CHARLOTTE, N.C., April 6, 2018: One of the fascinating things about doing research on the internet is that one subject frequently leads to two or three others. Case in point: In a recent column, we offered our readers a fact concerning three earthquakes that occurred in 1811 and 1812. Each temblor allegedly measured more than eight on the Richter scale.
The problem is, the Richter scale wasn’t developed until 1935, more than a century after the earthquakes in question. So how did scientists know the exact magnitude of the three quakes under discussion?
We can trace part of the confusion back to yours truly, the author of the article. Your intrepid triva expert failed to explain that the quakes “would have measured eight on the Richter scale if the Richter scale had existed at the time.”
So exactly how did seismologists conclude that each cited early 19th century temblor measured eight or more on the Richter scale?
Estimating historical earthquake intensity
When estimating the power of earthquakes that happened before 1900, scientists examine historical records. Such records first assess the amount of damage the temblor has done to buildings. They also log the distances at which people felt tremors and any geological reports of changes in the soil.
For each location where information is available, seismologists assign a score based on something called the Mercalli intensity scale. That scale incorporates whole numbers to determine the effects a temblor likely had on a particular place.
The Mercalli scale and the Richter scale. In reality, the media often misreports this fact when they describe the intensity of a quake.
The Mercalli intensity scale
Developed by an Italian vulcanologist named Giuseppe Mercalli, this scale revised the previously used Rossi-Forel scale and changed the original 10-degrees of measurement to 12. Scientists still use the Mercalli intensity scale today. But people often mistake Mercalli readings for those of the more familiar Richter scale.
The Mercalli intensity scale determines something called the “moment magnitude.” That measures the amount of energy released by a temblor. However the magnitude of a quake does not fully determine its intensity.
The Mercalli scale quantifies the effects of an earthquake on the Earth’s surface, human beings, natural objects and man-made structures on a scale of 1 to 12. (One = “not felt.” Twelve represents total destruction)
Using the Mercalli scale, a score of six means people have difficulty walking and items will fall from shelves and cabinets, but there no major structural damage has occurred.
A reading of 12 however, occurs when the ground actually moves with a visible rippling effect. The result: total destruction.
Even more temblor trivia
Studies show that when Mercalli values can be assigned to several locations near the epicenter of the earthquake, investigators can estimate magnitudes by comparing modern Richter measurements with earlier Mercalli values.
All of this sounds great, except the system is not entirely reliable due to a lack of available data. According to the results of the 10 deadliest earthquakes occurring before the Richter scale came into use, experts have only been able to assign a maximum magnitude of three.
As for those earthquakes in 1811 and 1812, the series of natural phenomena near the current state of Missouri actually caused the Mississippi River to flow backward for a time. These temblors are still said to have been the most powerful in the history of the United States. Though they were not measured by modern technology, the largest quake was projected to register about 8.6 on the modern Richter scale.
Fortunately, these quakes all took place in a sparsely populated area. That minimized the death toll and the destruction to property.
Fun facts about Dolly Madison and the Big Muddy
Just to keep the trivial aspect of the story alive, there are two interesting side notes.
- First, First Lady Dolly Madison was said to have been awakened by the 1812 temblor in far distant Washington, D.C.
- Second, The earthquakes themselves officially ended on March of 1812. But aftershocks occurred for a number of years after those three major seismological events.
We present one final item regarding pre-1900 readings of earthquakes. When two major events occur within proximity of each other and within a few years of each other, they become more difficult to measure. That’s due to the inability to accurately determine the resulting damage from each cause.
Oddly enough, the Mississippi River has reversed its flow twice since 2005. The first time occurred during Hurricane Katrina and it lasted only a few hours. However, following Hurricane Isaac in 2012, the river ran backward for a full day.
And that completes the rest of our story.
About the Author:
Bob Taylor is a veteran writer who has traveled throughout the world. Taylor was an award winning television producer/reporter/anchor before focusing on writing about international events, people and cultures around the globe.
Taylor is founder of The Magellan Travel Club (www.MagellanTravelClub.com)
Read more of What in the World and Bob Taylor at Communities Digital News