Final answer:
A seismograph is the instrument that makes a continuous permanent record of earth motion, essential for measuring earthquake arrival times and estimating the epicenter's distance.
Step-by-step explanation:
The instrument that makes a continuous permanent record of earth motion is a seismograph. Seismographs are essential in measuring the arrival times of earthquakes and can determine the distance to an earthquake's epicenter with precision. They do this by comparing the arrival times of S-waves and P-waves, which travel at different speeds through the Earth. By calculating the time difference between these waves, geophysicists can estimate how far away the earthquake occurred.
Additionally, geophones are devices that convert ground movement into voltage, recorded as seismic responses to analyze Earth's structure. While seismometers are indeed used to measure ground motion, the precise term for the recording device that produces a permanent record is a seismograph. This precision plays a crucial role in fields such as global plate motions measurements and can have significant implications in situations such as detecting nuclear bomb tests that violate test bans.