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The September 28th 2018 tsunami (3 – 5 m high) triggered by a 7.5 M earthquake on the island of Sulawesi, Indonesia teaches us some lessons, perhaps the two important ones are:
I have attached an impressive image of this event published by the New York Times – prepared by USGS scientists.
Apart from amplifications resulting from long-wave transformation processes (e.g. shoaling and funneling), many inlets and bays respond in resonant modes to an incident tsunami wave. Subject to further investigation, it is likely that the tsunami episode at the head of Palu Bay might have occurred in some resonant modes (e.g. in sub, peak or super resonant frequencies). Among others, my own research (published by World Scientific in 2006, https://doi.org/10.1142/9789812709554_0135) for the case of 1964 Alaska tsunami, showed a sub-resonant response from the 64-km long Alberni inlet. The small-amplitude incident tsunami at the inlet entrance, amplified to 3 times at the head of the inlet – causing substantial damage to the Port Alberni in British Columbia.
While we will know more in the coming days and weeks, I invite all to share insights and experiences. Our thoughts and prayers are for the family of the deceased and other affected people of Indonesia.
It is nice reading the interesting discussions contributed by ASM Ahmed, JF Chapman and D Sherman. As some of you are aware, preventative mitigation measures such as configuring, sizing or modification of bays or embayments are indeed done in small scales. For example, harbors for marina and ship berthing are usually sized to avoid resonant amplifications of waves in the infragravity range – and are configured with the help of breakwaters and dampers to diffract waves in such a way that shadowed tranquil areas are maximized, and the exposed illuminated areas and undue reflections are eliminated or minimized.
However, any such modifications to mitigate the effects of large-scale events like tsunami or storm surge can be very overwhelming and challenging (perhaps and I hope that some cutting-edge research is going on somewhere) – in part because such long-period waves would tend not to see or feel the effects of some known engineering measures – to deter them from causing havoc (or having only marginal deterring effect). We have seen how the stone-wall tsunami barrier failed in Japan during the 2011 tsunami. I invite interested readers to have a look at some pieces in my website – the Science & Technology has a piece on Tsunami and Tsunami Forces.