On August 23, 2011, a magnitude 5.8 earthquake hit Virginia in the early afternoon. Even though the epicenter was over 100 miles away from me, I still felt it. I remember it felt like a truck was driving by, but too close and for too long. You may have felt it, too, if you were on the East Coast, as it was felt as far north as Quebec and south as Atlanta. In fact, it was felt by more people than any other quake in US history, and may be most memorable for damaging the Washington monument, which was then closed to visitors for several years.
Much of what we know of the structure of the Earth, and just about everything we know (we think we know) about its inner layers are calculated from measurements made during earthquakes. The Journey to the Center of the Earth by Jules Verne is just Science Fiction; in real life, nobody’s actually been there. The deepest we’ve ever drilled was about 7.6 miles, and there were a number of surprises (read: things we knew that turned out to be wrong), including how hot it would be at that depth, making it unsafe to continue. The center of the Earth would only be about another 3,950 miles.
In discussions of earthquakes, we often hear about tremors. When we talk about measuring earthquakes, we talk about seismometers. Although they can also register tremors, the device specifically designed to measure tremors is a tromometer, and that branch of study is tromometry. Since I can feel some trucks going by on the main road not that close to my house, and you don’t have to be very close to the tracks to feel a train go by, and even kids jumping in the house can cause a pretty good shake, how can a tromometer possibly measure Earth’s tremors with accuracy and precision? How is “vibration pollution” filtered out?
Tremors are often mentioned as precursors to full-blown earthquakes. There were almost continuous tremors, as well as a series of earthquakes before the massive eruption on May 18, 1980 which blew the top 1/3 off of Mt. Saint Helens in Washington State. In spite of the earthquakes and tremors and the growing bulge on the north side of the mountain, and the heat from magma melting the ice and boiling away the water in some places, some people just didn’t think much of the risk.
Most famously, 83 year old Harry R. Truman refused to evacuate, saying, “…the mountain is a mile away, the mountain ain’t gonna hurt me.” In fact, he became a bit of a folk hero, living life on his own terms. But when the mountain blew, it headed his direction at nearly the speed of sound, and buried him (presumably) under 150 feet of volcanic debris. It puts an ironic edge to his statement, “That mountain’s part of Truman and Truman’s part of that mountain.”
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