One Doublet in Two Slabs: The 2018 Mw 8.2 and 7.9 Fiji Deep Earthquakes

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

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 (M_w) 8.2 and 7.9 struck the Fiji area, providing a rare opportunity to interrogate the behaviors of great deep earthquakes in their still enigmatic mechanism. We image the rupture process of the doublet by subevent inversion, where we invert waveforms of teleseismic P, SH, and depth phases pP for multiple subevents’ centroid times, locations, durations, and moment tensors. The subevent model for the M_w 8.2 earthquake shows two stages of rupture on multiple faults that features different focal mechanisms, rupture area and aftershock productivities. The M_w 7.9 event has ~30% non-double-couple component, which is also reflected by the diverse subevent focal mechanisms. By cursory examination, the doublet rupture dimensions and aftershock productivities are similar to the 1994 Bolivia M_w 8.2 earthquake in a warm slab, instead of the 2013 Okhotsk M_w 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 M_w 8.2 ruptured mostly in the warmer rim of the Tonga slab and the M_w 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.

Presenting Author: Zhe Jia

Authors