Dynamically triggered slip on a splay fault in the M_w 7.8, 2016 Kaikoura (New Zealand) earthquake

Abstract

We investigate the Mw 7.8, 2016 Kaikoura (New Zealand) earthquake 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 co-seismically (3-4 m). The global CMT centroid indicates both thrust and right-lateral slip, and is located ~100 km NE of the mainshock epicenter. The significant non-double-couple component of the gCMT (25%) suggests the mainshock 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 mainshock epicenter, and (2) at ~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, it provides a rare example where the contribution of slip on a splay fault can be clearly isolated in the seismological waveforms.