Basically, you're wanting a seismic sensor or seismic detector; Forrest M. Mims III describes how to build such a homebrew device in his Engineer's Mini-Notebook "Science Projects"; this particular mini-notebook is out of print, but it might still be described in this newer edition:http://www.amazon.com/Science-Communication-Circuits-Projects-Forrest/dp/0945053320
Essentially, it involved suspending a magnet using 2-4 feet of wire or nylon fishing line from a fixed sturdy beam in your house. This magnet was then suspended above a pickup coil made from the coil of a telephone pickup, or a 9-volt relay; the voltage induced in the coil as the magnet swung from seismic events would then drive a 741, which was cascaded to a second 741, which would then cause an LED to flash. It was also suggested that for a permanent installation, to bolt (using L-brackets) a piece of 2-4 foot metal or plastic pipe to concrete, put the magnetic pickup in the bottom, drill an "observation port" in the side of the pipe near the magnet, and suspend the line from a pipe cap on the upper end.
I suppose if you built such a thing, it could be interfaced to an analog input on the Arduino in some manner, to record voltage swings.
Something else you could do, along the same lines:
Get three cheap 6-8 inch bass speakers, and carefully cut away the speaker cones (don't cut the lead-in wires to the coil!) leaving 3-4 "strips" running from the coil to the cone edge (you might try cutting the entire cone away, leaving the coil suspended by the "spider"). Using epoxy, mount a large mass to the center coil, keeping it well centered. A good mass would be round lead fishing weights (if you have the means, recasting the lead-weights into a proper round shape would be best - takes nothing more than a steel pipe-cap, a propane torch, and a simple sand cast mold). You want them to be fairly heavy, but not so heavy they can warp the spider on the speaker (causing the coil to rub against the permanent magnet).
Make a 3-side steel corner from four pieces of 12 inch x 12 inch plate steel (ok, you'll need to weld some), plates should be orthogonal to each other. Then mount the speakers to each plate, with the baskets facing inward (use epoxy). The whole steel plate construction should then be bolted to the concrete floor, with one of the vertical speaker axis along a north/south line, the other along an east/west line, the third facing up, of course - these will be your x/y/z axes.
You'll need to use a similar 741 or other op-amp system to amplify the voltage produced by each speaker, but from there, you can read using the Arduino and the analog pins the three voltage levels, and be able to sense the vibrations in three dimensions. Three (or more) such sensors, spaced widely apart (as in hundreds or thousands or more feet apart) could be used (maybe?) to triangulate the source of vibrations.
The basic idea, though, is the detector is a device whereby a mass is suspended and moves independently of a detector of some sort (in this case, the mass is a magnet/lead weight combo, and the detector is a coil - the the mass could be anything, and the detector nearly anything - you could even do an optical sensor if you wanted; I imagine there are other methods possible as well). The biggest problem with detecting such vibrations is that the sensor needs to be relatively large, and bolted/mounted to something relatively stable (bedrock is ideal, but if you can't mount to that, then a concrete foundation is the next best thing).
Good luck with your project, and I hope this helps...