On Friday, March 11th, 2011 at 2:46 P.M., a magnitude 9.0 earthquake struck the eastern shore of Japan. According to written historic records, this particular earthquake is the 4th largest earthquake in the world. When trying to understand why an earthquake is large or devastating, it is helpful if you know where the hypocenter and epicenters are located. The hypocenter of an earthquake is the location where the earthquake originates underground. The epicenter of an earthquake is the point on the earth’s surface that is directly above the hypocenter. For the 2011 Tohoku earthquake, the hypocenter was about 29 km (18 miles) underground and the epicenter was about 70 km (43.5 miles) to the east of Tohoku.
The earth, as we know it, is controlled by plate tectonics(Figure 2). When two plates meet, they create a plate boundary. There are three different types of plate boundaries: 1) Convergent: when two plates meet and one subducts under the other one. The convergent plate boundaries are also known as megathrust fault. Japan is located near a convergent plate boundary. The second type is Divergent: when two plates are moving away from each other. The last type is Transform: It is when two plates slide past each other in a lateral direction; for example, the San Andreas fault.
Now that we understand the three different plate boundaries, let’s look at why Japan was hit with such a powerful earthquake. The largest earthquakes occur on subduction zones at convergent plate boundaries. Why? A subduction zone by definition is a place on earth where two tectonic plates collide and the denser plate moves downward under the other plate, or subducts below the other plate (see Figure 1.) Now keep in mind that these plates are not soft or lubricated – they are composed of hard, rugged, and brittle rock. When the two gigantic plates collide, they are not going to slide peacefully past each other. Instead, they will become stuck or locked in place. Now, when these plates are locked in place at the location where they meet, the rest of the plate wants to keep moving forward at the same pace. This causes stress to build and eventually the amount of stored stress will surpass the amount stress the rocks can stand. The end result is the plates slipping and generating an immense amount energy – the earthquake!
Let’s get back to the 2011 Tohoku earthquake. The 2011 Tohoku earthquake was so powerful that it shifted the Earth on its axis of rotation by redistributing its mass. This led to the shortening of the length of our day by a microsecond. This earthquake even shifted the island of Japan closer to the western coast of the United States by 2.4 m (8 feet). This earthquake was so big, that it was followed by over 5000 aftershocks. Some of these aftershocks were as high as a magnitude 7.8.
The shaking during the earthquake lasted a whopping 6 minutes! Imagine hiding under a table during the earthquake for a whole 6 minutes. That’s almost as bad as waiting for your hot pockets to cook in the microwave. I always feel like microwave minutes are much longer that regular minutes, right? Anyway, 6 whole minutes of shaking! Can you imagine thinking “Wow, I’m glad that the shaking has stopped and my house is still standing! Now what do I do?” Then you suddenly hear the Tsunami siren blaring outside your house. Japan has so many Tsunami’s that the word Tsunami is a Japanese translation of Tsu =harbor Nami= waves. A Tsunami is a wave that is generated when there is displacement of water. Since the subduction slipped in a semi-vertical direction underwater, the ocean floor changed in height and displaced the water to create a series of giant waves. The majority of the damage in Japan was due to the tsunami; tens of thousands of deaths were attributed to the tsunami. This was a true tragic disaster that was brought to us by our mother earth.
I thank you all very much for taking your time in reading this. Any questions, comments, or any specific earthquake you would like to read about? Please ask, and I would be more than happy to answer them.
Bryan C. 😉