A slight change in the eruption rate of Yellowstone National Park's Old Faithful Geyser tipped off a park official to an impending earthquake, he said.

Rick Hutchinson, a National Park Service research geologist, said he was able to predict in November the earthquake that rocked the Yellowstone area shortly before Christmas thanks to a shortening in the interval between the geyser's eruptions."The average was dropping," he said. "That suggested some seismic activity was on the way."

In the first 21 days of November, Hutchinson measured the time between the geyser's eruptions at an average of 74.8 minutes, almost 4 minutes less than the average interval of 78.9 minutes recorded in the spring of 1989.

"In the past, when the average interval has dropped, there tended to be, in the relatively near future, local seismic activity," he said.

On Dec. 23, an earthquake measuring 3.8 on the Richter Scale shook West Yellowstone, Mont.

The earthquake's epicenter was about 26 miles northwest of Old Faithful.

Hutchinson said that in the past, Old Faithful's eruption intervals have lengthened following earthquakes.