Signal processing techniques for an accelerometer signal?
I'm wondering why the STFT pops out. To me, wouldn't a simple threshold on the signal itself or on its envelope do better / just as well, after removal of the g offset?
Once you decide what &quot;measure&quot; is best to detect your event, you can apply the work of Basseville and Nikiforov, that I answered here.

The classic reference for that problem is Detection of Abrupt Changes - Theory and Application by Basseville and Nikiforov.  The whole book is available as a PDF download.
My recommendation is that you read Chapter 2.2 on the CUSUM (cumulative sum) algorithm.