Researchers setting out to sea from the Virginia coast this weekend hope to find answers to a new scientific riddle: Could tsunamis, the large destructive waves that have terrorized Japan and the rest of the Pacific for centuries, pose a threat to the mid-Atlantic coast of the United States as well? According to latest research they might. Scientists have discovered a 25-mile series of cracks along the edge of the continental shelf, about 100 miles east of Virginia Beach and the mouth of the Chesapeake Bay.