How is the intensity of an Earthquake measured?
Well for measuring the intensity/magnitude of an earthquake we usually use Mercalli's intensity scale.
For weak earthquakes we usally use a manner in which the earthquake is felt by people as a measure.
The higher numbers of the scale are based on observed structural damage.
You can check the Mercalli's intensity scale here:
Who is Mercalli?
Giuseppe Mercalli (May 21, 1850 – ✝March 19, 1914) was an Italian* volcanologist and Catholic priest.
He is well remembered of his Mercalli intensity scale with which we measure the intensity of earthquakes.