Before every earthquake, there’s the inevitable rumble.  In his latest book, historian Jay Sexton focuses on the rumblings–foreign threats, economics, slavery, immigration and other global concerns–that brought “violent earthquakes,” periods of crisis, to our shores. He believes that centuries of disruptions have forged our nation more than anything we could have done by design. What is the nation’s narrative?   What will our unpredictable future bring? The answers might be found by revisiting our turbulent past.