Monday, March 14, 2011

Summary of Japan Earthquake in Terms of Plate Tectonics

In light of the earthquake and tsunami that hit Japan last Friday, I thought I would talk about the geology that helps explains what happened. While I was in graduate school, the theory of plate tectonics was hitting full stride. Many of the geology department’s special seminars at that time were presented by leading figures who were active in flushing out the details of the theory. 

The new theory of plate tectonics replaced the old continental drift theory that never gained widespread acceptance by the geological community. Continental drift recognized how the east coast of North and South America seemed to fit nicely into the west coast of Europe and Africa. However, the theory of continental drift proposed that the continents somehow were able to float or push through oceanic crust. No one knew how this might occur. How do you explain how rock can drift through other solid rock? However, some felt it was a good theory and they felt that eventually the scientists would discover the mechanism by which this could occur. 

During the late 1960s and early 1070s, the results of various studies began to come together in support of a new theory; one in which the continents were not drifting through the Earth’s crust, but that the continents were fixed to crustal plates that drifted over the Earth’s surface. According to plate tectonic theory, the Earth’s crust consists of approximately 30 rigid plates varying typically from about 10 miles thick for oceanic crust to about 120 miles thick for continental crust.

 Figure 1. The Earth’s Major Tectonic Plates (from USGS)

 There are three kinds of relative motion occurring between these tectonic plates - spreading centers (divergent), such as along mid-ocean ridges; transform faulting, where the plates are sliding past each other; and subduction zones (convergent), where one plate is being overridden by another plate. 

Figure 2.  The Three Types of Tectonic Plate Boundaries (from Rapid Uplift).

Some of the early studies that supported the theory of plate tectonic theory showed that new oceanic crust was being formed along oceanic ridges, such as the mid-Atlantic ridge. This was verified by the study of the age of the rock obtained by deep-sea drilling, at various distances from the ridge. The further away from the ridge, the older was the rock. Additional evidence that was being found about the same time dealt with the pattern of paleomagnetism*. On average, about every 200,000 years the Earth’s magnetic field reverses. The pattern of reversals can be seen in magnetic surveys perpendicular to the oceanic ridges. The reversal patterns on both sides of the oceanic ridge match and can be dated from the rocks that were cored and dated. The reversal patterns show the same pattern as the rock ages; the further one proceeds from the ridge the older the rock.

I like the analogy of an egg to describe plate tectonics. Imagine an egg in which the shell has been cracked into a number of pieces that are still attached to the egg.  Spreading centers occur where two of the egg shell fragments are being pulled away from each other. Perhaps you could think of the whites of the egg seeping up between the spreading shell fragments, sticking to the edges and hardening to form new shell. However, if this was the only mechanism, the size of the egg would slowly begin to increase. The size of Earth, however, has not been increasing in size. To negate this, there are other places on the egg where one shell is being overridden by another. This would be analogous to a subduction zone where one of the Earth’s plates is subducting or diving down under a plate that is overriding it. On other parts of the egg, one shell is simply sliding past another shell.

A short video demonstrates plate tectonics and shows the motions of the Earth's plates over the past 200 million years.

Japan is located above a subduction zone. The northern part of Japan is actually located on a part of the North American Plate (figure 1) that is overriding the Pacific Plate. The Pacific Plate is diving down and to the west under Japan. Friction along the plate boundary is not smooth and most of the time the rock on the two plates sticks together. When the forces between the differential motion of the two plates become strong enough, rupture between the two plates occur and the earth moves. This is when an earthquake occurs. In this latest earthquake in Japan the overriding plate is thought have been lifted as much as 30 feet. The uplifting sea floor also lifted the overlying column of ocean water. Gravity then caused the waters to flow westward toward Japan and eastward toward North America. Although this animation was created to show what happened during Sumatra earthquake and tsunami of 2004, the same mechanism caused the devastation that happened in Japan.

* paleomagnetism – Paleomagnetism is the study of the magnetism left in rock by the Earth’s magnetic field when the rock is formed. As molten volcanic rock rises along the mid-ocean ridges, the iron in the rock aligns with the Earth’s magnetic field and is frozen into the rock as it cools and hardens

Other references:
  1.  Rapid Uplift Blog by Suyrat Kher.
  2.  Japan earthquake: The Explainer, Chris Rowan blog.
  3.  Plate Tectonic Animations, Paleomap Project, Christopher R. Scotese.
  4.  Observe animations of processes that occur along plate boundaries, Exploring Earth Visualization, McDougal Littell.

1 comment:

  1. So, Sendai is near a triple-plate boundary like Cascadia, eh?