The device which measures earthquakes is called the Richter scale.

The word measures means to determine the size, amount, or degree of something.

In the given sentence, the Richter scale is used to measure the intensity of earthquakes.

Option C: gauges is the correct synonym because it also means to measure or estimate the magnitude or extent of something.

Option A: prevents means to stop something from happening, which is unrelated.

Option B: weighs refers specifically to measuring weight, not general measurement like earthquake intensity.

Option D: calculates means to compute mathematically, which is not the exact meaning in this context.

Therefore, the best synonym for measures here is gauges.