Shake Rattle & Roll

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By Linda | April 21, 2008

In the predawn hours of this last Friday Morning (4/18/08) I awoke in confusion. At first I thought I had overloaded the washing machine and imagined that the unbalanced spin cycle was causing the house to vibrate. As I began to shake off sleep I remembered that I hadn’t turned on the washing machine - Next, I reasoned that our house was being buffeting by some really strong wind gusts. I pulled aside my bedroom curtain expecting to see the trees whipping around in a violent spring storm but the wind was barely stirring. By this time I could no longer hear or feel whatever it was that had awakened me so I groggily got back into bed, taking a moment to glance at my bedside clock before dropping off to sleep again - 4:39 am.

The next morning (actually later the same morning) I didn’t give the incident another thought until the newscaster on the national syndicated news show I was watching reported that there had been an earthquake measuring 5.2 in the midwest. The events of earlier that morning came flooding back to me, suddenly making sense. I had experienced an Earthquake! Those folks living in earthquake prone regions such as California and Washington experience small tremors frequently but for a Midwesterner this is a quite a novel event! I only wish that I had been more awake and aware when it occurred so that I could have fully appreciated, understood and properly observed the event as it happened.

We did experience an aftershock measuring 4.5 later that morning but I was driving around in my car by then and didn’t feel it at all. My car radio was tuned to the local radio station when this aftershock hit and the announcer was excitedly chattering about how the microphones and other equipment in the studio had moved and shaken. At this point the radio jock announced that the aftershock had measured in at a 4.3 - he went on to comment that this was not much less that the initial 5.2 seismic event. I was sure that this couldn’t be correct and after looking it up, discovered that indeed, the broadcaster must have had no idea how the Richter scale works. I thought this a timely moment to provide some information on this topic so here it is…

The magnitude of most earthquakes is measured on the Richter scale, invented by Charles F. Richter in 1934. The Richter magnitude is calculated from the amplitude of the largest seismic wave recorded for the earthquake.

The Richter magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole number you go up on the Richter scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. Using this scale, a magnitude 5 earthquake would result in ten times the level of ground shaking as a magnitude 4 earthquake (and 32 times as much energy would be released). To give you an idea how these numbers can add up, think of it in terms of the energy released by explosives: a magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases as much energy as detonating 6 million tons of TNT. Pretty impressive, huh? Fortunately, most of the earthquakes that occur each year are magnitude 2.5 or less, too small to be felt by most people.

The Richter magnitude scale can be used to describe earthquakes so small that they are expressed in negative numbers. The scale also has no upper limit, so it can describe earthquakes of unimaginable and (so far) unexperienced intensity, such as magnitude 10.0 and beyond.

Earthquake Magnitude Scale

Magnitude	Earthquake Effects	Estimated Number Each Year
2.5 or less	Usually not felt, but can be recorded by seismograph.	900,000
2.5 to 5.4	Often felt, but only causes minor damage.	30,000
5.5 to 6.0	Slight damage to buildings and other structures.	500
6.1 to 6.9	May cause a lot of damage in very populated areas.	100
7.0 to 7.9	Major earthquake. Serious damage.	20
8.0 or greater	Great earthquake. Can totally destroy communities near the epicenter.	One every 5 to 10 years