Is the USA ready for any such disaster as a major earthquake, volcano, or even tsunami?
First of all, we have to realize that the United States, as a whole, does not need to be ready for such things. Any disaster of the sort that you mention is fairly localized and does not affect a very large region of the country. For this reason, most disaster planning is done at the local and regional level. Different parts of the United States can be more ready or less ready, depending on how much planning they have done.
Second, there is no way to answer this question in an objective way. We simply do not know until it happens. For example, parts of the city of Seattle could be hit by things like lahars if Mt. Rainier were to erupt. The city of Seattle could also be hit by a very large earthquake since it is in a seismically active zone. The city has plans for these sorts of eventualities. In that sense, they are ready. But we cannot know how ready they really are until they need to actually put those plans into action.
When it comes to disaster planning, we cannot know how ready we are until our plans are forced into action. Parts of the US that are in danger from these sorts of natural disasters have plans for dealing with them. But we do not know if their plans are good enough for us to say that they are “ready.”