What are the nations involved in the United Nations?
There are 190 countries. 15 countries in the Security Council of the United Nations. And 5 permanent countries in the Security Council (China, France, Great Britain, Russia, and the USA).
Should the United Nations become involved in mandating laws that require all buildings to be earthquake safe?
The United Nations as a whole does not have real power unless it is given to it by the member nations. The United Nations can agree on trying to make all building earthquake proof but there is no way to enforce such a mandate. Can they become involved? Yes. Can they enforce this kind of mandate? No.