The system is based on the ADXL335 accelerometer that has a detection range of ±3 g. It can measure the static acceleration due to gravity , as well as dynamic acceleration resulting from motion, shock, or vibration.
A metal ring mounted on the sensor ADXL335 contains perimeter loose steel spheres and magnifies the weak vibrations of the building creating a more intense shock to the sensor

the vibration of the building will turn on the device and it will sound alarm
the LED will turn red

will also activate wireless receivers alarms in other rooms at frequency 433Mhz

https://www.youtube.com/watch?time_continue=1&v=2_NDtMLdBQY