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We investigate the Mw 7.8, 2016 Kaikoura (New Zealand) earthquake by using optical satellite imagery and seismology to reveal the main features of the rupture process. Correlation of Landsat8 images reveals a 30-40 km surface rupture on the Kekerengu Fault and Jordan Thrust, with up to 12 m of right-lateral slip. A previously unrecognized conjugate strike-slip fault, the Papatea Fault, also slipped coseismically (3-4 m). The global centroid moment tensor (gCMT) centroid indicates both thrust and right-lateral slip and is located similar to 100 km NE of the main shock epicenter. The significant non-double-couple component of the gCMT (25%) suggests that the main shock is not well represented by a single planar fault. Back projection of teleseismic P waves reveals two main bursts of seismic radiation: (1) at 10-20 s, near the main shock epicenter, and (2) at similar to 70 s, close to the observed surface ruptures. We determine a finite source kinematic model of the rupture from the inversion of seismic waveforms. We use two faults in our model, defined to match the observed slip on the Kekerengu Fault, and a deeper offshore fault with a lower dip angle to satisfy the long period seismological observations. We compute the equivalent moment tensor from our finite source model and find it to be remarkably consistent with the gCMT solution. Although little is known about the geometry of these faults at depth, if the Kekerengu Fault splays from the deeper thrust, then it provides a rare example where the contribution of slip on a splay fault can be clearly isolated in the seismological waveforms.

Plain Language Summary Through a combined analysis of satellite images and seismic waves, we investigate the recent Mw 7.8 earthquake near Kaikoura, New Zealand. We observe major surface ruptures, with up to 12m strike-slip displacement, located far from onset of earthquake slip. Properties of the seismic waves indicate that the earthquake slipped on a dipping thrust fault at depth and propagated to the northeast for 120 s. Halfway through the rupture, the earthquake triggered simultaneous slip on a neighboring fault, which possibly splays from the main thrust, and which was responsible for the observed surface offsets. Splay faults are commonly observed at converging plate boundaries globally and are suspected to pose a seismic and tsunamic hazard. This is the first time where simultaneous slip on a possible megathrust and associated splay fault has been directly observed in the seismic data.