This isnt a detector, but its an interesting screensaver showing where they have been.
I thought it might interest some body.

http://www.mgshareware.com/qsmain.shtml

Sadly, we can only estimate the statistical likelihood of earthquakes, and even that on about 100 year timescales. The estimates are mainly based on historical activity in the region.

An instrument to predict earthquakes does not exist. Take it from a geologist (in training): it's a sham.
Or take it from the USGS .