Here is a large aftershock M7 near the island closest to the mainshock M8. USGS site page Here is a summary poster of the tectonics in the region: USGS site page USGS poster (68MB pdf)
M8 in the Santa Cruz Islands region (Vanuatu subduction zone, New Hebrides trench)
M8 Earthquake following a couple weeks of increased seismic activity in the region. located at the northern end of this subduction zone. Here is a map of the subduction zone: Here is the USGS web page for the earthquake. Here…