Hmm, why does this needs to be an app and not the built-in alert notification system? Outsourcing critical infrastructure and emergency services to private parties is always a terrible idea.
This wasn't big enough in Tokyo to send out one.
That would include Apple and Google.
Under these constraints it is effectively impossible to send automated alerts at scale with low latency as demonstrated here. A private app does not operate under such constraints.
For major quakes I think over communication is probably warranted.