Zhe Jia, Zhichao Shen, Zhongwen Zhan, Chenyu Li, Zhigang Peng, Michael Gurnis

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. By cursory examination, the doublet rupture dimensions and aftershocks are similar to the 1994 Bolivia Mw 8.2 earthquake in a warm slab, instead of the 2013 Okhotsk Mw 8.3 event in a cold slab. This appears 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.