I honestly don't know, but I heard something similar when I was there. Earthquakes are certainly an issue in the region. You made me curious, so I did a little bit of searching. In my (pretty cursory) search, I wasn't able to find any scholarly article claiming that the pillars were an earthquake warning system. However, I was able to find some suggesting that the pillars' design made them more earthquake resistant than other columns of the time. (For instance, this article, available through Google Scholar.) So whether or not they were thought of as a warning system, they did apparently make the buildings more earthquake resistant. (Of course, when they did fall down, the building materials were reused in other buildings. I find it interesting that Jerash was rebuilt as a Roman ruin instead of a Byzantine or Ottoman one, which would be equally viable - but that's another topic.)

Here's hoping that someone can tell us more about whether they were a warning system!