It is my understanding that at the epicenter the time between "start of earthquake" and "full force" is negligible. When the earth slips, it slips. The reason why there feels like there is a ramp-up and delay to full intensity is that some waves travel faster than others, and therefore at moderate distance from the epicenter you start getting one type of wave before the other.

You're assuming that an impending major earthquake can be detected before it happens. I don't think that is possible.

All 'early warning' systems rely on having sensors close to the epicenter that detect the waves before they propagate out to populations further away, but by that time the earthquake is already happening.

I think it should be possible in principle... but I had a professor whose casual academic output basically consisted of publishing rebuttals to other academics who thought they had found an early warning sign (lightning phenomena is a common approach). A particularly amusing one he shared with us was just a case of bad data analysis in Matlab, the author had used 'filter' instead of 'filtfilt' to get their graph and so were trying to derive their results from the distorted phase...

Parent seems to be assuming, rather, that earthquakes take time to reach "full force". I weakly suspect that's not the case, but it's a different question than "before it happens".

From the videos I've seen of the 2011 Japan earthquake, the general reaction generally follows a pattern of "meh, just an earthquake... okay, this is a strong earthquake... HOLY SHIT THIS IS A STRONG EARTHQUAKE." I don't know how much of that is particular to the earthquake, though, and it's hard to tell why that kind of reaction seems to occur.

Small earthquakes are common enough in Japan that many people will just brush them off. My personal reaction to anything 4 or less on the intensity scale is just to stand/sit still. Basically, if it’s not strong enough to trigger the early warning system, I don’t really worry.

Even with the early warning, sometimes the earthquake will still be on the weaker end of the spectrum, so you brace yourself but not necessarily panic.

The 2011 earthquake started off moderately, so no one in my office ducked for cover until it really started to shake, which was already about half a minute in.