The instrument used to study earthquakes is called a seismograph.

Here's how it works:

* Detecting Ground Motion: A seismograph has a sensor that detects the vibrations of the Earth's surface caused by earthquakes.

* Recording the Data: The sensor's readings are then recorded on a device called a seismogram, which is a visual representation of the earthquake's seismic waves.

* Analyzing the Data: Scientists analyze seismograms to determine the earthquake's magnitude, location, and depth.

There are different types of seismographs, but they all perform the same basic function: to detect and record ground motion.