I think it is not very difficult to answer - federal entity in Washington DC. Empire is both a material and cultural product - it is a way of organizing material resources in culturally distinctive way that maintains its identity.
When an empire collapses, it does not mean that the material resources on which it was build disappear into the thin air - but that the hitherto culturally unique and identity-defining principle no longer holds. Rome collapsed, but Romans, its buildings, technology, organizational know how etc survived and became building block of another entity. Ditto for the Soviet Union.
What I mean by the collapse of the US empire is the ability of the federal entity in Washington DC to maintain national unity and at the same time maintain both domestic and international policy as that unity. That means serving as a currency reserve, regulatory agency, distribution of resources, law enforcement, waging wars, etc. What I am arguing is that this capacity of the federal entity may be challenged if the internal situation deteriorates to the point that most of the US populace will see that they are not getting much in return for their federal taxes.
Of course that does not mean that the US and its material resources will disappear but that the organization of these resources will undergo reorganization that will form a radical departure form the way they were organized in the past. That may or may not result in the replacement of the federal entity by another form of confederation and confederate authority; it may or may not result in administrative split into several separate entities (say, the United States of Canada and Jesusland) - or perhaps some other form of restructuring. While none of them would wipe out of the material resources now under the US control - most of them would change what Is now the US political identity.
Wojtek