Earthquake waves sensor
If you live in an earthquake zone, a device like that may be helpful for you, though it is not a full protection. When an earthquake occurs, two kinds of waves are emitted from the centre:
a primary wave and a secondary wave. The primary wave causes a small shaking, but it is faster. Therefore the primary wave is the first sign of an earthquake if your house is located some kilometres away from the centre.
In the images below you can see a simple quake waves sensor, which I built by myself.
That device consists of an high sensitive lever, which is hooked to a spring. A weight is fixed at the end of the lever. When the sensor shakes, a screw fixed on the lever will touch a second spring, and it will close a circuit which drives a piezoelectric bell. A delay off circuit is used to send a stable signal to the bell.
The lever works with a frequency of about 2 Hz.