Scientists reported yesterday that fissures and holes formed by the eruption of Mount St. Helens have merged into a single giant crater that could loose a blast of fiery steam and volcanic rock at any time, incinerating everything in its path.

Geologists said a 3 1/2-mile-long lake on the northern slope of the volcano has begun to tilt. They said further movement would indicate a swelling action that could precede a violent eruption.

The sharpest earthquakes yet jolted the mountain early yesterday about 3,000 feet below the summit, and authorities said there was a "good possibility" that lava had climbed to within 3,000 feet of the top.

"I'm scared to death," said Dan Miller, a volcanologist for the U.S. Geological Survey who has studied southwest Washington's volcanic peaks for six years. "It is dangerous and I've heard about sightseers flying over the crater and I shudder."

Almost as if responding to Miller's warning, the volcano triggered another huge blast of rock and steam at 8:16 a.m., sending debris 3,000 feet above its 9,677-foot summit.

Aerial observers in Forest Service aircraft reported that new vents and fissures had been ripped across the top of the mountain and that the volcano's two craters were puffing steam, ash and debris as a single unit and for all practical purposes, had merged into one giant hole.