Fake user registrations

I am getting a new user registrations every couple of days from users that are not genuine. Most of them have email addresses and user names that look like a random character string, some are , some end in .ru, etc. So I started sending emails to new users asking them to reply and answer a simple question. If they don’t reply, I have been deleting the users. Is this an attempt to hack my site? Is my site already hacked? I haven’t noticed any problems with it. Thanks.

http://www.coalgasificationnews.com