Quake!

If the ground rocks and rolls but no one and nothing gets hurt, then it was a minor thrill ride, like the Jack Rabbit.  An earthquake in Pittsburgh is like snow falling in San Francisco:  Your adrenalin goes up in a big hurry, and when it's over, a few seconds later, you make sure that you have all of your things and wits about you. 

This was a quake.  When the ground moved in San Francisco in 1989, I was standing in the left field bleachers of Candlestick Park, waiting for a World Series game to start.  Everyone in the ball park that night watched the concrete lip of the upper deck undulate like the hips of a slow-motion hula dancer.  When the rolling stopped, a stadium full of people let out a monstrous cheer.  That was a major thrill ride, we're still here, and that was so cool!

Then the people in the stands turned on their portable TVs and tuned in to images broadcast from the Goodyear blimp.  Bridge sections collapsed.  Neighborhoods on fire.  SF police drove onto the field and told everyone to go home.  The lights in the stadium, and then the lights everywhere, went out.

When I moved to Pittsburgh, ten years later, a local real estate agent drove me around the city for a morning.  When she learned that I was moving in from the San Francisco area, she pointed out that all of the stone and brick buildings in Pittsburgh would hold up well in a (hypothetical) earthquake.  I think that the idea was that I should be comforted by all of these strong buildings after living through lots of ground-shaking on the West Coast.

Stone and brick and clay are, unfortunately, among the most brittle of building materials and among the least safe in an earthquake-prone zone.  Chris is surely right that a real seismic event in Western PA would damage the region's water infrastructure, probably critically, but in the developed areas of the region, infrastructure of all sorts likely would fail -- especially older bridges and tunnels, rail beds, all kinds of sizable buildings built on uncompacted fill, and scores of older houses, apartment buildings, and small businesses. 

Pittsburgh would be in Barney.  Rubble.  Trouble.