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Lake Tsunami in Alpine Fault Earthquake Events – Numerical Studies in Lake Tekapo

Presentation Date published: March 2023

Date published: March 2023

Authors: Wang, X., Holden, C., Mountjoy, J.J., Power, W.L., and Liu, Y.
Event: APEC Cooperation for Earthquake Science

Summary: This oral presentation explored what lake tsunami might occur in southern lakes of New Zealand in the event earthquake ground motions and/or seismically triggered landslides.

https://www.gns.cri.nz/news/aces/(external link)  

Recent research indicates that there is a 75% probability of a rupture on the central section of the Alpine Fault in New Zealand over the next 50 years, with an 82% probability that it will be a magnitude 8+ event. It is highly likely that such an earthquake would strongly shake Southern Lakes of New Zealand that are in close proximity to the fault, causing lake tsunami by the earthquake’s ground motions and/or indirectly by seismically triggered landslides. Our two numerical studies in Lake Tekapo, one on seismic seiches and the other on landslide tsunami, provide insights into what might happen in these lakes.  

To investigate seismic seiches in Lake Tekapo, we applied 220-sec synthetic ground motions of an Mw8.2 Alpine Fault earthquake to the 3-D lake terrain (topography and lakebed) to drive lake water motions in a tsunami simulate model – COMCOT. Our modelling results reveal that lake water oscillations are mobilized immediately by the ground movement and further amplified by cross-lake seiches. Maximum amplitudes of the lake oscillations reach up to 4.0 m above normal lake level in the lake’s narrow southern arm, up to 1.0 m along the shore of Lake Tekapo township, and about 1.5-2.5 m along many other parts of the lake shore (Figure 1).  

In contrast, vertical co-seismic displacements in the lake area, the conventional source mechanism used for tsunami generation, are too small to trigger tsunami waves of concern. In the other study, we numerically modelled landslide tsunami generated by individual subaerial, marginal, and subaqueous mass failures, reconstructed from identified mass transport deposits, as well as by multiple mass failures. The numerical simulations show that landslide tsunami could push lake water up to over 5m above normal lake level at the shore of Tekapo township, overtop the lake Control Structure located 400-500 m downstream of the lake outlet, and flood further downstream.  

We conclude that in an magnitude 8+ Alpine Fault earthquake both seismic seiches and landslide tsunami could pose a significant threat to people and infrastructure in the water of Lake Tekapo, on the lake shorelines, and possibly at locations further downstream of the lake outlet.  

Xiaoming Wang

Xiaoming Wang

Project Leader: Simulate & Tsunami Scientist

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