How do dogs sense earthquakes?

There was a series of small quakes (< or = magnitude 4.0) in my town last year. This series spread over a couple of months. I was sitting in the living room with my dog lying on the floor for two quakes, and did not notice that my dog was aware in advance. The dog's reaction when the quakes struck was to bark at them. Then the biggest quake, a 4.0, struck, and the dog was terrified from then on of the smallest quake. In fact he is still terrified today when strong winds make the house creak, the way it creaked during a quake.