In April 2012, a Mw 8.6 earthquake—the largest strike-slip and the largest intraplate earthquake ever recorded—occurred beneath the Indian Ocean about 400 kilometers off the shore of Sumatra, Indonesia. This event was part of a sequence that struck the Wharton Basin, where the seafloor fabric is dominated by large, left-lateral fracture zones from an Eocene age spreading center oriented north-northeast. Although researchers initially assumed that the main shock had ruptured one of these deep, old structures, several studies have since reported a cascading failure of multiple faults that included displacement (or “slip”) along a number of west-northwest trending, right-lateral faults presumed to be much younger. Which faults carried the majority of the slip, however, is a topic of debate.
To tackle this problem, Hill et al. improved constraints on the slip distribution and geometry of the faults involved in this earthquake sequence. Their study is the first to include near-field, high-rate GPS observations in a joint inversion with teleseismic data, source time functions from broadband surface waves, and far-field static GPS displacements. The team found that most of the energy release occurred on the younger, west-northwest trending faults, which are optimally oriented to accommodate the present-day stress field.
The results also demonstrate that the rupture extended to depths of at least 60 kilometers, about twice as deep as the expected limit for seismic rupture based on estimations of how much temperature increases with depth. The scientists were able to identify possible explanations for why the greatest and deepest slip—as well as the biggest relief in stress—occurred on the west-northwest trending faults. It may be a combination of strong lithosphere between the fracture zones and the deep brittle-ductile transition, where warmer temperatures allow rocks to creep and flow rather than fracture. It may also be due to an alternative failure mechanism like thermal runaway, a positive feedback of constantly increasing temperature.
The results of this study have implications for identifying the poorly constrained plate boundary between the Australian and Indian plates within the Indian Ocean. According to the authors, the location of this earthquake sequence and the evidence for deep rupture of the younger faults indicate that this region may be experiencing the formation of a new boundary between these plates. The fact that multiple faults ruptured during this sequence may be an indication that no large, single fault exists to relieve the stress created by the relative plate motion—yet. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2014JB011703, 2015)
—Terri Cook, Freelance Writer
Citation: Cook, T. (2015), Wharton Basin earthquakes: Evidence for a new plate boundary?, Eos, 96, doi:10.1029/2015EO039185. Published on 11 November 2015.