Two identical seismometers are oriented so that one detects ground motion in the…
PHYSICS · Single correct · A Level · Physics · Waves · Waves
Problem
Two identical seismometers are oriented so that one detects ground motion in the north–south direction and the other in the east–west direction. A distant earthquake sends waves that arrive simultaneously at both instruments. Only one seismometer registers a large amplitude signal, while the other shows almost no motion. Which conclusion is most consistent with this observation?
(a) The wave was a P-wave; because its longitudinal motion is confined to the propagation direction, only one horizontal component appeared on the instruments.
(b) Both P- and S-waves reached the station, but they arrived out of phase such that they destructively interfered on one seismometer while reinforcing on the other.
(c) The recorded signal was an S-wave whose transverse motion happened to be polarized along the axis of the active sensor, so the orthogonal sensor detected almost no displacement.
(d) Surface waves produced the motion; the mix of longitudinal and transverse components canceled along one axis but added along the perpendicular axis.