by Kirk Johnson, New York Times, February 2, 2009.
"Scientists said Monday that one of the biggest earthquake swarms ever recorded in the [Yellowstone National Park] took place in the last week of 2008 into early 2009, with 813 quakes in 11 days, most of them deep under Yellowstone Lake, and felt by almost no one. Only one other swarm, in 1985, was more intense. Records go back to 1973."
Apparently it wasn't a sign of a looming geological upheaval !!
An upheaval would have included dramatic things like blow-offs of steam and flows of lava, especially if the activity was under land.
It seems that a lot of people aren't aware of that, because the Yellowstone Volcano Observatory received hundreds of calls and email messages suggesting a connection with the end-of-the-world. Some quoted the ancient Mayans as an authority for end-of-world disasters.
It is true that much of the Yellowstone area is above a giant ancient volcano that last erupted about 640,000 years ago. However, since then the area has been relatively calm. The observatory keeps its "volcano monitor level" for the park at normal or green - the lowest level of concern. The 'normal' level at Yellowstone represents between 1,000 and 3,000 minor earthquakes each year.
A theory (not noted in the article), describes a situation where frequent minor earthquakes deep underground is good news - because stress is being relieved. Large, strong earthquakes happen when stress builds up over long periods and is released all at the same time, possibly causing damage at ground level. The damage would be limited in sparsely populated rural areas, like national parks.
The scientist-in-charge did express some frustration because the recent swarm of earthquakes was mostly under Yellowstone's largest and deepest lake - more than 300 feet deep. It is somewhat more difficult to observe changes in hydrothermal and magma activity when it could be underwater! I guess two of the things the scientists monitor would be the water levels and the temperature of the lake water. If the scientists do monitor those things, it would have been good if it had been noted in the article, otherwise some readers wouldn't be completely convinced that the earthquake swarm was harmless.
More information here :-
Idaho Press-Tribune: Yellowstone Activity Not Precursor To Eruption