Modeling the large runup along a narrow segment of the Kaikoura coast, New Zealand following the November 2016 tsunami from a potential landslide

In November 2016 an earthquake of magnitude 7.8 (Mw) hit the South Island of New Zealand. Consequently, it triggered tsunamis with maximum runups of up to 7m, that were witnessed along a narrow segment of the coast close to Kaikoura. Literature has it that ruptures occurred on multiple faults triggering the resulting earth movements. Nonetheless, to date, the debate about the source of the earthquake and the characteristics of the resulting tsunami continues. In fact, researchers have labeled this quake as the “most complex earthquake ever studied”. A complex sequence of ruptures occurred with the hypocentre (the point where the ruptures started) being at a depth of 15 kilometers. In preceding studies, several models have been proposed but none of them has been able to reproduce the large concentrated runup of ∼7 m. To generate the large tsunami peak, such as the one observed in Kaikoura tide gauge record and the observed runup height, offshore seafloor movement is necessary, but the offshore extension of the plate-interface rupture and its type, either seismic rupture or a landslide, is uncertain. In addition, dual source earthquake and submarine landslide tsunamis are a poorly understood hazard, because there are too few well-studied examples.

In a recent publication, a team of scientists, Dr. Mohammad Heidarzadeh from the Brunel University London, Dr. David R. Tappin at the University College London, and Dr. Takeo Ishibe from the Association for the Development of Earthquake Prediction in Japan, presented a study where they carried out a thorough and expert review of the source models for the 2016 Kaikoura tsunami. In particular, they focused on developing numerical models for an additional submarine landslide with the potential to successfully reproduce the near-field observed runup of 7m. As such, they proposed a submarine landslide in addition to the earthquake source. Their work is currently published in the research journal, Ocean Engineering.

Briefly, they proposed submarine landslide was delayed 10–20 minutes after the earthquake rupture as opposed to the offshore plate-interface rupture model. In addition, the location of the submarine landslide source was determined based on the iterative numerical tsunami modeling of various scenarios of dual earthquake-landslide sources.

The authors observed that, notwithstanding the uncertainty over an offshore plate-interface rupture and its location, the proposed submarine landslide offered a viable alternative in comparison to previous models. Moreover, it was established that the proposed model was consistent with tsunami waveforms and field runup data. In fact, the dual source was validated using the obtained data while as the landslide component was seen not produce significant seismic signature on the seismic network.

In summary, in order to explain the large and concentrated runup height of 7m witnessed during the 2016 tsunami, a dual, submarine landslide-earthquake mechanism was proposed by Dr. Mohammad Heidarzadeh and his colleagues as an alternative to previously-proposed offshore plate-interface rupture. The presented model comprised of the earthquake source and a theoretical landslide source located offshore Kaikoura. Altogether, the study highlighted the importance of considering observed runup data for earthquake/tsunami source studies through runup inversions. So far, the dual source model presented for the 2016 Kaikoura tsunami is the only source model that reproduces both tide gauge records and the observed near-field runup heights.