Electronics enthusiast 'veedo' has created a device that monitors for seismic activity and other vibrations which displays the data via a simple Arduino-based local web server that can be accessed from any Internet-connected device. Using a film-tab piezo sensor and analogue circuity, the system detects frequencies of around 1 to 20 Hz.
For more details, circuitry, code and so on - visit the instructions here. To recreate your own version - you can save time and money by using our Freetronics EtherMega - as it is a fully Arduino Mega2560-compatible board that also includes a microSD socket and Ethernet:
Furthermore the sensor circuitry can be built onto a matching Mega-sized Protoshield: