Have a read of this article;
The accelerometer chip in iPhones can detect seismic movement and may even provide a few seconds of warning before the most violent shaking strikes
www.smithsonianmag.com
Thanks a lot Helmsley for that amazing article/link!
[part of that article:]
->There are some limitations: the iPhone accelerometers aren’t as sensitive to weak vibrations, so during the tests,
->they were only able to record movements that correspond to earthquakes that would register as magnitude 5 or
->higher
.
That's also my experience. Unfortunately....