A team of scientists announced yesterday that massive underwater eruptions of gas are the likely causes of mysterious rifts in the ocean floor off Virginia, bolstering the researchers' theory that tidal waves could someday menace the mid-Atlantic coast.The three scientists, who just returned from a two-week research mission at sea, said they were surprised by the volume of gas they discovered along a 25-mile stretch of the continental shelf east of the Chesapeake Bay. Neal.W. Driscoll and his colleagues, Jeffrey Weissel, of Columbia University, and John Goff, of the University of Texas, have prompted waves of concern by raising the possibility that tsunamis, which have bedeviled Japan and other parts of the Pacific for millennia, could pose a threat to the mid-Atlantic as well.