#low-frequency waves

By Andrew E. Kramer, NY Times, February 17, 2013 CHELYABINSK, Russia--The shock wave from a meteor that exploded above Siberia last week somehow sheared the roof off a brick and steel factory building while leaving a nearby glass facade unscathed.

In some high-rises in this city, the first modern urban community to have felt the breath of a cosmic close encounter, every window blew out on the top floor; elsewhere, the ground floors suffered.

More ominously, reports came in to local news media over the weekend of stranger phenomena: behind unshattered apartment windows, glass jugs were said to explode into shards, dishes to crack, electronics to die. Balconies rattled. One man said a bottle broke right in his hand.

Anna V. Popova was at home with her daughter when she saw the flash, then heard explosions, then found the windows of her enclosed balcony blown in; her neighbor, with identical windows, escaped without property damage.

Scientists believe the space rock that tore through the atmosphere on Friday morning and blew apart here was the largest to have entered the atmosphere since 1908 and that it was unusual as well for the scale of its effects: more than 1,200 people injured and broad property damage.

Indeed, the event is providing a first indication of the type of structural and infrastructural costs meteors can exact from a highly industrialized society. NASA scientists say a meteor of this size strikes the Earth about once every hundred years.

Shattered glass caused most of the damage and injuries here in Chelyabinsk, a sprawling industrial city of about a million people.

What shattered the glass, scientists say, was both the explosion as the meteor fragmented and the waves of pressure created as it decelerated. Such low-frequency waves--called infrasound--are sometimes detected by cold-war era nuclear blast sensors in remote parts of the Pacific Ocean or Alaska, according to meteor experts.

The waves can bounce off buildings and be stronger in some places than others; they can also resonate with glass, explaining why bottles and dishes might have shattered inside undamaged kitchens, as if crushed by the airy hand of the meteor itself.

"A shock wave is like a ball," Aleksandr Y. Dudorov, director of the theoretical physics department at Chelyabinsk State University, said in an interview. "Throw a ball into a room and it will bounce from one wall to another."

Infrasound waves have not previously been studied in a cityscape, Richard P. Binzel, a professor of planetary science at the Massachusetts Institute of Technology and an author of a textbook on asteroids and meteorites, said in a telephone interview. But he noted that the apparent randomness of the damage was consistent with the way such waves function.

"A shock wave can be coming from a particular direction, and if you face that direction you are more susceptible," Dr. Binzel said.

"One building might shadow another, or you may have a street that is optimally aligned to channel the wave, either in a fortunate or unfortunate way."

Peter Brown, a professor of physics at the University of Western Ontario, wrote in an e-mail that an infrasound wave "is very efficient at traveling long distances," and that "windows, structures or even glass jars susceptible to resonate at this frequency could be a factor to seemingly random damage at widely disparate locations."

Alekdander V. Anusiyev, the spokesman for the governor of Chelyabinsk region, characterized the damage here as without a discernible pattern. "It is impossible to say more glass broke in one part of the city or another," he said. "Glass broke everywhere."

The roof of the zinc factory that collapsed was reinforced with a lattice of steel beams and supported by concrete joists that are now broken, jutting upward with mangled re-bar protruding. Windows on a neighboring house blew in with such force that the frames went with them.

Yet a few yards away on Sverdlovsky Street, the cosmos spared a seemingly vulnerable Hundai dealership, a three-story cube sheathed in glass, with glistening display models inside. Not a window broke.