From the intro on Instructibles:
Living in California, earthquakes are a part of life. Sooner or later, you'll feel one, and the first time can be pretty scary! If it's a small earthquake, which thankfully most are, it's actually hard to know for sure until you check the news later on. Sometimes a painting shakes on the wall, or a hanging lamp starts swinging, or you hear a rumbling sound - most small earthquakes are much more difficult to notice than you would expect.
In order to detect low magnitude earthquakes, we wanted to build a very sensitive Seismometer. This project was quite easy, mostly some carpentry, and the detector was a PocketLab sensor, using the magnetometer function. An iPad was used for data collection.