Most phones today would already be able to feel an earthquake via the built-in accelerometer they have. Actually, there is a network of people sharing this information with each other already, called MyShake: https://myshake.berkeley.edu/
That app has great potential, but it's not really ready for prime time. Its rating in Google Play is 3.1 based on over 3,500 votes.
Today, they are apologizing for a problem wherein the app reported a 6.1 quake as being located somewhere in the United States, which is not very helpful.
Other reviewers have complained that the app is a power hog that drains a phone's battery pretty quickly.
> "Other reviewers have complained that the app is a power hog that drains a phone's battery pretty quickly."
I've seen it suggested before, but if this app or one like it only worked while the phone was plugged in, that would eliminate power consumption concerns and also provide cleaner data (a phone plugged in is usually sitting flat on a table, counter, etc, and is thus in a better position to collect seismic data than if it were in somebody's pocket.)
