The phrase "subsequent tsunami" refers to another tsunami that happens after a previous one. It means a second or following tsunami that occurs later in time. Full definition
We have all heard about the devastating earthquake and subsequent Tsunami in Japan. (7binaryoptions.com)
Japan's scientific community has been quietly taking stock of how extensively the magnitude - 9 earthquake and subsequent tsunami on 11 March damaged facilities and experiments. (sciencemag.org)
On August 13, 1868, an earthquake and subsequent tsunami destroyed most of Arica. (southamerica.cl)