How is that proof of anything artificial... what's the belief, here? That anything that repeats rhythmically is automatically unnatural? There are lots and lots of things in nature that follow very predictable rhythms

The consensus on that seismic wave in 2018 seems to be activity of an underwater volcano.

Later that underwater volcano became more active. Here is an article that gives some information on the later activity. https://www.sciencealert.com/the-largest-underwater-volcanic-eruption-ever-recorded-gave-birth-to-a-new-volcano