[saagarjha]

Actually, it takes around 0.3 to double to power.

[bhrgunatha]

USGS (and most reports use the moment magnitude scale - https://en.wikipedia.org/wiki/Moment_magnitude_scale :

Each point is 10 ^ 1.5 (about 31)

Every 2 points is 100

The Richter scale is almost never used now but that'd where the factor of 10 per point comes from.