What is the source of that chart? I believe Alaska 1964 was a 9.2.
All of the sources I have found list it as 8.5. Having said that, it is possible that the Alaskan quake was greater that 8.5 because the Richter scale only goes to 8.5. The Kanamori scale (only invented in 1979) can measure larger quakes.
The source of the chart is the South Dakota Geologic Survey.
The most authoritative list is the USGS Moment Magnitude list.
The strongest quake known since 1900 and instrumentation is the 1960 Chilean 9.5. The second is the Alaskan 1964 at 9.2