https://earthquake.usgs.gov/earthquakes/eventpage/nc73799091...
Santa Clara had 2 seconds of advance warning, and Sunnyvale had 4 seconds. San Francisco got 18 seconds of warning, though was outside the "ground motion" zone.
Though I wish it would have overridden silent mode. 10s of advanced warning isn't all that helpful if I only find out about the notification 10 minutes later.
Apparently my phone supports notification history, but it was also turned off by default... Bad defaults strikes twice in a row.
Earthquakes move through the ground at the speed of sound. Once earthquake detectors on the ground near the epicenter detect it they send the information out at the speed of light, so even factoring in a little bit of delay for calculations, advance warning on the order of several seconds is possible depending on how far away the recipient is.
(In this case it seems like the app worked.)