From http://gsa.confex.com/gsa/2001ESP/finalprogram/abstract_7493.htm: "Erosion of volcanic oceanic islands by large scale landsliding is a well known phenomenon. Many hundred km3 of rock can be removed in a single event, leaving towering headwall scars, amphitheatre shaped depressions (many previously regarded as calderas), and massive dumps of debris offshore. Here we summarise one single event which removed a substantial portion of the Canary Island of El Hierro about 12-17,000 years ago. The initial landslide involved dislocation of about 180 km3 of rock from the north-western flank of the island; about 50% of the material was derived from subaerial sources. Avalanche deposits extend to 4000 m waterdepth, cover an area of 1500 km2, and have a thickness of 250-350 m. Blocks up to 1 km across occur on the seabed up to 65 km away from the island."
An event like this could represent a significantly disruptive situation on the US eastern seaboard.
A Canary Island Landslide based tsunami would be devastating but we would have several hours warning.
That’s enough time for folks in coastal major cities to vertically evacuate (it would take days to evacuate inland).
There would still be deaths but the death toll in the US would be greatly minimized due to the Advanced Warning.
Worst case scenario for a Tsunami would either be a large localized underwater landslide near shore or the sudden undetected impact of a meteor at least the size of half a foot ball field in size crashing into the ocean.
You would have a wave of some size with no warning.