Friday, March 11, 2011

Explaining Japan's Earthquake

Updated November 2016

As a geologist, I have always been fascinated with seismic activity of all types, particularly earthquakes.  My one bitter disappointment was the lack of any earthquake activity during a three week stint in one of the world's most earthquake-prone nations, Japan.

Japan is located along the western edge of the Ring of Fire, a zone of frequent earthquakes and volcanic activity that rings the Pacific Ocean.  Over 90 percent of the world’s total earthquakes and over 80 percent of the world’s major earthquakes take place along the active margins of the Pacific Ocean.  Here is a map showing the Ring of Fire:

Tectonic activity along the Ring of Fire results from the collision and subduction of the world’s tectonic crustal plates that are “floating” on the underlying mantle layer.  The massive earthquake and tsunami off the coast of Indonesia and the earthquakes associated with the San Andreas faults are related to their position along the Ring of Fire.  Here is a diagram showing the internal structure of the earth:

Here is a map showing the world’s major tectonic plates:

Japan is an island arc created by the activity of the earth’s crustal layer as it moves on the underlying semi-solid mantle layer.  Unfortunately, in the case of Japan, the country lies at the intersection of four tectonic plates; the North American Plate, the Eurasian Plate, the Pacific Plate and the Philippine Sea Plate.  These plates all meet on the island of Honshu, the largest of the many islands that comprise the country of Japan. This makes the country extremely vulnerable to earthquakes, in fact, today’s earthquake took place off the coast of northern Honshu to the north of the city of Tokyo.

Here is a map showing Japan’s tectonic plates and the smaller faults (in red) associated with the larger plate boundaries:

It is the location of these plate boundaries that result in the massive number of earthquakes in Japan, estimated at over 1500 annually.  Here is a map showing the annual frequency of earthquakes with a magnitude of five or greater in Japan:

In Japan, the Pacific Plate is sliding under the North American Plate creating a subduction zone as the crust of the Pacific Plate is subducted (or goes under) into the earth’s mantle.  This is termed a convergent zone by geologists.  Because the crust is not perfectly smooth, it will lock in place as it is subducted and then suddenly release once pressure builds up and friction between the plates is overcome.  It is this sudden release that created the earthquake on March 11th and the massive earthquake that killed 6400 people in Kobe Japan in 1995.  In the case of the March 11th earthquake, scientists believe that the plates moved between 60 and 80 metres, a huge single event movement.  Japan also experienced an even more massive earthquake in 1923; the Great Kanto earthquake killed over 140,000 people.

Here is a diagram showing how, in general, a crustal subduction zone works:

As the more dense oceanic crust of the Pacific Plate is subducted under the less dense continental crust of the North American Plate, the oceanic crust is forced downward into the hotter earth’s mantle, where the oceanic crust heats up and melts, creating magma.   This magma, which is lighter than the surrounding rock, rises to the surface and results in the creation of volcanoes.  In Japan, Mount Fuji is a prime example of a perfect cone volcano formed as the melted and less dense earth’s crust rose back to the earth's surface.  As well, the over-riding plate is crushed along its leading edge, resulting in the formation of mountain chains as the crust is compressed and forced upwards. The sudden release of energy and movement of the earth’s crust during an earthquake displaces overlying ocean water.  This results in the creation of a tsunami which, interestingly enough, is a Japanese world meaning “harbour wave”.

Here is a diagram showing how the crustal plates interact with each other in Japan:

I hope that this very basic primer in crustal geology will help explain why Japan experienced such a massive earthquake on March 11th, 2011.  A very similar geological scenario exists along the west coast of North America, most particularly in the Pacific Northwest and it is just a matter of time before the “Big One” hits that portion of the Ring of Fire.  Fortunately for Japan, the country has undertaken a massive program of earthquake preparedness over the past 50 years which will minimize the loss of life and property from this massive catastrophe.



  1. Wow ,excellent Article

  2. Good work, Glenn. I learned a few things.

  3. Many many hearty condolences and prayers for my poor Japanese friends from India. I remember the moment when these ghost waves hit my place Kerala, south end state of India erupting from Indonesia in 2006.

    Dont worry our Indian crew has already offered help to Japan for all support and relief operations. We are one and have only one mind whenever disasters happen to any part ofearth.

    Out of 25000 Indians in Japan, i know many people from here who are either studying or working in Tokyo. Many Japanese tourists were here recently. I am seeing only Japanese news here in media's after earthquake.

    Hope every one is safe and sound. Prayers to God for no more nature plays. But take care...

    Our PM has offered all aid, help and assistance to Japan.

    A Japanese lady professor named Takaku who also speaks "malayalam" language of our place Kerala, southend state of India yesterday came in our local news TV channel to describe about this earthquake and her relatives and friends in Japan. She said she couldnt contact anyone and noone is taking any call there in Japan. Her home is in Sendai or Miyagi.

  4. Cherie Downton-WhittleMarch 12, 2011 at 2:31 AM

    This is a great in-depth study and bring great understanding as to why these earthquakes occur. Can you explain why the Pacific Plate is constantly on the move, and the possible ultimate outcome?

    Thank you very much.

  5. Cherie,

    All plates are constantly on the move. The viscous material in the mantle circulates and forms plumes that rise and fall toward the core of the earth. It is this motion that causes the plates, that are floating on the mantle, to move around and collide with each other.

    In the middle of the Atlantic Ocean, there is a long ridge known as a spreading centre where new oceanic crust is being created. This action is actually pushing the margins of the ocean away from each other - in fact, the Atlantic Ocean is becoming wider (very slowly) as we speak.

  6. our planet is really dynamic! it's evolving so to speak! in fact, the himalayas now were once a part of the african continent

  7. copied the picture to use in my high school science class. Thanks. Just read a newspaper article that said the Pacific Plate was going under the North American Plate and caused the equake. Reporters can sure foul it up for the public.

  8. Heartfelt condolences to Japan!

    Great article, thanks Viable Opposition.

    We are teaching a class on Japan, earthquakes and tsunamis in Laos, would you mind if we used some of your article and translated some of it into Lao?

  9. Village Science:

    Please feel free to use the text as you wish. I'm more than happy to help educators explain plate tectonics to students. The diagrams come from various sources, mainly the United States Geological Survey.

    Thanks for your comment and I hope that my writings help explain why Japan is so prone to earthquake activity.

  10. that explains everything... since Japan is a place waiting for a disaster to happen, why are people of Japan relocating to safer parts of the world...

  11. if tectonic plate shifts is responsible for earthquake in japan then should there be not 2 earthquakes on opposite sides of the plate. 1 quake on the rifting side and 1 more quake on the subducting side.

  12. Anonymous, earthquakes are much more complicated than that. Certainly, over the long term, 1+1 will eventually work out to 2; with earthquakes, though, 1 equaling .1+.1+.1+.1+.1+.1+.1+.1+.1+.1, with the former being subductive faults and the latter rifts, is more common. Thus, the mid- Atlantic rift, where the world's major expansion occurs, receives more frequent but milder quakes, whereas the "ring of fire" receives rarer but larger ones. Another interesting point is the relationship between subduction and volcanoes. I live on the island of Kyushu, surrounded by volcanoes (due to, apparently, a series of rift zones), and yet we receive few and relatively minor quakes. Lucky us.

  13. Im using this for my geography homework. Thanks! I feel lucky we don't live in a place like Japan, and I think it can only get better for them from here. I feel deeply sorry for the people effected in any way from the 9.0 earthquake!


  15. Great article, reminded me of all the things I've forgotten from college. Nice and straightforward I can use this to provide an intelligent answer to all my 7 year old's questions about how these earthquakes happen.

  16. This reads like my geography textbook when I was 15, except that it's much more condensed and helpful, and with better graphics! Thanks :)

  17. Very interesting primer on earthquakes. March 11 was much, much larger than the Great Kanto Earthquake (9.3 vs an estimated 8.4). Most of the estimated 140,000 deaths in 1923 were a result of fire -- lots of smaller fires caused by collapsed buildings grew into massive infernos because of strong winds and this is what destroyed a lot of Tokyo and killed so many people.

    Thanks for the interesting article.


  19. i have learned more in the 10mins it took to read this post, than in the four 1-hour-long lessons at school about earthquakes.

    thats 4 hours of my life ill never get back! :D

  20. Below your 6th Figure, you wrote:
    "As the more dense oceanic crust of the Pacific Plate is subducted under the less dense continental crust of the Eurasian Plate..."

    I think it supposed to be North America Plate instead of Eurasian Plate, since in the upper figure the Pacific plate is converge with North America Plate.

  21. What a lovely way of putting geographical occurances in a classroom like lecture. It is not so scarry when we know what happened. It is bad enough to have casualties. But the Cosmic energies should not be underestimated. Have Faith, Hope and Love is my motto. According to my knowledge Japan is hit this year with the total loss energy. My prayers and thoughts are with the victims and families. Thank you. It is very educational. @Lady4Lynn

  22. Excellent information, thx for the info.

  23. If this how it really is , why on earth would you build a nuclear power plant in Japan ? I thought the Japanese were smart people .

  24. thank you for this . Japan build these plants so we can have a normal electricity . Usually building are built to survive the earthquake. however this tsunami destroyed these buildings. Japanese people are smart and organized. Earthquake is part of our lives in JAPAN. ganbare nippon!

  25. The Japanese are smart. They built the plants so their people could have what they needed. The government built the plant to withstand earthquakes.The tsunami took it out not the earthquake.

  26. This was very helpful for my science hmwk! Thanks :)

  27. Global warming could change Earth Axis, control Groundwater Depletion to Save our Earth.

    We missed to Study some more factors. There is a Correlation in Groundwater Depletion and Global Warming.

    Un-Pumped Lost Groundwater is still not estimated in the study of Groundwater Depletion.

    Deeper Multi-Aquifer Tube Wells are Responsible for Groundwater Depletion & Global Warming - “That is Curable”

    Please find Official Letter sent to President, Prime Minister and Chief Minister of Rajasthan from the link -

    This letter also has been sent to President/Prime Minister of various Nations and UNFCCC.

    FOIA & RTI applications have been submitted at NASA, EPA and USGS and Prime Minister of India to get the required data


    The correct root cause of Global Warming should be :- A Concept of Global Warming.

    Relevant Suggestion to the "Concept of Global Warming" in Durban - UNFCCC -

  28. BULLDUST!!!

    Read a book... "about tectonics" that is...

    You will find a good one here.

  29. huh. i ahve tp do my geography project on this thx for the info thx :)

  30. The Leaking of the waste water is very dangerous and this plant needs to be shut down right a way be for it kills more people, this is very sad that this plant was not shut down the day it happened and torn down. Get a grip Japan you are poisoning the world with your nuc plant and killing people slowly. this article is excellent and hopefully people will push to understand these faults do not lie.