magnitude

Earthquakes are measured with the Richter Magnitude Scale developed by Charles F. Richter of California Institute of Technology in 1935. Here is a summary of the Richter scale:
Magnitude less than 3.5: Generally not felt, but recorded.
Magnitude 3.5-5.4: Often felt, but rarely causes damage.
Magnitude under 6.0: At most slight damage to well-designed buildings over small regions.
Magnitude 6.1-6.9: Can be destructive in areas up to 100 kilometers across where people live.