The seismograph and the seismoscope are the two main instruments used to measure the strength of earthquakes. The seismoscope is a simple instrument that measures the time that an earthquake takes place. The seismograph records the motion of the ground during an earthquake.
A seismograph's main component is a hanging mass — as heavy as 1,000 pounds — that is connected to a pen, and this large pendulum is situated just above a paper surface. When the ground moves, so does the paper, which rubs against the pen and marks the movement. Seismographs are isolated and connected to bedrock to ensure that the data they receive is not affected by the movement of surrounding objects. Advanced technology improves the accuracy of the seismograph and makes it sensitive to minute ground movements.