Italy, Myanmar quakes show Earth's seismic volatility

The powerful earthquakes that rocked Italy and Myanmar only hours apart Wednesday were in no way related, but show just how seismically volatile our planet is.

Earth is rocked each year by more than 100,000 earthquakes with a magnitude-3 intensity or greater, and hundreds of smaller quakes move the ground beneath our feet every day — many too small for humans to even feel.

The shaking is caused when the planet's massive tectonic plates suddenly slip along a fault line. The plates are always slowly moving, but sometimes their edges become stuck due to friction. That stress puts pressure on the plates that is then released in waves that travel through the earth's crust and up to the ground beneath our feet — an earthquake. Humans typically don't feel the shaking until it reaches a magnitude-3 or greater.

It's not unusual for powerful quakes of a magnitude-6 or greater to occur in two vastly separate places in the world on the same day, said U.S. Geological Survey geophysicist John Bellinni. With about 100 quakes at that strength or higher each year worldwide, the number of these powerful temblors averages out to about two per week.

Wednesday's quakes occurred in two completely different seismic zones, Bellinni said, and at locations more than 5,000 miles apart. Italy's quake struck at a magnitude-6.2, killing scores of people. The one in Myanmar measured even higher at 6.8, but so far only three people are reported to have been killed.

The disparity is a result of the fact that deeper quakes tend to do less damage. That's because the strength of shaking from an earthquake diminishes the farther you get from its source, meaning the strength of shaking at the surface from a 300-mile deep quake is considerably less than a 12-mile deep one, the USGS said.

Italy's quake occurred at a depth of about six miles below the surface, while Myanmar's took place some 50 miles underground, said Susan Hough, a USGS seismologist. Italy is a seismically active region, and that quake occurred along a boundary where the tectonic plate of Africa is crashing into the European plate, Bellini said.

"Italy has a history of such tragic, moderately large events," said John Vidale, director of the Pacific Northwest Seismic Network at the University of Washington. "It is not the most active area in the world, but this is no surprise," he said.

As for whether one quake can impact another, Hough said it is possible for some massive earthquakes to send out shockwaves that influence fault lines in other areas. A magnitude-7.3 earthquake in California, for instance, triggered a separate quake some 250 miles away in Nevada in 1992. However, Italy's quake Wednesday was far too small to have that effect, Hough said.

A lot is known about earthquakes, but Hough said it's possible there are still interactions between two shaking events that we still don't fully understand.