FAQ Answer

How do we estimate earthquake magnitude from intensity measurements for older earthquakes that were not recorded with seismic instruments?

We can use earthquakes with similar magnitudes that also have intensity measurements to calibrate a relationship between an earthquake's magnitude and its associated intensity distribution with distance from the earthquake.

Another method is to use estimates of ground motion decay with distance to then estimate intensity from the ground motion estimates. In the central and eastern United States, the ground motion conversion to intensity is good up to intensity six (VI).