Is my use of "knee-jerk reaction" correct?

Solution 1:

Getting under the table, although probably a conditioned response, is mostly a deliberate one (you might use "immediate", "initial" response; or, although you did probably give it a nanoseconds' thought, "without thinking I...").

Knee-jerk reactions, are more reactive, less deliberate, and equally prone to conditioning but more toward the edge of irrational predispositions of personality. Knee-jerk reactions are frequently emotional, confrontational, and less mediated by an end consequence (i.e. surviving the earthquake).

Solution 2:

If you're interested in using an idiomatic phrase, this might work better:

The big earthquake made me gun-shy. Now, whenever I feel a slightly big quake, I immediately put myself under a table.