6.4: Recording Earthquakes
Recording Earthquakes
The formal study of earthquakes, called seismology Links to an external site., began with the development of instruments that were capable of detecting earthquakes. This instrument, called a seismograph or seismometer Links to an external site., can measure the slightest of Earth’s vibrations (Figure 6.9). A typical seismograph consists of a mass suspended on a string from a frame that moves as the Earth’s surface moves. A rotating drum is attached to the frame, and a pen attached to the mass, so that the relative motion is recorded in a seismogram (Figure 6.10). It is the seismograph frame, attached to the ground, that moves during an earthquake. The suspended mass generally stays still due to inertia. Watch the following NASA video about Newton's Second Law, inertia, with Astronaut Randy Bresnik aboard the International Space Station (ISS).
Figure 6.9: A USGS seismograph at the Point Reyes visitor center, near the San Andreas Fault. (CC-BY-SA 3.0; Dvortygirl Links to an external site.).
Figure 6.10: The P-waves are the first to arrive at each station, followed by the S-waves. Because the P-waves travel faster than S-waves the great distance between the two (S-P interval), the further away the earthquake epicenter. (CC-BY-SA 4.0; Benjamin J. Burger Links to an external site., modified by Chloe Branciforte).