This animation shows a drum-style horizontal seismograph station that records back- and-forth (N-S, E-W) movement. The suspended mass remains at rest as the spring and pivoting knife absorb most of the motion during an earthquake. The instrument shifts back and forth due to the oblique travel direction of the P wave and the shearing movement of the S wave. The stylus (pen) affixed to the mass, records the relative motion between itself and the rest of the instrument, thus recording the ground mo