The 2018 Fiji Mw 8.2 and 7.9 deep earthquakes: One doublet in two slabs

The cold Fiji-Tonga subduction zone accounts for >75% of cataloged deep earthquakes but none of the largest ten in the last century. On 19 August 2018 and 06 September 2018, a deep earthquake doublet with moment magnitude (Mw) 8.2 and 7.9 struck the Fiji area, providing a rare opportunity to interrogate the behaviors of great deep earthquakes in cold slabs. While the aftershock productivity of the 2018 Mw 8.2 event is similar to the 2013 Okhotsk Mw 8.3 event in a cold slab, the inferred compact rupture dimensions of both the Mw 8.2 and 7.9 events appear to be similar to the 1994 Bolivia earthquake in a warm slab. This seems to contradict the traditional view that slab temperature controls deep earthquakes. However, we find that neither event was confined within the cold Tonga slab core: the Mw 8.2 ruptured mostly in the warmer rim of the Tonga slab and the Mw 7.9 occurred in a warm relic slab leaning on top of the Tonga slab. The Fiji doublet demonstrates local slab temperature as the critical factor for deep earthquakes, and reveals complex interaction of subducted slabs in Tonga.