context: At the opening of the movie we meet the seismologist Dr. Amy Lane (Brittany Murphy) in the midst of giving a speech about how we are ill prepared for earthquakes. At that very moment, a big earthquake hits.
  Parting from her husband Sebastian (Jack Goldenberg) and little daughter, Dr. Lane is flown by helicopter to the epicenter of the quake, which is a mining field in West Virginia. There she finds and rescues a Charley 'Boomer' Baxter (Eriq La Salle), whose car was dragged down into a crack in the ground with him still inside. Boomer is feeling bad as he believes he triggered the quake when he set off some explosives, but Dr. Lane assures him it wasn't his fault. New tremors occur, and a massive crack in the ground starts eating its way across America.
  Meanwhile, Sebastian and Amys daughter are being flown home to Colorado when the quakes knock out the airport beacons and the next moment the plane collides with another one and crashes to the ground. Sebastian and Amys daughter escape from the crash alive and catch a ride from a tanker truck. Then they get chased down the road by the suddenly appearing megafault, while stuff explodes all around them.
  In the meantime Dr. Amy Lane does a remarkable job focusing on how to stop the megafault in its path. Apparently, the way to do this is to use some kind of beam from an orbiting satellite to freeze the groundwater, thus triggering a second quake that will somehow neutralize the first one. But in its first attempt this fails, and now the megafault is headed towards a dormant supervolcano in Yellowstone National Park. The new plan is to blast a trench the size of Grand Canyon into the ground in order to prevent it.
question: How do they decide to block the fault's path?
answer: Blasting a trench the size of the Grand Canyon in its path.