This web page http://www.chcp.org/seismo.html describes the mechanism inside the ancient first seismometer (see this post) of the first century AD.
From seismo.html: "Our seismometer is intended as a demonstrator. The visitor shakes the table to simulate an earthquake. Our pendulum is "tuned" to this input. The crust of the earth absorbs the high frequency content of a quake, the signal from a distant earthquake is in the sub-audio range. In order to detect actual earthquakes the pendulum would need to be several feet in length."