How are Richter magnitudes of past earthquakes estimated? In reading about historical major earthquakes, in particular, the Great Shaanxi Earthquake that killed approximately 830,000 people in July, 1556, there is a claim made about the approximate Richter scale magnitude:


  Later scientific investigation revealed that the magnitude of the quake was approximately 8.0 to 8.3


despite this earthquake obviously pre-dating the development of the Richter Scale.

What do geologists look for to estimate the magnitude of past major earthquakes?
The approximate magnitude of ancient historic earthquakes may be estimated using a Magnitude/Intensity comparison based on modern earthquakes.  Earthquake intensity may be estimated from the descriptions written down by people. The Chinese officials made detailed records of the damage caused by large earthquakes.