The February 6th, 2013 M 8.0 earthquake in the Santa Cruz Islands occurred as a result of shallow thrust faulting on or near the plate boundary interface between the Australia and Pacific plates. In the region of this earthquake, the Australia plate converges with and subducts beneath the Pacific plate, moving towards the east-northeast at a rate of approximately 94 mm/yr. 

The February 6th earthquake is located approximately 700-750 km ESE of the Mw 8.1 Solomon Islands earthquake of April 1, 2007, and the Mw 7.1 Solomon Island earthquake of January 3, 2010. It is over 900 km to NNW of the February 2, 2012 Mw 7.1 Vanuatu earthquake, and approximately 200-300 km north of a series of earthquakes in October, 2009 along the Vanuatu Trench that included two earthquakes larger than magnitude 7 (Mw 7.8, Mw 7.4). 

This earthquake is located adjacent to a complex section of the Australia-Pacific plate boundary, where the Solomon Trench to the west is linked to the New Hebrides (Vanuatu) Trench to the south by a short segment of dominantly strike-slip plate motion. The February 6th earthquake is located at the northern end of the New Hebrides (Vanuatu) segment. To the north and west of this event, the plate boundary changes in character, is oriented more west-to-east, and connects the segment ruptured by this event with the continuation of the subduction zone along the Solomon Islands.

Over the month leading up to the February 6th earthquake, there have been dozens of earthquakes in the epicentral region – over 40 M4.5 or larger in the preceding 7 days alone, 7 of which were larger than M6. Faulting mechanisms for these earthquakes suggest a mixture of strike-slip, normal and thrust faulting events. Within an hour of the February 6th M8.0 mainshock, there were also two large aftershocks with magnitudes greater than M6. See a map of current aftershocks here.

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This figure highlights the activity in the region of the February 6th, 2013 M8.0 earthquake over the week leading up to the event. Several earthquakes of M6 or larger, and dozens of smaller shocks, occurred in the same region over that time period. The upper panel plots the magnitude of these events versus time. Their locations and USGS W-phase moment tensors are plotted in the map below, colored by depth. Historic seismicity (all events prior to January 30th, 2013) from the EHB and USGS PDE catalogs are plotted with white circles, sized by magnitude. Slab contours for the Vanuatu slab, from the USGS Slab1.0 database, are shown with dashed lines, at 20 km intervals. (Click on image for larger version.)

This figure illustrates the dramatic transition in tectonics around the bend in the plate boundary adjacent to the February 6th, 2013 M8.0 earthquake. South of this earthquake (upper panels and cross-section A-B), the Australia plate subducts beneath the Pacific at the New Hebrides (Vanuatu) Trench. The location of the February 6th earthquake (center panels, cross-section C-D. 02-06-13 earthquake marked as a star) is very close to what could be considered the edge of the Vanuatu slab. Further west (lower panels, cross-section E-F), the plate boundary is oriented approximately W-E, plate motion is nearly parallel to the boundary, and fault motion is dominantly strike-slip.

In each map, historic seismicity from the EHB and USGS PDE catalogs are plotted with white circles, sized by magnitude. Gray circles are highlighted in cross-sections to the left, where moment tensors from the global CMT catalog are also plotted for larger events.

More information on regional seismicity and tectonics