It has been unclear whether the 1993, 2001, and 2002 Guam earthquakes (Mw 7.8, 7.0, and 7.1, respectively) in the southern Mariana subduction zone were interplate events or intraplate ones within the Pacific slab. We relocated their mainshocks, aftershocks, and numerous long-term background earthquakes simultaneously with high relative accuracy using the modified joint hypocenter determination (MJHD) method. As a result, their mainshocks and three-day aftershocks are clearly distributed within the background slab seismicity. Therefore, we conclude that the 1993, 2001, and 2002 Guam earthquakes were not interplate events, which scarcely occur in the Mariana subduction zone, but intraslab earthquakes within the subducted Pacific plate. The 1993 fault plane is inferred to be the near-horizontal nodal plane of the mechanism solution, though the 2001 and 2002 fault planes remain unknown. The relocation of target events simultaneously with background earthquakes has turned out very effective for discriminating between large interplate and intraslab earthquakes in subduction zones.

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