> a supershear strike-slip earthquake that occurs beneath a narrow, shallow bay can trigger a massive tsunami
Basically, the horizontal shock waves from the fault are reflected vertically by the shape of the ground, which then causes the tsunami.
I did some numerical analysis on sub-Raleigh and supershear fault rupture propagation 15 years ago. In strike-slip earthquake, plates don’t slide against each other a rigid blocks, but rather un-zip rapidly along the fault. Depending on the energy in the fault (how hard the plates are pressing against each other, to grossly oversimplify), the rupture will reach one of two stable velocities along the fault. Sub-Raleigh is the most common and slower speed and super-shear is much faster. It moves fast enough to generate a shock wave, which generated the tsunami in this case.
Super-shear ruptures are a big deal because (surprise, surprise) they are much more destructive, even without a tsunami.