Complex and Diverse Rupture Processes of the 2018 M w 8.2 and M w 7.9 Tonga-Fiji Deep Earthquakes

Deep earthquakes exhibit strong variabilities in their rupture and aftershock characteristics, yet their physical failure mechanisms remain elusive. The 2018 Mw 8.2 and Mw 7.9 Tonga-Fiji deep earthquakes, the two largest ever recorded in this subduction zone, occurred within days of each other. We investigate these events by performing waveform analysis, teleseismic P wave backprojection, and aftershock relocation. Our results show that the Mw 8.2 earthquake ruptured fast (4.1 km/s) and excited frequency-dependent seismic radiation, whereas the Mw 7.9 earthquake ruptured slowly (2.5 km/s). Both events lasted similar to 35s. The Mw 8.2 earthquake initiated in the highly seismogenic, cold core of the slab and likely ruptured into the surrounding warmer materials, whereas the Mw 7.9 earthquake likely ruptured through a dissipative process in a previously aseismic region. The contrasts in earthquake kinematics and aftershock productivity argue for a combination of at least two primary mechanisms enabling rupture in the region. Plain Language Summary Physical mechanisms of deep earthquakes are poorly understood as their ambient environments inhibit brittle slips, which operate shallow earthquake rupture processes. On 19 August 2018, a moment magnitude 8.2 deep earthquake occurred in Tonga, and 18 days later, another moment magnitude 7.9 deep earthquake occurred about 280 km away. These two events are among the largest deep earthquakes that have ever been recorded. We investigate these two events with a variety of seismological techniques and find that these two earthquakes show distinct rupture characteristics and aftershock productivities. The Mw 8.2 earthquake ruptured fast, whereas the Mw 7.9 earthquake ruptured slowly, despite they both lasted similar to 35s. Our observations show that Tonga can host two types of deep earthquakes with diverse and complex source properties, which is rarely observed. More importantly, our observations suggest that multiple physical mechanisms enabled the rupture propagation for the Mw 8.2 earthquake, and the Mw 8.2 and Mw 7.9 earthquake likely ruptured through different physical processes.