Provided is an earthquake response system using a seismic motion sensor. The seismic motion sensor of the earthquake response system includes: a sensor unit measuring a sensor value including peak ground acceleration (PGA) of the ground due to shaking; a seismic motion sensing unit sensing seismic motion on the basis of a seismic motion sensing result value calculated from an artificial neural network that uses the peak ground acceleration as input when the peak ground acceleration satisfies a seismic motion sensing condition; a seismic motion determiner determining generation of final seismic motion on the basis of a seismic motion determination parameter calculated from the sensor value on the basis of the seismic motion sensing result; a shaking grade calculator calculating a shaking grade of the determined final seismic motion through the peak ground acceleration; and a communication unit notifying the shaking grade of the final seismic motion.