I also think the earthquake and the water rising are directly connected to each other but that Lulusia kingdom may only have tangential connection e.g. the incident triggered/accelerated some separate geologic process.
My idea was that the Red Line is consuming land mass from underneath in order to grow, causing all islands to sink. This could be either from the existing Red Line rising up even higher or even better, a new Red Line forming that will even further divide the world and displace even more water.
It's also unclear if whatever is happening is a natural process influenced by events of the story (Lulusia) or is entirely artificial/man-made, where some technology is being used to geoengineer/terraform the planet. Maybe this is what Imu meant by another cleansing where they as god decided to flood the world (there are multiple real life myths/religions around this kind of thing).
From what I recall, this is absolutely correct.
The destruction of Lulusia triggered earthquakes which raised the sea-levels.
At first, I thought the sea-levels rise by 1m each time an Island is destroyed, however, I'm beginning to think the sea-levels rising isn't the problem.
My reasoning initially was each Island destroyed means sea-levels will be up by 1m, so 10 Islands would be 10m rise in sea-levels.
However, I think the real issue is the earthquakes.
Perhaps, the greater the change in tectonic plates, the greater the chances of earthquakes and possible the greater the magnitude of earthquakes.
Thinking about it, we know tectonic plates move towards each other, and apart or slide by each other... but when you change the shape of plates so drastically, that would surely makes things a lot more chaotic.
Furthermore, I think this chapter said the world will sink... not that sea will envelop the world. Makes sense if earthquakes cause the destruction of many Islands and result in these Islands sinking.
Another thing I found interesting was when Imu ordered the destruction of Lulusia, he said the country being destroyed must be above Sea-levels... that is very specific. Not sure why that is the case, but hopefully we get some elaboration.