What is the underlying principle of seismograph construction?

A heavyweight suspended within a moving box needs to overcome inertia which leads to a slight delay in the motion of the weight as the box moves.

Explanation:

A seismograph is an instrument used to record earthquake waves

A heavyweight suspended within a moving box needs to overcome inertia which leads to a slight delay in the motion of the weight as the box moves.

The first earthquake waves arrive at a seismograph station, a short time after the earthquake occurs.