Predicting the fault beneath a newly-created earthquake-related landform: A case study of Leader Fault rupture during the 2016 Kaikōura Earthquake, New Zealand

In outcrops, the hanging-wall and/or footwall structure around a fault are often exposed, while the underlying fault is poorly resolved. In these cases, it is desirable to estimate the location and shape of the fault at depth, especially if it belongs to an active fault system prone to large earthquakes. The Mw 7.8 Kaikōura earthquake occurred two minutes after midnight on 14^th November 2016, causing at least 17 faults in the northeast South Island of New Zealand to rupture, including a number of faults that had not been previously mapped. One of these smaller new faults is the Leader Fault, which at the surface displaces Mesozoic interbedded greywacke and argillite. In outcrop, the fault rupture caused an over 3 m high, 20-30 m wide, and over 120 m long hanging-wall fold to appear at the surface.

In September 2022, we used a differential global navigation satellite system to map the topography of the fold. We collected a total of 1493 points over a map area of 4526 m², i.e. an average point density of ca. 1 point per 3 m². The data were meshed into a three-dimensional triangular surface, which was then sectioned into ten cross-sections, each 10 m apart and perpendicular to the fold axes. We present fault-prediction modelling of two of these sections. In the Move^tm software (Petroleum Experts), we used two methods of fault prediction; constant heave and constant slip. Both methods require implicit information about the hanging-wall shape, the position of the fault at the surface and the “regional”, i.e. the position of the hanging wall before deformation. Before the modelling, all this information was known apriori; i.e. we mapped the shape of the ground surface, we knew the fault to outcrop at the break of slope at the front of the leading edge, and the regional is an extension of the undeformed footwall. Both modelling techniques require a seed, i.e., a small portion of fault at the surface with a certain angle of dip. We use a horizontal and a 60° dipping seed.

We can estimate the fault geometry down to a depth of 20-25 m. For both sections, we predict the fault is steep, greater than 60°. Using a flat seed gives a slightly listric fault geometry, but in any case, the fault is steep down to 20 m depth before flattening out slightly. Compared to a small (15 cm) outcrop of the fault plane (dipping 75° WNW) at the surface at the northern end of the outcrop, the best matches are given by modelling with constant slip. The steep fault geometry is governed by the basement rock that has steep bedding that also dips ca. 70° WNW.