The scale for measuring earthquake strength (moment magnitude) is logarithmic; ie: each unit is tenfold the strength of the previous unit; eg: 7.0 is ten times the strength of 6.0 etc.
This brief USGS video offers an excellent visual comparison of the relative strengths of recent quakes. Each circle's area indicates relative energy release, color indicates tsunami potential (see http://ptwc.weather.gov/ptwc/about_messages.php), and labels indicate each earthquake's moment magnitude (according to USGS/NEIC), location, and the year they happened.
The 1906 quake that struck San Francisco was between 7.7 and 8.25 in moment magnitude.
http://www.youtube.com/watch?v=ooXt6p35Kzw