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

Abstract

We improve constraints on the slip distribution and geometry of faults involved in the complex, multisegment, M w 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.

The 2 March 2016 Wharton Basin Mw 7.8 earthquake: High stress drop north‐south strike‐slip rupture in the diffuse oceanic deformation zone between the Indian and Australian Plates

“…The M w 8.6 earthquake is the largest recorded intraplate event and also the largest recorded strike-slip event. Hill et al [2015] estimate an average stress drop of 17 MPa for this event. The Wharton Basin extends southeast of these events and has experienced other large, predominantly strike-slip earthquakes such as the 18 June 2000 M w 7.9 earthquake (Figure 1) west of the Investigator Fracture Zone [Robinson et al, 2001;Abercrombie et al, 2003].…”

“…The 11 April 2012 M w 8.6 and 8.2 great strike-slip earthquakes ( Figure 1) ruptured a mix of north-south and east-west trending strike slip faults in the Cocos Basin (north Wharton Basin) [e.g., Duputel et al, 2012;Satriano et al, 2012;Meng et al, 2012;Yue et al, 2012;Ishii et al, 2013;Wei et al, 2013;Hill et al, 2015], and there are many other strike-slip events along the outer rise ( Figure S1). The M w 8.6 earthquake is the largest recorded intraplate event and also the largest recorded strike-slip event.…”

Co-seismic offsets due to two earthquakes (M w 6.1) along the Sumatran fault system derived from GNSS measurements

“…Seismicity map is based on BMKG (Agency for Meteorology, Climatology and Geophysics) database (lower detection limit is approximately M w 4.5). b Coseismic offsets resulting from the April 11, 2012 earthquake derived from AGNeSS (blue arrows) and two previous studies (gray arrows) (Yadav et al 2013;Hill et al 2015). Ishii et al 2013).…”

“…Ishii et al 2013). Previous studies (Yadav et al 2013;Hill et al 2015) reported co-seismic offsets around the Andaman-Nicobar GNSS network, which includes the Sumatran GNSS Array (SuGAr) and IGS sites. The AGNeSS also observed horizontal co-seismic offsets ( Table 1).…”

“…However, the vertical offsets were very small (∼1 cm); thus, we reported only the horizontal offsets at the AGNeSS site. Figure 1b shows the horizontal displacements from the GNSS sites, which consisted of AGNeSS (blue arrows), SuGAr, and IGS sites (gray arrows) (Yadav et al 2013;Hill et al 2015). These offsets contained the effects of both the M w 8.6 sequence and the M w 8.2 aftershock.…”