Toward Ecosophy

I've been having to ask for more help than usual these days -- many thanks to my readers for pitching in with details! -- and here I am again, with a question for the geologically minded.  

A current fiction project has, as part of the backstory, a major volcanic eruption from an undersea volcano, the Kikai caldera, which is just south of Kyushu Island, the southernmost major island in Japan. It's a big one:  an ultra-Plinian eruption of VEI (volcanic explositivy index) 7, with a total of 126 cubic miles of tephra blown into the atmosphere, a column collapse event resulting in a pyroclastic surge that spreads as far as southern Kyushu, then a caldera collapse and resulting tsunami, and of course a high-altitude cloud sufficient to cause a "year without a summer" like 1816, and crop failures across much of the northern hemisphere. 

The details I need to know are (a) how big would the tsunami be  when it hits nearby land areas, (b) how large would it be when it gets to the far side of the Pacific and slaps the west coast of the US, and (c) how thick of a layer of volcanic ash would land on the Japanese islands. Presumably vulcanologists know these things, but I don't, and I haven't been able to find the appropriate resources online. Any help would be greatly appreciated.