The 2012 Mw 8.6 Wharton Basin sequence: A cascade of great earthquakes generated by near‐orthogonal, young, oceanic mantle faults

Abstract

AbstractWe improve constraints on the slip distribution and geometry of faults involved in the complex, multisegment, Mw 8.6 April 2012 Wharton Basin earthquake sequence by joint inversion of high‐rate GPS data from the Sumatran GPS Array (SuGAr), teleseismic observations, source time functions from broadband surface waves, and far‐field static GPS displacements. This sequence occurred under the Indian Ocean, ∼400 km offshore Sumatra. The events are extraordinary for their unprecedented rupture of multiple cross faults, deep slip, large strike‐slip magnitude, and potential role in the formation of a discrete plate boundary between the Indian and Australian plates. The SuGAr recorded static displacements of up to ∼22 cm, along with time‐varying arrivals from the complex faulting, which indicate that the majority of moment release was on young, WNW trending, right‐lateral faults, counter to initial expectations that an old, lithospheric, NNE trending fracture zone played the primary role. The new faults are optimally oriented to accommodate the present‐day stress field. Not only was the greatest moment released on the younger faults, but it was these that sustained very deep slip and high stress drop (>20 MPa). The rupture may have extended to depths of up to 60 km, suggesting that the oceanic lithosphere in the northern Wharton Basin may be cold and strong enough to sustain brittle failure at such depths. Alternatively, the rupture may have occurred with an alternative weakening mechanism, such as thermal runaway.