little quakes - making the earth shake

Some of you live in areas prone to earthquakes.  The USGS map is a great resource for checking out what's been shaking in the past day.  It goes down to magnitude 2.5 - close to the smallest you'd notice if you were in the vicinity.  You'll notice that smaller quakes are much more common than larger ones.  But how small can you go?

 

We're conditioned to think "Richter Scale" when we hear earthquake. It's a measure of the amount of shaking recorded on a specific seismometer Charles Frances Richter built at Caltech in the mid 1930s to standardize measurements around the world. Richter's seismometers gave a measure in short order and the media quickly latched on.

 

Geophysicists have learned a lot since the thirties and Richter Scale was obsoleted decades ago.  The problem was new measures were more difficult to describe.  For awhile Richter numbers were given, but then one of the new measures was  normalized to give similar numbers as the old scale for garden vanity mid-sized quakes.  These days you'll just hear the term magnitude..  if the reporter says "on the Richter Scale.."  well .. 

 

There's an issue of what it really means.  We're used to 3.0 being mostly benign, 4 causing some damage, 5 getting into real damage and 6 and about being severe.. depending on a lot of things, of course.  The scale is logarithmic.  A magnitude 5 quake has ten times the shaking of the seismometer needle as a 4 and just a tenth of a 6.  The energy released goes a bit more dramatically.¹ These numbers are still a bit abstract.  So let's consider smaller shakings..

 

Modern seismometers record down to about magnitude 2.0 or 1.5 . Below that more sensitive instruments called geophones are used. It's common practice to prospect for minerals by setting off a blast (a bit of TNT or dynamite) and "listening" to how the ground shakes  These blasts are in the magnitude 1 range.  You can keep going down. Let's consider the world at a human scale.

 

There was a big football game recently.  If one team ran as fast as they could into a brick wall the energy released would be about the same as a magnitude 0 quake.²  A high school wide receiver running flat out into your tree (I've seen something like that) would be a -1.   If Puff jumps to the top of the tall newly waxed dresser and skids off, you have the equivalent of a -2 quake when she hits the floor.  Puff glares menacingly and appears to be plotting a cat scheme.  Later she jumps up and pushes off your iPhone.  It strikes the hardwood floor at about magnitude -3 and probably needs a new screen.  You can keep going.. snowflakes hitting the ground, your voice striking a microphone.  Things shake. 

 

Back to football and shaking at our scale.  The first recorded football earthquake came in 1988 when LSU scored a last minute winning touchdown against Auburn sending the fans leaping for joy and then jumping in place for about a minute

If the jumping gets coordinated it can be dangerous and arena sections have collapsed.   Here's an example of cheering in a Frankfurt football match that caused structural damage.

For completeness something on the other end of the scale.  The energy required to break the Earth apart is roughly magnitude 15.  Logarithmic scales are compressed and very handy if you want to look at something that changes a lot in scale.  Some of our senses are logarithmic, but that's for a different post.

__________

¹ The energy released goes as the 3/2 power of the amplitude. A magnitude 6 quake releases 1000 times the energy of a 4 and 31.6 times that of a 5.

² These are just to get a sense of scale - approximations of energy transferred in the collision.  The amount of shaking would be much more complex and depend on the dynamics of the collision.

 

Comments

[Jheri]

When I slip on the ice and fall on my ass, I can say it’s beteeen -1 and -2;)