December 30 - Happy Birthday, John Milne

Posted on December 30, 2021

This is an update of my post published on December 30, 2010:

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 around 55 earthquakes worldwide EVERY DAY!

So, basically, there is 100% chance that there will be an earthquake today. Because there are many, every day. But we still cannot predict where and when BIG earthquakes hit.

People think that getting in a doorway is safe during an
earthquake, but it’s better to get under a sturdy piece of
furniture like a desk or table, protect head and neck, and
HOLD ON until the shaking stops.

How do we measure the strength of an earthquake?

A seismograph is 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 video explains how seismographs work.

By the way, a modern seismograph is sensitive enough to sense mine explosions, falling trees, or even elk footsteps!