Why do lighter bridges generally withstand earthquakes better than heavier bridges?

Lighter bridges generally withstand earthquakes better because the horizontal forces exerted on a structure during ground acceleration are directly proportional to its mass.

This relationship is governed by the fundamental Newtonian law of motion, which states that force equals mass times acceleration ($F=ma$). Because the seismic force acting on the bridge is a product of its mass, reducing the dead weight of the structure effectively minimizes the total force it must endure during a seismic event.1

Engineers often utilize materials and techniques, such as prestressed concrete, to create more efficient, lighter designs that maintain structural integrity while reducing the overall load.2