| Summary: | Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 46(5), (2019):2434-2448, doi:10.1029/2018GL080997. 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 ∼35 s. 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. We thank the Editor Gavin Hayes and two anonymous reviewers for their helpful comments that improved the quality of the manuscript. The seismic data were provided by Data Management Center (DMC) of the Incorporated Research Institutions for Seismology (IRIS). The facilities of IRIS Data Services, and specifically the IRIS Data Management Center, were used for access to waveforms, related metadata, and/or derived products used in this study. IRIS Data Services are funded through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation under Cooperative Agreement EAR‐1261681. W. F. acknowledges supports from the Postdoctoral Scholar Program at the ...
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