Why is a mild earthquake often mistaken as dizziness or vertigo?

I'm from a place that never has earthquakes and just about two years ago, I moved to San Jose, Costa Rica which experiences minor earthquakes daily 2. Since I've moved, I've experienced strong episodes of vertigo due to even the minor earthquakes.

I have been trying to research this topic without any success. From what I've found the evidence is usually about the after affects that occur after major earthquakes 1 or they speak mainly upon how "earthquake sensitive people" try to predict earthquakes.

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.

I live in Southern California (Inland Empire, near the San Andreas and San Jacinto faults). I did not grow up here. I don't think I've ever mistaken an earthquake for dizziness. During mild ones that have a short 'signature' (a jolt) and that occur while I'm at work, I often wonder first whether someone dropped something on a floor above me.

For the few stronger ones that last longer, I pretty quickly realize it's an earthquake--maybe because I don't often have dizzy spells and don't drink alcohol.

I've missed some of the biggest ones as Northridge (1994) happened a few months before I moved to California.

When a bus or truck goes by, you typically hear it before you feel it, so you already know what it is before any noticeable ground motion starts, and your brain accepts the shaking as being due to the vehicle.

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:

1. The ground is shaking, and the building, and, therefore, you, with it.
2. 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.

Having experienced many minor earthquakes (Mag 3 to 5 in the East African Rift Valley, Samoa, Solomon Islands, Philippines and Indonesia), and one major life-threatening earthquake (Magnitude 8, Nepal), I can make a few personal observations about physiological reactions. First, You are quite correct about the body picking up any swaying feeling, and interpreting this as dizziness. I was in an 11th floor hotel room in Bangkok when an aftershock of the Aceh earthquake occurred. My first thought was "Did I drink too much beer this evening"? I wasn't alone. When I opened the door into the corridor, the occupants of nearly every room were looking out and wondering much the same thing.

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'.