This invention improves seismic resistance of structures by transition from stiff to non destructible flexible state at a threshold earthquake level higher than prior art maximum design earthquake level of stiff structures. Functional characteristics of the category of auto-reversing stiff-to-flexible seismic structures comprise: a limited six degree of freedom motion; a laterally-stable limited rising-twist-sway ascent; a self-centering diagonal-untwist auto-descent; a multidirectional flexibility; and a multi-phase split-force-impact seismic protection. Seismic construction technologies of the category of structures comprise: base split-force-impact technology; cluster split-force-impact technology; tuned segment split-force-impact technology; and tuned spine split-force-impact technology. The auto-reversing stiff-to-flexible seismic joints of the structures are low-cost, simple and easy to manufacture, and especially suitable for mass industrial application.