Who hasn't been jarred awake at night wondering what went bump!?

Hank Heasler doesn't worry too much about it...unless his cellphone starts chirping. For Ranger Heasler, the staff geologist at Yellowstone National Park, that impatient phone message means something's going on in the park's seismic world.

That chirping cellphone shouldn't come as too great a surprise to us. After all, Yellowstone is set atop a simmering volcano. Complementing -- or perhaps complicating -- that is the fact that the park is near the boundaries of some of the "tectonic regimes" associated with the North American tectonic plate.

Those regimes include the Yellowstone "hotspot," a plume of magma burning like a candle beneath the North American plate, and the Basin and Range Province just to the west, a landscape that has been defined ever-so-slowly (in human time) by the stretching of the Earth's crust and upper mantle. With the exertions of these various forces, it should be no surprise that Yellowstone is one of the more geologically active spots in North America.

Which brings us back to Ranger Heasler's cellphone.

"I'm alerted only at magnitude 2 and greater," says the geologist, who had a lot of restless nights early this year when a swarm of about 900 earthquakes rattled the park between December 26 and January 8. "I got enough sleep, but it wasn't a solid eight hours."

At last count, seismologists affiliated with the Yellowstone Volcano Observatory had determined that at least 111 of those quakes carried a magnitude of 2 or more. Eighteen were magnitude 3.0 or better, and the strongest was recorded at 3.9 on December 28.

"Right now," Ranger Heasler said the other day, "the analysis is ongoing as to what caused this. Probably in a true scientific sense there will be one or two possibilities."

For example, he explains, the swarm could have been tied to strains and stresses in the lithospheric plate beneath the park. In other words, the quakes could have been tectonic in nature. Or they might have been "hydrothermal" in nature, as the hot geothermal waters under the park can fracture rock. Or they might have been volcanic in nature, as the movement of magma can fracture rock.

While scientists with the observatory, which is a collaboration between Yellowstone's geologists, the USGS, and the University of Utah Yellowstone Research Group and Seismograph Stations, pore over the seismic footprints left by that swarm, an elaborate system spread throughout the park is listening for, and ready to record, the next geologic burp.

"There's quite an array of scientific equipment for the monitoring of the Yellowstone volcano and earthquakes," explains Ranger Heasler.

In a very basic sense, this "array" includes seismographs to measure earthquakes, GPS instruments to measure ground movement, and relatively new "borehole strainmeters" (thanks to a National Science Foundation grant) that measure ground deformation. Additionally, the park has stream-gauging stations that not only measure water flows but also water temperatures and, in some cases, the chemical constituents of the water.

The data churned out by these devices are all available in near-real time via links on the observatory's web page.

"We consider it very positive," says Ranger Heasler, "but it can also be a curse because sometimes individuals who aren't trained in how to interpret that data can misinterpret the data and get very excited."

Yellowstone's geologist hasn't gotten very excited lately, but he and his colleagues at the observatory -- Dr. Robert Smith from the University of Utah and Dr. Jake Lowenstern, the scientist-in-charge at the observatory -- have put in some very long days and nights recently.

"Anytime something unusual begins, we just start carefully monitoring that situation to see if there are any indications of danger to the public or to the (park) employees," says Ranger Heasler. "This (recent swarm) was highly unusual, but not out of the normal range of activity for Yellowstone."

For instance, he notes, back in 1985 the park endured a swarm of about 3,000 earthquakes, including some that peaked at a magnitude of 4.7. That 1985 swarm, points out Dr. Lowenstern, went largely unnoticed by the general public simply because the real-time data was not available via a click or two of your mouse.

"While that swarm would have gone largely unnoticed by the public," Dr. Lowenstern said from his Menlo Park, California, office, "this one they could have watched on practically a minute-by-minute basis."

Now, all the data being made available to the Yellowstone Volcano Observatory are crunched so Ranger Heasler can relay to his superiors in the park and to the Homeland Security offices in Wyoming, Idaho and Montana just how serious, or not serious, the earthquakes are.

"All the 'what-if' questions start going through our minds and the seismologists and volcanologists can give me a better assessment" of the possible risks, he says. "It's an interesting challenge, and it's a very, very important one from my perspective."

Is there a magic number of earthquake magnitude that would spur an evacuation of Yellowstone?

In a word, no.

"Any evacuation depends on a variety of factors," says the park geologist. "That's why the monitoring system is constructed the way it is. No single line of evidence is conclusive."

Large earthquakes, he explains, aren't necessarily precursors to significant volcanic activity. For instance, the Hebgen Lake earthquake that struck shortly before midnight on August 17, 1959, just outside Yellowstone's western border measured between 7.3 and 7.5 in magnitude. It killed 28 people and caused an estimated $74 million in damage in 2006 dollars.

While Yellowstone was evacuated in the aftermath of that quake, it was not due to a volcanic threat. Indeed, there was no associated volcanic eruption, says Ranger Heasler. Rather, the evacuation was ordered due to damage to the park infrastructure.

Understanding that, if there were a magnitude 7 quake in the park tomorrow there wouldn't necessarily be an evacuation unless there was serious damage to Yellowstone's infrastructure or other signals pointing to larger problems, he said.

Now, if such a quake had been preceded by thousands of smaller earthquakes leading up to that magnitude 7 quake, or if there was immense ground deformation noticed in the park, or substantial changes in the park's famous geothermal plumbing, then an incident command team would be set up and the Yellowstone Observatory scientists would interpret the data they were receiving.

Whether such a roll-up to a substantial volcanic eruption would play out over a number of days, weeks, or months, is hard to say.

"Yellowstone is a very active place. That's why it has this phenomenal hydrothermal system," says Dr. Lowenstern, who also has an alarmist cellphone. "There's a tremendous amount of heat that is rising off this deep magma system, and it causes a lot of activity. ... Basically, the lesson there is it takes a lot to make Yellowstone erupt. It hasn't erupted for 70,000 years, and a lot of activity can take place without leading to an eruption.

"If we saw the same (recent) behavior that we see at Yellowstone on one of the Cascade volcanoes (such as Mount Rainier), we would strongly suspect that an eruption was in the offing," the volcanologist adds. "Because Yellowstone has magma that is more shallow, it has a very active tectonic system, there are lots and lots of faults that are well-developed and active, it has this aquifer of boiling groundwater, which generally causes its own earthquakes, the system is just far more active than a typical dormant volcano.

"Yellowstone is dormant, but it has all sorts of activity. Most volcanoes that are dormant show no signs of activity."

All that said, Dr. Lowenstern said it's also possible that there would be no precursor to an explosion from Yellowstone's hydrothermal plumbing. While every year there is some such explosion somewhere in the park that tosses rocks into the air, according to the geologist, it's been a long, long time since there was one of any spectacular substance.

"In the last 15,000 years there have been explosions that created holes in the ground that are over a mile across. So they can be big," he says. "But since the park has been founded, the largest explosions created holes on the order of 5-10 meters (across)."

Mary Bay, on the north end of Yellowstone Lake, was created by such an explosion about 13,000-14,000 years ago. The resulting crater is roughly 2 miles wide, says Dr. Lowenstern.

Such geologic possibilities led to a meeting last November in which officials discussed the need to develop protocols and procedures that would be followed during times of "geologic unrest." That document should be ready for public review in about six months.

Traveler bonus: Listen to Dr. Lowenstern's analysis of the recent swarm in this podcast, and watch this videocast from Yellowstone to further your understanding of the park's geologic underpinnings.