applicability to oceanic lithosphere. Futhermore, the limited geological and geophysical constraints on large oceanic earthquakes hinder robust assessment of how deformation, fault growth, and stress accumulation takes place in the oceanic lithosphere. The 2012 M_w 8.6 Wharton Basin earthquake, the largest instrumentally recorded strike-slip event, challenged prevailing views of intraplate deformation in the Indian Ocean by rupturing a complex network of faults at high angles to one another. Seismological and geodetic analyses revealed a deep centroid depth, high stress drop, and multi-fault rupture, yet the offshore setting severely limited constraints on fault geometry and rupture propagation. Here, we bridge short- and long-term deformation processes by integrating high-resolution bathymetry, multichannel seismic reflection, and sub-bottom profiler data. We present the surface and near-surface deformation along one of the faults ruptured during the M_w 8.6 earthquake, which runs ESE-WNW and initiates near the epicenter of the M_w 8.2 aftershock. The ~100-km-long fault displays well-preserved dextral offsets accumulated since ~4 - 5 Ma and an en-echelon segmented pattern forming a positive flower structure rooted in the oceanic mantle. We estimate slip rates of ~0.4 to 0.8 mm/yr suggest long recurrence intervals for large intraplate earthquakes. Coulomb stress modelling indicates substantial coseismic stress loading on the N-S fault that subsequently ruptured during the M_w 8.2 earthquake, thus establishing a mechanical relationship between the two events. Overall, our study shows that the oceanic lithosphere can deform slowly and extensively over long time scales, accumulating strain along slow-slipping faults that can produce very large, cascade-style earthquakes. Furthermore, our study offers key inputs for earthquake cycle and dynamic rupture models in oceanic settings by providing geological constraints on fault geometry and slip rates.