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Prime Minister advocates little-known earthquake measurement system

The Prime Minister tabled the Meteorology Geosciences and Ozone Bill 2020 in Parliament this week but his advocacy for a little-known system for measuring earthquakes' intensity went largely unnoticed. 

Unlike the generally accepted Richter Scale, which measures an earthquake based on magnitude the Modified Mercalli scale is a competing scale that measures the strength of an earthquake based on its intensity.

The legislation passed its second reading stage on Tuesday. It covers a wide range of areas relating to weather, geosciences and ozone protection, guiding, by way of law, the meteorology division’s scope of work.

Perhaps the most important part of the meteorology bill for the public is the Multi-Hazard Early Warning System which provides for speedier issuance of warnings for earthquakes, tsunamis, cyclones and natural disasters including volcanic eruptions.

The Prime Minister introduced the bill in his capacity as the Minister of Natural Resources and Environment (M.N.R.E.).

When he spoke of earthquakes, Tuilaepa said the Mercalli Scale is a useful tool for Samoa to gauge whether a quake warrants the population to run to higher ground or not.

“For measuring earthquakes, there is the Modified Mercalli Scale. I thought there was nothing like this…the Modified Mercalli Scale…when an earthquake strikes and [framed] photographs fall off the shelves [and] dishes crack, especially for residential areas near the ocean, that is the time to run for your life,” Tuilaepa said.

“You don’t wait. The only thing [you do is] look for a car, if there is a car. If not, then you head by foot to higher ground.”

A quake of lesser intensity, under the Mercalli Scale, is when “you hear dishes rattling but they don’t crack,” the Prime Minister said.

“But for the stronger earthquake, when dishes fall and crack you don’t wait [for a warning]. But with the weaker earthquake, the dishes will rattle…it tells you to stop fishing and to prepare,” he said.

On the Richter Scale - the most commonly used measure of an earthquake’s power - a reading of 7.5 or higher can trigger a warning for people to head for higher ground.  

On the Mercalli Scale, an earthquake of equivalent intensity would be a Level 5. 

The scale has twelve levels, from Level 1 when an earthquake is not perceived to Level 12 where complete damage is wrought. 

The Mercalli Scale was developed in 1931 by American seismologists Harry Wood and Frank Neumann.

Unlike the Richter Scale that measures magnitude, the Mercalli Scale measures focuses on intensity, explains Dr. Gregory Moore, a Professor of Marine Geophysics at the Department of Earth Sciences at the University of Hawai’i at Manoa.

He told the Samoa Observer that “earthquakes are the result of a sudden release of energy in the Earth’s crust that creates shock waves that travel through the Earth and shake the ground.”

Dr. Moore spent more than four years on the research lab staff at the Scripps Institution of Oceanography and is a fellow of the Geological Society of America.

“Earthquake intensity [is] what you feel,” Dr. Moore explains.

He notes intensity is based on damage. It has one value for each neighbourhood for each earthquake so the range of intensities for each quake can be used for historical earthquakes.

Magnitude, explains Dr. Moore is roughly based on energy. It has one value for each earthquake and is a more modern and accurate measure. The Richter Scale is the most famous example of a measurement system based on magnitude. 

“The effect of an earthquake on the Earth's surface is called the intensity. The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally – total destruction,” according to the United States Geological Survey (U.S.G.S.).

Numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli scale. 

It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects, the U.S.G.S. continues:

“The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place.”

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