The size and type of earthquakes a given fault system may produce remain poorly understood for most major fault systems. Recent superquakes, such as the March 2011 magnitude-9 off Japan and the December 2004 magnitude-9-plus off Sumatra, have been far larger than what most scientists expected those faults to produce. The problem is that current models rely on short historical records, and even shorter instrumental records. Today, scientists are working to rewrite these models based on new paleoseismic and paleotsunami data to create a more comprehensive picture of earthquake activity through time. What they're finding might alarm you.