Wednesday, February 10, 2010
No, I did not feel it.
I awoke to something mumbled about an earthquake. Honestly, I didn't pay a whole lot of attention to the reports. The earth is a vibrant entity. It moves. I had no idea.
4 a.m. this morning there was a rumble. I didn't feel it. Rodney says he felt it and it woke him. It doesn't seem to have shaken anything in my house, at least nothing looked awry when I got up. Perhaps the cats noticed things but they didn't become agitated and wake me up, although this could explain why Pilchard came meowing into the bedroom this morning wanting ear scratches. She just doesn't currently do that.
I walked into the office and the boss says, "Did you feel it?" "Feel what?" was my response, which garnered some rather incredulous looks from my co-workers. We got a foot of snow yesterday that blew around overnight. I'm more concerned with getting out of my driveway.
Yes, for those of you who have emailed me, four of the seismographs my company has in the western suburbs of Chicago picked up the quake. Above is the record from one of the machines.
On the left is a plot of the data graphed to the United States Bureau of Mines' (defunct and rolled into the Department of Interior) "Z" curve. This graph, based upon decades of gathering data from mining, says if quarry blasting stays below the lines shown, the chance of doing even cosmetic damage to a building is essentially nil. Everyone uses this and it has been proven again and again to be the platinum standard for judging intensity of blasting operations.
On the right is the actual waveform record of the quake. All vibrations have 3 components, a side-to-side motion, an up and down motion and the motion of the vibration waves as they pass by the sensor. The purple line above is sound and we aren't concerned with that here. The right side recording would be that machine everyone sees, the one that makes the up and down lines on paper when there is an earthquake, except all our data is stored electronically.
Our machines are located approximately 9 miles east of the quake's epicenter. We've been fortunate before. In the 10 years I've worked for this company, this would be the third earthquake we've captured. The first one I remember was in June of 2004. The second was in April of 2008. It's always interesting to see what our machines capture when this happens.
So how does what you see correspond to the widely used Richter Scale. The Richter Scale is a measure of damage. The bigger the number, the more likely there will be damage. Our machines are not a measure of damage. They measure the velocity of the vibration, the displacement of the ground as measured in inch per second. We deal with pieces of inches. Our machines showed the quake to measure 0.393 inch per second in displacement or velocity.
The other component of a vibration is its amplitude or frequency. We explain that by having you visualize a still pond. You toss a rock into this still pond. The resulting waves emanating from that rock can be thought of as vibration waves. The distance between waves is the frequency. The height is the intensity. All you see when a machine goes crazy and scribbles frantically across a piece of paper, is the intensity. That's all people really care about, even though you are less likely to notice a stronger vibration if it doesn't take its time to pass you by.
And that's the key to earthquakes, they take a long time to finish. Mother Nature releases a lot of energy over a long duration. This energy is released in waves that are spread out. You feel the shaking for a "long" time. One of the structural engineers in my company did an analysis of blasting vibrations versus earthquake vibrations and came to the conclusion that to produce even a 1 on the Richter scale would take millions and millions of pounds of explosives. Beyond a nuclear weapon, it's impossible.
Earthquakes are in the low frequency area of vibrations. You can see all the dots on the left graph clustered at the bottom and near the 10. This is the area where people are really going to feel things. The waves are not moving fast and most people's perception is in this range. If you can visualize this, the front part of a structure is moving up while the back part is down. You've seen this as an anchored boat goes up and down with a water wave. As the parts of a building are anchored, they have little flexibility to move. This up and down motion, called racking and sheering, twists the building components apart. That's what does the damage and, in earthquake prone areas, what buildings must be designed to either absorb or move with.
In the end, for the next few days, we are popular within the company, as offices become aware of the earthquake and call us asking if we recorded it. I kind of wish I would have been awakened by it. I've monitored enough quarry blasts that I'd like the comparison.