Was at lunch when it hit. I felt it sitting down at a table. I looked at others in the store and I didn't see anyone reacting to it at all. It was kind of subtle but it lasted for 5 or 6 seconds. Just a little bit later, the earthquake reporting apps on my iPhone and iPad show a notification. Nearly 15 minutes after it happened, I get an alert on my phone from the state emergency services department reporting the shake. Quite useless, I'm afraid. A 5.1 is the largest we've had around here in sometime.
If you're close to the epicenter there's no way for any app to give you advance warning.
Any app is going to make use of the fact that internet messages travel faster than earthquakes, so the farther you are from the epicenter the more warning you'll get. Close to the epicenter is going to be no warning regardless of whose app you use.
