Concrete is an extremely strong building material, but has a notoriously weak tensile strength.  In order to resist tension, bending, and shear forces, steel rebar or other reinforcement materials are added either prior to the placement or into the mix.  Even with reinforcement, concrete is still extremely rigid and prone to cracking.  In the event of a major earthquake, the uneven and horizontal forces can cause structures to crack and, in the worst case, cause failure.