Yeah, there's something very suspect about this data. Just off the top of my head, I'm quite sure there were more than 3 earthquakes with magnitude between 5.0

Yeah, there's something very suspect about this data. Just off the top of my head, I'm quite sure there were more than 3 earthquakes with magnitude between 5.0 and 6.0 for the 80 year span between 1900 and 1980. Also, there's a notable discontinuity of exactly magnitude 7.0 earthquakes starting at the year 2000 that doesn't look like raw data.