They do. Waves travel at the local speed of sound (in water, that's several hundred miles per hour).
Like I said, I'll buy the shockwave from an earthquake causing both the landslide and the tsunami but I don't buy the landslide alone.
When I was a kid we'd drop H100's in a lake and every once in awhile stun some fish if the fuse was long enough for them to sink. Drop a boulder in and you don't stun squat.