Yosemite Earthquake Aftershock or the Garbage Truck?

On Wednesday morning, three earthquakes in the mid-5’s magnitude range shook Yosemite Valley. Scientists predict aftershock tremors of magnitude 3 in the Bay Area over the next couple weeks, KQED News reported.

The difference between these magnitudes at face-value is relatively small, only 2.5 or so. A 2.5 magnitude earthquake is just about the smallest tremor humans can notice. But as a measure of size, force and potential damage, a 2.5 magnitude difference is huge.

According to the USGS magnitude calculator, recent tremors in Yosemite Valley will be 500 times bigger than any aftershocks felt in the Bay Area. The energy released from the Yosemite earthquakes will be 11,000 times greater. In terms of damage, a 5.5 magnitude earthquake shook Tokyo’s skyscrapers in 2013. Meanwhile, the USGS describes an earthquake of magnitude 3 like “the vibrations of a passing truck” – it could maybe knock a collector’s plate off the edge of a shelf.

How can just 2.5 units of magnitude determine whether a building will wobble or just a plate will break? It’s in the formula.

Most scales we’re familiar with change at the same rate, by the same unit of measurement. For a literal example, a dog that weighs 40 pounds is twice as heavy as one that weighs 20. And a dog that weighs 80 pounds is twice as heavy as the 40-lb pooch. The scale is linear.

But the scale we used to measure earthquakes isn’t so straightforward. Just like its retired predecessor, the Richter scale, moment magnitudes are logarithmic. Every one unit increase means the force is 32 times more powerful…

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