Maybe you thought a truck was rumbling by. Or maybe you thought it was a terrorist attack. But then, you checked Twitter and saw that dozens of people were all asking the same thing: “Was that an earthquake?” A decent amount of shaking hit the Eastern seaboard on Aug. 23, jostling D.C. with a 5.8 magnitude quake. With few injuries and limited damage, it was easy to giggle about a trembler here in the seismologically placid mid-Atlantic. But the spectacle of flummoxed local governments and a mad rush-hour dash out of town was something else entirely. Ten years after Sept. 11, we’re still not up to speed on handling panicked crowds.