The first signs of the impending disaster are entirely sub-terranean and highly classified.
The Seismic Ring: The United States Geological Survey (USGS) begins tracking a bizarre chain of micro-earthquakes. These tremors don't follow natural fault lines; instead, they perfectly trace the outermost continental shelf of North America, cutting through the northern border of Canada and the southern tip of Mexico.
The DARPA Investigations: The Department of Defense establishes a classified task force to investigate unexplained magnetic fluctuations. Submarines patrolling the Gulf report their compasses spinning wildly when crossing specific longitudinal coordinates. DARPA scientists theorize it is an experimental, localized magnetic weapon being tested by a foreign adversary.