It has been a shaky few months here. More than 1,200 earthquakes have rumbled Yellowstone National Park this year, up from 800 last year and 179 the year before.
While this park is world famous for its skyrocketing geysers, simmering hot pools and bubbling mudpots, what is less well known, because it is invisible, is the underground volcano that created these geothermal attractions. Slumbering fitfully beneath sagebrush-studded meadows and lodgepole-pine forests, this volcano, known as a caldera, is believed to be causing the strings of earthquakes, known as swarms. It is by far the largest "hot spot" in North America and one of the largest in the world.
The surrounding region is one of the most seismically active in the country, and some of the quakes there have been quite large.
On Aug. 17, 1959, for example, a quake of magnitude 7.5 shook an area west of Yellowstone, jarring loose some 80 million tons of rock -- half a mountain -- and burying 25 sleeping campers at a national forest campground. It also dammed the Madison River and created Quake Lake.
In 1983, the 7.3 magnitude Borah Peak quake, centered about 100 miles west of the park, shook Idaho, killing two children, and lengthening the period between eruptions of Old Faithful, from an average of about 69 minutes to 77 minutes.
With the help of locational satellites and tomographic imaging, which uses sound waves to create a subterranean profile, scientists have determined that there is partly molten rock five to six miles below the surface, beneath a crust that has been repeatedly fractured by eruptions for two million years. The movement of that molten rock, called magma, or of hot water or gasses beneath the geologically unstable region, is probably causing the quake swarms, said Robert B. Smith, a seismologist at the University of Utah in Salt Lake City who, with Lawrence W. Braile, published a paper about the caldera last year in The Journal of Volcanology.
With five major geological fault lines buried in it, the caldera has a much more widespread effect in the Northern Rockies than was previously thought, they say. The movement of the molten and semi-molten rock could cause earthquakes as far south as the Wasatch fault in Utah, as far north as Bozeman, Mont., central Idaho to the west and Cody, Wyo., to the east, Dr. Smith said.
Earthquakes are most often thought of as a sharp jolt followed by a series of aftershocks, the largest of which is usually an order of magnitude smaller. The aftershocks then trail off over time. The swarms that occur in volcanic regions "start with one quake, build to a crescendo and then die off," Dr. Smith said.
"A swarm can last hours, days or a month," he said.
Dr. Smith said not enough was known about the subterranean structure of the caldera to say if the recent quake swarms suggest that more quakes, or larger quakes are to be expected. There is no reason to believe that a major earthquake is going to shake Yellowstone anytime soon, he said, or that the caldera is ready to explode, although both are always possible. The park has 24 seismic stations that are constantly monitored.
Late in June and in early July, a series of 550 small earthquakes, some with a magnitude as high as 3.1, rattled the Mount Haynes area a few miles east of the town of West Yellowstone. Then, starting on Oct. 6, more than 100 earthquakes up to magnitude 4.3 shook an area near Mount Sheridan and Lewis Lake, in the park's southern end.
"I doubt we could have volcanism here without plenty of advance warning," said John Varley, research coordinator for the park. "The earthquake guys can't say that."
The existence of the caldera was not known until the 1950's. And it was not until the mid-1980's, when a park geologist was comparing aerial photographs taken a decade apart of Yellowstone Lake, in the center of the park, that clues to the enormous movement of the caldera began to emerge.
The photos showed the water level at the south end of Yellowstone Lake was rising. Scientists discovered that the caldera, which sits under the northern portion of the lake, was pushing up the lake's northern tip, causing water to run to the southern end.
Measurements by Dr. Smith and others show that from 1923 until 1977, the ground above the caldera was pushed up two feet. From 1977 until 1985, the ground gained an additional foot, rising at what geologists consider a remarkable rate of more than an inch a year.
Then, during a one-month period in 1985, a large swarm of some 2,000 quakes as strong as magnitude 4.5 shook the park's northwest side. A short time later, the caldera stopped rising and started to drop, and it continues to do so at the rate of about half an inch a year.
Dr. Smith theorized that the dropping of the landscape might be one of the reasons for the earthquakes. As the caldera, and thus the park, sinks, it occupies volume, and might be causing magma and hot water to squirt out laterally into the highly fractured crust. "As this stuff intrudes through the broken-up piece of crust," Dr. Smith said, "it increases the pressure and causes a lot of little earthquakes."