As the numbers of buried or dead continue to climb from today's 6.9-magnitude earthquake in China, an event so close on the heels of the devastating Chile and Haiti earthquakes, you might wonder if Earth is shaking more lately. Perhaps, scientists say, but not unusually so.
Seismic activity may be higher in recent years than the long-term average, but it's still not out of the normal range, the experts contend.
"Relative to the 20-year period from the mid-1970's to the mid 1990's, the Earth has been more active over the past 15 or so years," said Stephen S. Gao, a geophysicist at Missouri University of Science & Technology. "We still do not know the reason for this yet. Could simply be the natural temporal variation of the stress field in the Earth's lithosphere." (The lithosphere is the outer solid part of the Earth.)
News of the Yushu earthquake in China, which followed large temblors in the United States and Mexico, Haiti, Chile and the Ryukyu Islands of Japan — all in this young year alone — make it seem Earth is becoming ever more active. Much of this has to do with perspective, however.
"From our human perspective with our relatively short and incomplete memories and better and better communications around the world, we hear about more earthquakes and it seems like they are more frequent," said J. Ramón Arrowsmith, a geologist at Arizona State University. "But this is probably not any indication of a global change in earthquake rate of significance."
As the human population skyrockets and we move into more hazardous regions, we're going to hear more about the events that do occur, Arrowsmith said.
And when large earthquakes strike in populated regions, news travels fast.
"What happens is when a lot of people get killed there's a lot of reporting of it, and if an equally big event occurs somewhere out in the middle of nowhere it doesn’t attract the attention," said G. Randy Keller, professor of geophysics at the University of Oklahoma.
Even rare events are normal.
"The [8.8-magnitude] earthquake down in Chile, that's an unusual event, those don't happen too often," Keller said. Even so, seismologists would expect such an event at some point. "It sounds cold-blooded but it's an earthquake that occurred in a place that you'd expect it to sooner or later."
In fact, both the Ryukyu and Chilean quakes occurred within the Ring of Fire, which is a zone surrounding the Pacific Ocean where the Pacific tectonic plate and other plates dive beneath other slabs of the Earth. About 90 percent of the world's earthquakes occur along this arc. (The next most seismic region, where just 5 to 6 percent of temblors occur, is the Alpide belt, which extends from the Mediterranean region eastward.)
When abnormal is normal
Or the series of earthquakes could just be part of the natural cycle for what is arguably a dynamic planet.
"If you look at it globally the occurrence of earthquakes is confined to zones we already know have earthquakes but it's a largely random process and so sometimes it's a little quieter than normal and sometimes it's a little more active than normal. But it doesn't mean anything, because on a global basis these things aren't connected," Keller said.
He added, "We're having a few more than the average, but nothing particularly remarkable. This was a magnitude 6.9 earthquake so on the bigger scheme of things it's not that big."
China on the whole has suffered staggering numbers of lives lost due to earthquakes, particularly in a region far to the east of the current quake, Keller noted. That's where he and his colleagues have set up seismic recorders to figure out why that area is prone to such large temblors. For instance, in 1556 an 8.0-magnitude quake that struck Shensi, China, killed an estimated 830,000 people.
And in 1976, a 7.5-magnitude quake in Tangshan, China, killed 255,000. These areas are some 500 to 1,000 miles to the east of the current earthquake.
China quake not unexpected
What is different about the earthquake that struck the area of Yushu in Qinghai Province, China, early morning local time is that it occurred in the middle of one of Earth's tectonic plates, instead of at the junction between them as many earthquakes do. The China earthquake occurred along a fault along the northeastern part of the plateau, though scientists have yet to pinpoint that fault.
"It's in the northeastern part of the Tibetan Plateau, which is a very active tectonic feature," Keller said.
The Tibetan Plateau experiences continued uplift from the processes that originally created it, and is also being squeezed by other forces, resulting in numerous faults in the area.
"Because of this big stress that's a result of India plowing into Asia, there's been a whole series of these big faults that are from older events," Keller said, adding that the faults can get reactivated and cause earthquakes like the recent one. These "old faults" are on the order of some 100 million years old.
"It's like a slow bus piling into a building and the driver still has his foot on the gas and it's just a continuing process," Keller said today in a telephone interview.