This app is incredible! From MyShake:
MyShake is a free app for Android phones that has the ability to recognize earthquake shaking using the sensors in every smartphone. The app runs “silently” in the background on your phone using very little power – just like the step-tracking fitness apps. When the shaking fits the vibrational profile of an earthquake, the app sends the anonymous information to our central system that confirms the location and magnitude of the quake.