Marine Geophysical Investigation of a Fault Ruptured During the 2012 Mw 8.6 Wharton Basin Earthquake

Abstract

The 2012 Mw 8.6 and 8.2 Wharton Basin earthquakes, within an intraplate deformation zone in the Indian Ocean, are the largest instrumentally recorded strike-slip earthquakes on Earth that challenged the prevailing view that great earthquakes occur only at well-defined plate boundaries. Seismological and geodetic observations for the Mw 8.6 earthquake indicate a deep centroid depth (∼25–30 km) and high stress drop (>20 MPa) across a complex network of faults intersecting at high angles, including previously unidentified faults. The rupture of the MW 8.2 event was relatively simple, lying on a single NNE-trending fault located ∼160 km away from the Mw 8.6 epicenter. Because the earthquakes occurred in the deep ocean far from land-based stations, constraints on the fault geometry and rupture propagation remained poor. Using high-resolution bathymetry, multichannel seismic, and sub-bottom profiler data, here we present direct evidence of active deformation along a WNW-ESE oriented fault that ruptured during the Mw 8.6 event and subsequently triggered the Mw 8.2 earthquake. The ∼170 km-long fault exhibits well-preserved surficial lateral offsets since ∼5–7 Ma and shows en-echelon segmentation forming flower structures rooted in the oceanic mantle, indicating distributed deformation. Estimated slip rates of 0.2–0.9 mm/yr imply recurrence intervals of 20–90 Kyr for intraplate earthquakes of Mw ∼8.3 along this fault in the Wharton Basin, while Coulomb stress modeling indicates substantial coseismic loading on the fault that ruptured during the Mw 8.2 event.