That said, I read a bit earlier that it likely would not have warned them about this one because it originated in a volcanic eruption, not an earthquake.
What was troubling was the distribution of recent earthquakes in the area.
Krakatoa (west not east of Java) was the scene of one of the largest and most deadly volcanic eruptions of recent times.
Which evidently was accompanied by all sorts of electrical phenomena.. (so are earthquakes! Ive actually seen that and its really strange!)
The following is really worth reading because of its description of phenomena that give hints that the Krakatoa eruption may have been simply the most visible to human eyes of a number of related volcanic events over a large distance.
https://www.theatlantic.com/magazine/archive/1884/09/the-volcanic-eruption-of-krakatoa/376174/A really huge volcanic eruption near the Equator could cause a global food and energy catastrophe by reducing the amount of light that reached the Earth in both the Northern and Southern hemispheres for several years.