More Than a Quake? Exploring a multi-mechanism source of the 2021 East Cape Tsunami

The M7.3 East Cape Earthquake on 5th of March 2021 generated a tsunami whose observed signals challenge earthquake-only modelling approaches. The earthquake occurred at just after 2 am (NZT), at the northern end of the Hikurangi Subduction Zone of the East Cape of Aotearoa New Zealand, followed by a modest tsunami detected on GeoNet’s DART and coastal gauge network. This study investigates the tsunami signals of this event.

The earthquake was initially challenging to characterise, with a large variety of depths and focal mechanisms determined by various seismic agencies. Subsequently, Okuwaki et al. (2021) and Xie et al. (2022) took seismological approaches to better understand the mechanism, concurring that the rupture comprised two related subevents. 

Our tsunami modelling suggests that none of the various proposed earthquake solutions reproduced the full spread of observed waves detected, particularly the varied amplitude persistence across different sensors. We use the COMCOT tsunami model with a nested array of bathymetric grids. We perform a methodology validation on the 2016 Te Araroa earthquake and tsunami, which occurred in a very similar location. With a slight origin relocation similar to Kubota et al.’s (2016) proposal, we are able to match the observed signals well. The identified limitations of earthquake-only source characterisation lead us to explore the potential contribution of an additional landslide source.

We therefore test alternative mixed source mechanisms, exploring a potential partial reactivation of the historical submarine Ruatoria Debris Avalanche only a few kilometers away from the proposed earthquake locations. Mixed source tests are currently ongoing with full results to be presented at the meeting. Our work highlights the importance of understanding the hazard of potential submarine landslides near the coast of New Zealand and the importance of considering multi-mechanism tsunami sources in real time during tsunami response.