How to refer to the position where you bend to the floor with your hands covering your head?

Solution 1:

During the Cold war, in the USA, this movement was called "duck and cover".

It was popularized in this public service video.

This video was made for atomic bomb damage mitigation, not earthquakes – but you may find the term applicable.

You might simply consider the verb, to "duck"

Solution 2:

I think you are looking for crouch but that is not specific to an earthquake nor to covering your head:

crouch (krouch)
v. crouched, crouch·ing, crouch·es
v.intr.
1a. To stoop, especially with the knees bent: crouched over the grate, searching for his keys.
1b. To press the entire body close to the ground with the limbs bent: a cat crouching near its prey.

2. To bend servilely or timidly; cringe.
v.tr. To bend (the head or knee, for example) low, as in fear or humility.
n. The act or posture of bending low or crouching.

In the context of airplane safety instructions at least, the position you describe is called brace position:

a position for an emergency landing of a plane, where the passenger sits bent forward with their hands behind their head

Solution 3:

The current suggestions of duck and crouch imply to me that you would still primarily have your feet on the ground with most of your body just off the ground.

While I don't know what position one would take during an earthquake, I wonder whether you might be more lying on the ground under the table, in which case, you may:

• curl up (into a ball)
• adopt the foetal (BrE) / fetal (AmE) position