Tsunami hazard assessment of coastal South Africa based on mega-earthquakes of remote subduction zones

After the mega-earthquakes and concomitant devastating tsunamis in Sumatra (2004) and Japan (2011), we launched an investigation into the potential risk of tsunami hazard to the coastal cities of South Africa. This paper presents the analysis of the seismic hazard of seismogenic sources that could potentially generate tsunamis, as well as the analysis of the tsunami hazard to coastal areas of South Africa. The subduction zones of Makran, South Sandwich Island, Sumatra, and the Andaman Islands were identified as possible sources of mega-earthquakes and tsunamis that could affect the African coast. Numerical tsunami simulations were used to investigate the realistic and worst-case scenarios that could be generated by these subduction zones. The simulated tsunami amplitudes and run-up heights calculated for the coastal cities of Cape Town, Durban, and Port Elizabeth are relatively small and therefore pose no real risk to the South African coast. However, only distant tsunamigenic sources were considered and the results should therefore be viewed as preliminary.The Nuclear Structural Engineering (Pty) and the National Research Foundation through the Technology and Human Resources for Industry Programme project (THRIP) TP2011061400009.https://link.springer.com/journal/242019-04-01hj2018Geolog

Depth-inversion problem in shallow water

Two Depth Inversion Algorithms (DIAs) were developed and validated using on results of computations for the shoaling of periodic waves over mild slopes, in a two-dimensional numerical wave tank, based on fully nonlinear potential flow theory. The first algorithm, DIA1, uses sets of values of wave celerity c, height H, and spatial wavelengths Lc and Lt, simultaneously measured at a number of locations xi (i = 1, ..., N) in the direction of wave propagation (e.g., using remote sensing techniques), to predict the depth variation h(xi). The second algorithm, DIA2, uses spatial wave asymmetry s2/s1 calculated from wave phase, instead of H. Results indicate that state-of-the-art depth inversion methods based on the linear dispersion relation may lead to large errors (50-70%) for the depth prediction in very shallow water, whereas the present methods are 3-10 times more accurate

Implementation and validation of a breaker model in a fully nonlinear wave propagation model

A spilling breaker model is implemented in a two-dimensional fully nonlinear coastal wave propagation model. A maximum surface slope breaking criterion is used to identify breaking waves within the incident wave train. Energy dissipation is achieved by specifying an absorbing surface pressure over breaking wave crest areas. The pressure is proportional to the normal particle velocity on the free surface. The instantaneous power dissipated in each breaking wave is specified proportional to the dissipation in a hydraulic jump of identical characteristics. Computations for a periodic wave shoaling and breaking over a plane slope are compared to laboratory experiments. The agreement is quite good, although more work remains to be done in refining the breaker model parameters

Modeling of tsunami generation by an underwater landslide in a 3D-NWT

An existing three-dimensional (3D) Numerical WaveTank (NWT) solving fully nonlinear potential flow theory with a higher-order Boundary Element Method (BEM) is modified to simulate tsunami generation by underwater landslides. New features are added to the NWT to model underwater landslide geometry and motion and specify corresponding boundary conditions in the BEM model. Also, new snakeabsorbing piston boundaries are implemented to remove reflection from the onshore and offshore boundaries of the NWT. Two cases of tsunami generation are presented and results of the first one are validated using experimental results. Numerical accuracy is examined and found to be excellentin both cases. KEYWORDS : tsunamis, landslides, numerical wavetank, nonlinear wave transformations, boundary element method

Quasi-singular integrals in the modeling of nonlinear water waves in shallow water

The model by Grilli et al.,5,8 based on fully nonlinear potential flow equations, is used to study propagation of water waves over arbitrary bottom topography. The model combines a higher-order boundary element method for the solution of Laplace\u27s equation at a given time, and Lagrangian Taylor expansions for the time updating of the free surface position and potential. In this paper, both the accuracy and the efficiency of computations are improved, for wave shoaling and breaking over gentle slopes, in domains with very sharp geometry and large aspect ratio, by using quasi-singular integration techniques based on modified Telles17 and Lutz11 methods. Applications are presented that demonstrate the accuracy and the efficiency of the new approaches. © 1994