Final answer:
A seismometer is the device that measures the size of seismic waves, which is essential for studying earthquakes. Seismometers are highly precise instruments, allowing scientists to determine the location of an earthquake's epicenter by comparing the arrival times of the S- and P-waves. Devices like Geiger counters are intended for measuring radiation, not seismic activity.
Step-by-step explanation:
The machine that measures the size of seismic waves is known as a seismometer. A seismometer, or seismograph, records the motion of the ground during an earthquake, explosion, or other earth-shaking event. Seismographs are sensitive instruments that can detect and record the vibrations caused by seismic waves, which are generated by earthquakes or artificial means like explosions.
The precision of a seismograph can be quite high, being able to measure the arrival times of earthquakes with a precision of 0.100 seconds. When it comes to determining the distance to the earthquake's epicenter, the arrival times of the S- and P-waves, which have different speeds (P-waves typically travel at 4.00 km/s and S-waves at 7.20 km/s in the regions considered), are compared. This allows scientists to calculate the distance to the source quite precisely.
Devices such as Geiger counters, scintillators, and dosimeters are used to measure radiation, not seismic waves. Photography of a Geiger counter shows a metal box with a read-out screen and a sensor wand, which is significantly different from the equipment needed to monitor seismic activity.