If concrete mixed with plastics is a good idea, then it should be done in a controlled environment (factory), not taken from the sea. I don't see what this has to do with earthquake resistant building. If you want some "flex" in building materials, we already have steel. If plastic can replace steel, or can replace it partly, that would be great. Mixing plastic with concrete is environmentally a bad idea in the long run. How are you going to undo that mix later?