The 2011 Northern Kermadec earthquake doublet and subduction zone faulting interactions

Abstract

AbstractA large intraslab earthquake doublet (6 July 2011, Mw 7.6; 21 October 2011, Mw 7.4) occurred in the Pacific plate beneath the outer trench slope of the northern Kermadec subduction zone, seaward of an interseismically‐coupled region of the megathrust. The first large event, a shallow (~24 km centroid depth) normal‐faulting rupture, was followed by intraslab aftershocks within minutes and by aftershocks near the megathrust within hours. The second large event, a deeper (~45 km centroid depth) thrust‐faulting rupture below the northern portion of the first rupture plane, was also followed by interplate and intraslab aftershocks. The last large interplate activity in northern Kermadec involved an underthrusting doublet on 14 January 1976 (Mw 7.8, 7.9). Previous studies interpret westward movement of the northern Kermadec Arc relative to the Australian Plate (from GPS measurements) to be a result of interseismic coupling on the northern Kermadec megathrust. A locked plate interface in this region should tend to reduce trench‐slope extensional stresses. Coulomb stress change calculations using a finite‐fault model determined from teleseismic body wave inversions for the normal‐faulting event in the 2011 doublet favor activation of megathrust faulting. Coulomb stress changes for the October compressional event appear to reduce interplate thrust activity, and for a short time reversed the westward motion of the upper plate. The net effect of the doublet is a few bar increase of interplate thrust‐fault driving stress, which may have advanced a future large megathrust event. Intraslab normal faulting may serve as a harbinger, not only a response, to large megathrust ruptures.