Happy Birthday, John Milne
Born on this day in 1850, British geologist John Milne invented the first accurate seismograph.
The earth is always shaking, quaking, moving. Any day of the year, type “earthquakes” into Google, and you will see the most recent big quakes that happened that day, or the day before, somewhere in the world. But such a search only shows the big quakes, and there are many more small shakes than large quakes!
As a matter of fact, the earth experiences several million earthquakes a year!
Many of these tremors are so small or so out-of-the-way that they are not recorded, but the US Geological Survey National Earthquake Information Center does locate and record 50 earthquakes worldwide EVERY DAY!
As the USGS website says, there is 100% chance that there will be an earthquake today. Because there are many, every day.
How do we measure the strength of an earthquake? With a seismograph, an instrument that has a weight that can move relative to the instrument frame but is attached to the frame so that, if there is no motion, it will stay fixed relative to the frame.
The motion of the ground will move the frame, but the weight will not move because of inertia. By measuring the motion between the frame and the weight, the motion of the ground is also measured.
This short animation explains how seismographs work.
By the way, a modern seismograph is sensitive enough to sense mine explosions, falling trees, or even elk footsteps!
Learn more about earthquakes here.