Geology of a Natural Disaster



Japan-notice the trench on the right

 By now most of us are probably very aware of all of the devastation in Japan following the massive 8.9 magnitude quake that struck Japan last Friday.  As part of my science lessons that day I took some time to discuss the geology that went into that quake...quite fitting for my 8th grade class since plate tectonics is what we are currently studying.  I went a little off the beaten path in 7th grade (we're working on infectious diseases in that class), but it was well worth the detour. 

Now, I absolutely say that the good people in Japan deserve all the prayers and help that they can get.  There are plenty of places to go on the web to help out.  However, in my post today, allow me to put on my science teacher hat.  I am going to focus on the scientific side of the earthquake.  How exactly does an earthquake reach an 8.9 magnitude?  What does 8.9 magnitude mean?  How come this particular earthquake triggered a tsunami that raced clear across the Pacific Ocean? 

Today's post will consist of 3 parts:
1) Plate Tectonics of the earthquake
2) The meaning of the 8.9 magnitude
3) The tsunami

Let's get edumacated.  They pay me to do this stuff...no really they do.  Hopefully you'll learn something...after the jump :-).



1) Plate tectonics

This earthquake was a classic trembler of the Pacific Ring of Fire, the most seismically active collection of faults in the world.  All of Japan's seismic activity and volcanism is a result of the subduction of the Pacific plate beneath of the Eurasian plate.  The subduction is easily noticeable in the satellite photograph above as you can see the trench.  In fact, Japan is located not too far north of Mariana's Trench, home of Challenger Deep...the deepest location on Planet Earth. 

What is subduction?  The two tectonic plates in question (Pacific and Eurasian) are of very different density.  The Pacific Plate is extremely dense due to its thin structure (only about 6-10 km thick), and the Eurasian plate is less dense.  What happens when higher density things encounter lower density things?  They sink.  Just think about why ice (or anything for that matter) floats.  Ice floats because ice is less dense than the liquid in which floats, the water.  The same pattern is true here.  Since the Pacific plate is denser than the Eurasian plate, it subducts under the Eurasian plate.  This is one kind of convergent boundary, where the motion of the two plates is directly toward each other.  However, the driving force behind this motion (convection within the upper layers of the mantle) is extremely slow.  Therefore, plates have a tendency to "snag" against one another.  The same thing is true with the San Andreas Fault in California, except the stress in that case is shear, instead of compression of two different plates.  When the stress in the plates exceeds what the rocks within the plates are willing to stand, they rupture, and that rupture is the earthquake.  Seismologists call it the "elastic rebound."

Consider a rubber band.  Stretch that rubber band as far as it can go without breaking it.  Then, pull it just a little bit more.  What happens?  It snaps...sometimes quite painfully.  That is due to the excess force being placed on the rubber band.  At the moment of rupture, the matter quickly wants to establish equilibrium, so it accelerates rapidly.  The same thing is true about tectonic plate interactions, albeit on a much larger scale.  The Pacific plate is moving at a rate of about 8-10 centimeters per year closer to Japan.  If it gets stuck while subducting, the forces driving that motion (mantle convection) do not stop.  This adds more and more pressure to the plates, until eventually, they snap just like a rubber band.  The earthquake (as well as all of the foreshocks and aftershocks) is just the plate's way of trying to reestablish an equilibrium between its interaction with the Eurasian plate, and the forces of convection beneath it. 

As a quick side note, the subduction also causes the Pacific plate to melt, due to it entering the extremely hot mantle.  This melting rock is capable of rising through the surface and forming volcanoes like we see in places like Japan and the Pacific Northwest. 

2) Richter Magnitudes

This earthquake registered preliminarily at a magnitude of 8.9, and some seismologists are even considering rounding it up to a full 9.0.  This earthquake was powerful enough to accelerate Earth's rotation by a few microseconds (not nearly enough to notice) and shift its axial rotation by about 10 inches!  Seismic waves are extremely powerful. 

To put some things in perspective, the state of Maryland registered a 3 magnitude earthquake last Summer.  It woke me up.  Then I went back to sleep since it was at 5 in the morning.  When I woke up for the day, it was big news!  Meanwhile, California laughs at us for make such a big deal over nothing.  3 pointers are a regular occurence over there (and in other earthquake prone areas).  A magnitude 3 is 1/3 of the way to a 9, and 1/2 way to a 6.  Yet, if a 6 pointer were to happen here, it would cause massive destruction.  Let's not even discuss anything higher.  How come the changes become so much more drastic the higher up you go on the scale, yet you can go up in the same increments on the low end and barely notice it?  Sequentially, a 3 pointer is the same magnitude difference away from a 1 as a 9 is from a 7 (2 points).  However, the amount of damage caused is tremendously different. 

This is a problem I always consider with my students when I do earthquakes as a unit.  This is because the Richter scale is not linear.  It is a logarithmic scale.  Alright all you math nerds, time to get your scientific calculators out and bring back your old change-of-base formulas.  In Richter magnitudes, each unit corresponds to approximately 32 times more energy released.  So, a 3 pointer is 32 times stronger than a 2 pointer, and a 8 pointer is 32 times stronger than a 7 pointer.  OK, so what does that mean.  I like to look at a graph.



From tutorvista.com


This is the graph of the function f(x) = log x.  This is a base 10 logarithm (as opposed to a base 32 like the Richter scale, but the differences algebraically are slight).  The x-axis represents the energy released, and the y-axis represents the magnitude.  You'll notice that the closer you get to zero magnitude (the (1,0) point on the graph), the energy changes are not that much (ignore the graph portion in the negative quadrant).  However, if you were to expand this graph out further (try it on your graphers if you have one), the changes become far more extreme.  You need a lot more energy release to reach the next level of magnitude.  This is why you see tremendous damage for 8 point earthquakes and no proportional damage for earthquakes rated 4 or below. 

3) The tsunami

The biggest misconception about a tsunami is that is the same thing as a "tidal wave."  It is not.  Tsunamis have nothing to do with the tides.  The physics of a tsunami are pretty simple actually.  Imagine dropping a pebble into a pond with still water.  What happens?  The pond ripples outward.  Now, imagine the pebble being a jolting 9-point earthquake, and the pond is the Pacific Ocean.  As you can imagine, the ripples grow just a little bit.  However, they don't necessarily form the walls of water that we typically associate with "tidal waves."  By now we've surely seen the video.  What you are seeing in the video is more of a highly energetic pulse of water more so than a "tidal wave."  This pulse follows simple physics of the Law of Conservation.  This energy trapped in the tectonic plates is transferred to the water when the earthquake strikes.  This potential energy becomes kinetic energy, and the water moves.  The waves have so much kinetic energy that they are able to last all the way across the Pacific Ocean to affect places like Hawaii and the west coast of North America.  That's pretty much it.

Hopefully I did not bore you with my scientific explanation of the geology of the Japan Earthquake 2011 and now you have a little deeper appreciation for both what happened specifically, and the power of Earth in general.