IS IT AN EARTHQUAKE or simply a shock, asked Cole Porter almost 70 years ago. That question, or one almost like it, is puzzling economists today, as they try to decide whether the U.S. economy has merely hit a speed bump in the road to continued recovery, or driven into a ditch. The nervous types worry that high oil prices will scare consumers out of the malls, that a house-price bubble is about to burst, that the Chinese economy is about to implode, and that America’s huge trade deficit will cause a massive sell-off of the dollar, forcing the Federal Reserve Board’s monetary policy committee to raise interest rates to recession-causing levels. Not to mention the possibility of a terror attack. There’s more, but you get the idea.