I first saw them back to back. They've shown them separately a few times since then. They were on the Discovery Channel. It looks like they are the same as these two BBC shows:
[link|http://www.bbc.co.uk/science/horizon/2000/mega_tsunami.shtml|Mega-tsunami: Wave of Destruction]
[link|http://www.bbc.co.uk/science/horizon/1999/supervolcanoes.shtml|Supervolcanoes]
A link to the transcript of each show is at the bottom of each page.
I couldn't find any sign of the shows on the Discovery Channel's website or schedule for the next couple of weeks.