When a heavy bus or truck goes by at high-speed, the shaking in nearby ground is not mistaken as dizziness, rather the feet and body feel a vertical jerk (seemingly a transverse wave vertical to the ground).
But when a natural, mild earthquake takes place, it is not felt as a jerk or switch in a direction at all. Rather we feel it for a while as dizziness. I have experienced this, and heard of many others who have as well. In-fact it takes a few seconds to distinguish whether it is dizziness or an earthquake.
Is it due to the longitudinal waves? Is it because they are horizontal with the ground, matching the direction of dizziness?
Also I guess maybe there may be similarity between the frequency of seismic waves and this video from Wikipedia's article on vertigo:
Does this phenomenon's frequency act similarly to the motion and simulates dizziness?
Second, after experiencing a big one the body becomes hypersensitive - conditioned to pick up vibrations that could be the onset of a major quake. Now, at the onset of any vibration, my brain goes straight to "how big is it - should I run to the nearest exit"? This response is now much faster than the sensation of dizziness. Also I now pick up many vibrations that nobody else around me seems to notice.
Thirdly, I am not sure I agree with you about vertical motion being the trigger. It seems to me that lateral motion - moving one's feet sideways relative to the head, creates a slight imbalance, which the ears interpret as 'dizziness'.
In an earthquake, on the other hand, unless it's strong enough to actually start noticeably moving objects around (and I'm assuming that, when you specify a mild earthquake, you mean one that isn't strong enough to do that), your brain has no advance warning - suddenly, the vestibular system is reporting motion, while the eyes are reporting that you're stationary. Your brain can interpret this one of two ways:
- The ground is shaking, and the building, and, therefore, you, with it.
- The vestibular system is malfunctioning.
As temporary glitches in the vestibular system (i.e., dizziness) are (hopefully) a more common experience for most people than (even mild) earthquakes, your brain assumes that the culprit is the vestibular system, rather than the ground, and you feel dizzy until you realise that it's actually an earthquake.
For months now I've been experiencing vertigo symptoms "randomly" even when I, and the people around me, haven't felt any sort of earthquake movement at all; however, when I look up whether there has been an earthquake or not, the answer is always yes. Whether it was a strong earthquake that occurred in Mexico or a very mild one that occurred close by but not strong enough or close enough to feel, I can "feel" it just by how my body reacts. I seem to be extra sensitive than anyone around me, I've even felt small earthquakes that other people don't seem to feel.
Just now some time around 6pm Costa Rican time while laying in bed, I felt a strong nauseas feeling which made me think it might've been because I hadn't eaten in a few hours, so I eat but I don't seem to get better, so I started thinking maybe I had low blood pressure or low blood sugar. Both of which were incorrect because I had eaten a bit of sugar during my meal and my BP was at a normal 110/70. So then I decided to look up if there had been any earthquakes and lo and behold, the was a 3.1 magnitud earthquake at 6:23pm about 15km from me, however it wasn't even strong enough for me or my roommates to feel.