Elastic Rebound

Earthquake Underground blogger John Rundle talks about the “Elastic Rebound” effect. If you hang out with earthquake scientists or engineers, it’s a term that’s commonly understood. But what if you’re like me, a non-scientist, and terms like this are used to explain why the earth occasionally heaves beneath your feet? Let's pull the term elastic rebound  apart to better understand what it means.

Find a rubber band in your junk drawer (I’ve got a great junk drawer in my kitchen – full of rubber bands, paper clips, plastic bags, and other odd or useless things). Take the band and stretch it as far as it will go without breaking it. Then pull it a little more – beyond its capacity to stretch – and snap!! It breaks.

The gradual buildup of stress – in this case, stretching – causes increasing weakness in the band until it can’t resist the strain any longer. It then fails, or breaks with a sudden snap. (minor ouch!)

Now think about an earthquake fault. From the air, a fault looks like a line or crack in the earth. (To see what I mean, check out this photo that Dr. Ken Hudnut, USGS Pasadena Office, took from the air of the San Andreas Fault.) A fault is the boundary between two blocks of earth, or plates, that are constantly moving past each other (the Earth is always in motion). The movement causes stress to build between the blocks. Just like the rubber band, if enough pressure builds up, a sudden break will occur. An earthquake fault experiences the gradual build up of stress in the earth over tens or hundreds or thousands of years, slowly distorting the earth underneath our feet. Eventually the fault cannot resist the strain any longer and fails catastrophically. This is an earthquake. (major ouch!)

To see an animated illustration of how this works, click on this link, courtesy of the University of California at Santa Barbara. The animation shows a bird’s eye view of a country road that cuts through an orchard. Passing down the middle of the orchard and across the road is a fault zone. The animation shows how the earth is gradually distorted, eventually leading to sudden slip or displacement along the fault (what we call an earthquake). To avoid the major ouch!, PREPARE.