Thanks for your responses guys.
What troubles me is that the pages per visit is just 1, the avg time on site from these visits just 3 seconds and then a huge bounce rate, making me believe that it could be an attack.
However there is no common landing page - i thought this maybe the homepage if i was under attack.
In a normal month we will receive 10,000 direct visits across 3,000 pages - this month it's 24,000 visits across 9,500 pages!
Even if these were genuine US citizens surely the odd hundred or so would click on additional pages before realising it was the wrong site?
I don't understand it.