Surface rupture and slip distribution of the Jordan-Kekerengu-Needles fault network during the 2016 Mw 7.8 Kaikoura earthquake, New Zealand

Snapshot

Slip distribution and surface rupture morphology of the Kekerengu Fault during the 2016 Mw7.8 Kaikoura earthquake

 

During the 2016 Kaikoura earthquake, the Kekerengu fault caused the ground surface to move sideways by up to ~12 m, one of the largest observed permanent fault displacements to accompany an earthquake worldwide.

This is the first study to document a continuous ~83 km-long fault rupture that crosses a coastline, displacing both the ground surface onshore, and the seabed offshore. Onshore on the Kekerengu, Jordan Thrust, Upper Kowhai and Manakau faults, we used 117 natural (i.e. landforms such as river banks or ridgelines) and man-made markers (i.e. straight fence lines) to measure the fault movement in the field, as well as using airborne laser surveying techniques. Offshore on the Needles fault, we used sonar echo-sounding to observe permanent vertical movement of the seabed of up to 3.5 m. The average movement along the fault was 5.5 m, which is unusually large for an ~83 km-long fault rupture, compared to other events around the world. Fence lines that extend across the fault rupture show that it is not just a narrow crack, but that it varies in width between 13 – 122 m.

This research showed that the Kekerungu fault behaved unusually compared to other earthquakes around the world, but similarly to other New Zealand historic earthquakes. Understanding the level and type of rupture damage will help anticipate how other similar active faults may behave in future, and inform development planning around known active strike-slip faults (where two blocks of land move horizontally along a fault).

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Technical Abstract

During the 2016, MW 7.8 Kaikōura earthquake the Kekerengu fault ruptured the ground surface producing a maximum of ~12 m of net displacement (dextral-slip with minor reverse slip), one of the largest five co-seismic surface rupture displacements so far observed globally.

This study presents the first combined onshore to offshore dataset of co-seismic ground-surface and vertical seabed displacements along a near-continuous ~83 km long strike-slip dominated earthquake surface rupture of large slip magnitude. Onshore on the Kekerengu, Jordan Thrust, Upper Kowhai and Manakau faults, we measured the displacement of 117 cultural and natural markers in the field and using airborne LiDAR data. Offshore on the dextral-reverse Needles fault, multibeam bathymetric and high-resolution seismic reflection data image a throw of the seabed of up to 3.5±0.2 m. Mean net slip on the total ~83 km rupture was 5.5±1 m, this is an unusually large mean slip for the rupture length compared to global strike-slip surface ruptures. Surveyed linear features that extend across the entire surface rupture zone show that it varies in width from 13 to 122 m. These cultural features also reveal the across-strike distribution of lateral displacement, 80% of which is, on average, concentrated within the central 43% of the rupture zone.

Combining the near-field measurements of fault offset with published, far-field InSAR, continuous GPS and coastal deformation data, suggests partitioning of oblique plate convergence, with a significant portion of co-seismic contractional deformation (and uplift) being accommodated off-fault in the hanging-wall crust to the northwest of the main rupturing faults.