The M 7.1 earthquake in the southern Indian Ocean on December 4, 2015, occurred as the result of oblique-normal faulting within the oceanic crust of the Antarctica plate, nearly 700 km south of the Australia: Antarctica plate boundary in this region, which is defined by the Southeast Indian Ridge. At the location of the earthquake, the Antarctica plate is moving approximately southwestwards at the rate of 66 mm/yr with respect to Australia. Focal mechanisms indicate faulting occurred on an oblique-normal faulting structure moderately dipping towards either the west, or the southeast.
Earthquakes away from the Southeast Indian Ridge plate boundary in this region are quite rare; no other events larger than M 5 are known to have occurred within 400 km of the December 4, 2015 event over the previous century. Within 750 km, seismicity is predominantly focused on the ridge, with some off-ridge earthquakes on the southward extensions of the Vlamingh and Geelvinck Fracture Zones to the northwest of the December 4, 2015 event.
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