Magnitude: It’s Much More Than a Number

I ran into a little mystery today. It seems that an earthquake shook some of the residents in a suburb of Dallas, Texas at 11:05 PM “last night” CDT or 12:05 AM “this morning” EDT. The magnitude of this quake was rated on the Richter Scale at 3.4. Surely our little station M49A would have picked up the shock waves moving through our area…Or would it?

See below.

Image

Your eyes immediately spot the waves between 16:00 and 18:00….WHAT? That means our station picked up these waves at about 16:37 UTC or 12:36 PM EDT??? 12 HOURS LATER?…..LUNCH TIME?

I searched in the USGS website.

This M7.3 quake hit the country of Columbia about 6 minutes earlier.  So, WHERE is the Texas quake? Look close just to the right of 4:00 above.

See the first three tiny “blips” with a slightly thicker blue line?….That’s it.

What is going on? The answer is MAGNITUDE or how the waves of quakes are measured. Even with the amplitude reduction of the larger quake figured in, I had trouble finding the smaller Texas quake.

Here we go….

How much bigger is a 4.0M compared to a 3.0M?  10 TIMES.!!!! Each whole number on the scale means there is 10 times the amplitude of the resulting waves…It’s called a logarithmic scale, if you care…But, that is not the point here….

So how is your math? Let’s go from 3.4M to 7.3M…Each whole number step is 10 TIMES more…So that is 10 X 10X 10X 10! or…GULP…. almost 10,000 times more shaking amplitude from the stronger shock waves!

Give me a minute to calm down and explain….

I took advantage of our tax dollars and downloaded this from the USGS..    http://earthquake.usgs.gov/learn/topics/mag_vs_int.php

M 3.0 to 3.4 = Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated. This is what the folks in Dallas noticed (or missed.)

M 3.0 to 3.9 =  Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

Now let’s take it up a few notches…

What about M 7.0 and above????? Remember 10,000 times more wave amplitude. Naturally the effects depend WHERE the epicenter is and HOW DEEP it is….This one in Columbia was 168Km (104 miles) deep, (that’s good.) NOTE: The USGS has since downgraded it to M7.1( let’s skip the math here)….Little damage and no injuries reported….But LOTS of people felt it!

What IF the epicenter were closer to the surface (and the people)???? Scientists use levels to measure the degree of damage, starting at VIII. (The lower levels are for magnitudes less than 7.0)

VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. So build well, or pay later!

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

YIKES! I am sure you remember the last “Big One”. Check it out on the list of largest quakes recorded…

http://earthquake.usgs.gov/earthquakes/world/10_largest_world.php

3/11/2012 Japan. Magnitude 9.0

Only 177 Km (109 miles) off the east coast of Japan and a depth of only 30 Km (18.6 miles)

See? It’s not just the magnitude but the distance and the depth as well.

In any scientific investigation, there are always surprises. This one has revealed two…

1. Charles Richter (RICK-ter) of the “Richter Scale”, one of several scales used in seismology, made tremendous contributions to the WORLD in monitoring and preparing for earthquakes. He was however, a bit of a character that would make an interesting topic in a future blog.

2. Why did he use MAGNITUDE to measure the strength of quakes?  He was also fascinated with ASTRONOMY!  The brightness of objects in the sky is measured in magnitudes…Yet another possible blog topic.

“I feel the earth move under my feet, I feel the sky tumbling down, tumbling down.” – Carole King

MZH     of between the stones and the stars.

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About Mike

Retired science teacher that is learning to blog...

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