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Surface Rupture of the Hundalee Fault during the 2016 Mw 7.8 Kaikōura Earthquake

The Hundalee fault is one of at least 20 faults that ruptured during the 2016 M w 
Mw
 7.8 Kaikōura earthquake in the northeast of the South Island of New Zealand. Here, we document a 12‐km onshore section of the Hundalee fault that exhibited surface rupture from this event. To the northeast of our observations, the fault crosses the coast, and independent seabed surveys show that the 2016 rupture continued at least 2 km offshore. No surface rupture was observed across the southwestern section of the Hundalee fault, which crosses hilly vegetated terrain and poorly consolidated valley‐floor sediment. However, previous Interferometric Synthetic Aperture Radar (InSAR) analyses suggest that a 9‐km‐long section of the fault did rupture. Hence, the minimum length of the 2016 rupture along the Hundalee fault is 23 km. Field measurements indicate oblique dextral‐reverse slip along northeast‐trending Hundalee fault sections and reverse‐sinistral slip along north to north‐northeast‐trending sections. This is consistent with the regional principal horizontal shortening direction. Maximum vertical and horizontal offset measurements are 2.5±0.5 
2.5±0.5
 and 3.7±0.5  m 
3.7±0.5  m
, respectively. The discontinuous and irregular surface ruptures we observed are characteristic of a structurally immature fault, yet previous geological mapping indicates that the Hundalee fault is a regionally significant fault with >1‐km 
>1‐km
 late Cenozoic throw. Furthermore, a 60‐m‐wide sequence of fault rocks exposed by the rupture indicates that slip has localized into <10‐cm‐thick 
<10‐cm‐thick
 gouge zones, as anticipated for a mature fault. Therefore, a discrepancy exists between geological evidence of the Hundalee fault being a structurally mature fault and the characteristics of the 2016 rupture. We speculate that this signifies that the 2016 rupture was imposed on the Hundalee fault by movement across an inefficient multifault network rather than independent rupture of the Hundalee fault itself.