Reply #10: There have been five KNOWN 9.0 quakes in the 20th century.

[Cessna Invesco Palin]

Records before that are extremely sketchy, and in part must be inferred from the geologic record (from which it's basically impossible to determine magnitude.)

However, we have ample evidence of ocean-wide tsunami in the geologic record. We have historical documents that very strongly suggest that massive plate-rupturing quakes occur on subduction margins with regular frequency. The frequency of these events is only low with respect to a given human lifetime. Yes, they are rare in that respect, but they do happen. They have always happened for as long as ocean crust has subducted. They will continue to happen for the indefinite future.