How the power of an earthquake is estimated? At what strength it becomes destructive?

The power of an earthquake is expressed in terms of magnitudes on a scale called richter scale. Really destructive earthquake have magnitudes higher than about 7 on the richter scale. But bhuj and kashmir earthquakes had magnitudes greater than 7.5. Like many other scales in science (decibel is another example) richter scale is not linear. This means that an earthquake of magnitude 6 does not have twice the destructive energy of an earthquake of magnitude 3. In fact, an increase of 2 in magnitude means 1000 times more destructive energy. For example an earthquake of magnitude 6 has thousand times more energy that an earthquake of magnitude 4.