What is measured by a seismometer during an earthquake?

A seismometer is an instrument equipped to detect and record the vibrational waves caused by seismic events such as earthquakes. The primary function of a seismometer is to measure the actual displacement of the ground as seismic waves pass through it. This displacement refers to the movement of the Earth's surface, which can be quantified in terms of how much the surface moves or shakes during an earthquake.

When an earthquake occurs, seismic waves travel outwards from the focus of the quake, inducing ground motion. The seismometer captures these movements by using a mass that remains stationary while the ground moves beneath it, thus allowing it to record the degree of displacement over time. This data is essential for understanding the characteristics of the earthquake and for advancing seismic research.

While other aspects, such as the magnitude, frequency of waves, and intensity of shaking, can be derived from the information that a seismometer provides, the direct and primary measurement it makes is that of actual displacement. This makes the option about actual displacement the correct answer in the context of what a seismometer measures during an earthquake.