FLOREFFE, PA., JAN. 6 -- The Ashland Oil Co. storage tank that ruptured here last week and spilled diesel fuel into the water supply of 500,000 suburban Pittsburgh residents cracked so violently that it was blown 100 feet off its foundation, a federal engineer said today.

Richard N. Wright, director of the Center for Building Technology at the National Bureau of Standards, said the vertical crack in the 4 million-gallon tank may have been caused by defects in its welding, structure or material. EPA spokesman Ray Germann said the crack ran vertically from foundation to top.

Arctic temperatures in this western Pennsylvania town and the internal pressure that built up as the tank was filled nearly to capacity Saturday may have accentuated the flaws, Wright said. He was preparing today for what is expected to be a six-month investigation by the bureau requested by the State and Allegheny County.

"It was a very abrupt, high-energy failure," Wright said of the tank fracture. "This is the first such catastrophic, extensive failure of an oil tank in recent years."

As government and corporate officials began their search for the cause of one of the nation's worst inland water oil spills, residents received some relief from the effects of the 1 million-gallon discharge of diesel oil into the Monongahela River 25 miles southeast and upstream of Pittsburgh.

The slick, which had shrunk to about 17 miles long from 28 miles on Tuesday, slid downstream today to within 30 miles of Wheeling, W.Va., on the Ohio River, which begins in downtown Pittsburgh at the confluence of the Allegheny and Monongahela rivers.

But pollution levels had fallen low enough here in Allegheny County for the area's largest water supplier to resume partial service for its 500,000 suburban customers. Since Sunday evening, when the utility closed the intake plant that serves the county, they have lived off water left in distribution pipes.

Moreover, the city of Pittsburgh, which draws its water from the untainted Allegheny River, tripled its grant of water to the utility, the Western Pennsylvania Water Co.

"Everyone should have at least a trickle, enough to put a container under the faucet for clean, fresh water," West Penn spokesman Denis Casey said. The system's carbon filtration system will be stepped up and will provide 25 percent of its normal capacity, requiring residents to augment their supply from mobile water tanks provided by the National Guard, Casey said.

The 23,000 residents of Robinson Township and North Fayette, served by a smaller, municipal water authority that ran dry Monday night, had been forced to close businesses and schools. Water loans from a neighboring town will allow them to flush toilets but constitute "a drop in the bucket," chief plant operator Chuck Neil said. "No way our problems are over."

Cleanup of the Ohio and Monongahela continued as a small army of workers, using skimming barges, vacuum hoses and plastic containment booms sucked oil off the water's surface. Sheets of ice retarded the effort, hiding the pollutant and preventing use of equipment in some of the small pockets in which the oil has pooled, Coast Guard spokesman Todd Nelson said. Oil sheen was spotted on the Ohio as far away as Steubenville, Ohio, 90 miles away.

Nelson said small amounts of diesel fuel continue to seep into the Monongahela, possibly from contaminated ground water beneath the Ashland tank farm where the accident occurred. The oil is captured as soon as it enters the river, he said.

The Environmental Protection Agency plans to monitor the ground water, which could be a chronic source of pollution and require excavation if saturated by fuel.

Ashland permitted reporters their first look at the accident site, where pools of oil remained in a containment dike surrounding the tank. The oil was released with such force that it lapped over the 10-foot-high walls of the dam, flowed over the surface of the tank farm and entered nearby storm sewers that empty into the Monongahela about a half mile from the site, company spokesman Roger Schrum said.

The crumpled shell of the tank, which was 48 feet high and 120 feet in diameter, rested in pieces against a dam wall about 100 feet from its foundation. Oil streaked the sides of adjacent tanks 50 feet from the accident site.

Wright of the National Bureau of Standards said after a news conference that as the oil rushed out of the crack, the tank shell was apparently blown backward like a balloon after air escapes from it suddenly.

He said the investigation by the bureau, the nation's primary emergency and physical sciences measurement laboratory, will determine whether flaws in the metal, structural design, foundation or connections of the tank accounted for the container's failure to tolerate normal stress.

According to Ashland officials, the 40-year-old steel tank was tested for stress after it was transported here from a Cleveland facility and reassembled in August 1986.

Ashland's chairman John Hall acknowledged Tuesday that the company erected the tank without obtaining a permit from the county fire marshal or exposing the vessel to the standard tests.

Fire Marshal Martin Jacobs said today he has referred the case to the county's legal department. Had Ashland formally applied for a permit, he said, inspectors would have tested the site's geology and required the tank to pass all of the state code safety requirements.

Ashland, which hired a private consultant to investigate the accident independently and is conducting its own probe, will examine the 100 to 200 other fuel tanks it operates across the country, especially the rebuilt models such as the one that collapsed here, Senior Vice President Robert Yancey said.