Fun with the Mercali Scale

When one is looking for earthquake felt reports in the historic record the sometimes the easiest part is finding the report.  The hard job then becomes trying to figure out the magnitude or force of the earthquake that the report is detailing.  In the era prior to the Richter scale for instrumental earthquakes another scale was developed to help in quantifying felt reports.  This scale was the Mercali scale.  This scale was developed in the early 20th century from previous scaling measurements.  The developer was an Italian vulcanologist Guiseppe Mercali hence the name of the scale. It was later refined by Richter to become the Modified Mercali Scale or MMI in  scientific shorthand.

Let us look at the scale and how it is constructed and used.  The scale is from one to eight and is written in roman numerals .  It is based on the effects noted by eyewitnesses on buildings or the terrain caused by the earthquake.

Using this scale, gives eyewitness accounts a numerical value that can be used by seismologists to estimate the intensity of an earthquake where no instrumental data is present. This makes historic accounts usable for earthquake data that predates modern instrumentation.  It also means that historic accounts are important not only for descriptions of damage but for helping assign a magnitude to an historic earthquake.  In the next post I will give an example or two of how this is done with some historic accounts from the New Madrid era.