initialize() sets the accelerometer range to +-2g
In that case, you're looking at +-16384 for 2g.
You might need to use setFullScaleAccelRange to raise the sensitivity to +-4g
I'm not aware of any force-sensitive interrupt, are you? There's a few things that can trigger an interrupt, FIFO overflow, i2c interrupt, data ready.
That said, you might just want to analyze values over time. The DMP outputs at 100hz if enabled. Otherwise the device is adjustible via a divider setting and lowpass filter setting. The DMP is kind of nice. You'll get 10ms between samples. So if you find one sample has a high g force, and the LSB difference between that and the next is >=2g, that's a shock, I think.