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March 11, 2011 / Dave Gorham

When Plates Collide: Earthquakes of Japan

Japan is no stranger to earthquakes. And not only earthquakes but volcanoes, as well. Located along the Pacific Ring of Fire as well as along several boundaries of tectonic plates, Japan is in one of the most unstable regions found on Earth. According to Wikipedia, Japan can count 35 earthquakes, including today’s (and yesterday’s significant foreshock) since 684 A.D. Today’s quake is being classified as the strongest to ever strike Japan (8.9 moment magnitude), and perhaps the 5th or 6th strongest ever recorded anywhere.

Subduction Zones of the Nankai and Kurile troughs. Image Wikipedia

Additionally, this region of northern Japan near the capitol city of  Sendai, has experienced more than its fair share of dramatic quakes. These regional quakes include the 1964 Earthquake of Niigata, the 1978 earthquake of Miyagi, the 2005 Miyagi earthquake on the Oshika Peninsula, the 2007 earthquake of Niigata, and the 2008 earthquake of Iwate-Miyagi.

All the plates responsible for today's massive earthquake are indicated on this graphic: Eurasian, Pacific, North American.

Plates of the world Image: Wikipedia

Why is this area so prone to such frequent earthquakes? It’s what amounts to the Perfect Storm of tectonics. Five of Earth’s massive plates meet in this area and grind upon one another: The Okhotsk plate, the Pacific Plate, the Filipino Sea Plate the Eurasian Plate and the far western edge of the North American Plate all converge within this area broadly defined as the northwestern Pacific but more regionally defined as Japan and more locally as northern Japan. These boundaries are classified as transforming (when plates slide past each other), diverging (when plates move away from each other) or converging (when two or more plates are moving toward one another). The converging boundary of the Pacific and North American plates is responsible for the uprising of volcanic islands known as the Aleutian Islands of Alaska.

Subduction occurs as one tectonic plate passes beneath another. Image: Wikipedia

Today’s epicenter is on the Eurasian plate, however the  Pacific and North American plates are its immediate neighbors, and the Filipino Sea Plate is just to the south. In a process called subduction, the Pacific Plate is sliding under the Eurasian and Okhotsk plates. Traveling back to science classes of junior high school, we remember that the earth’s crust is in a constant state of movement. As these plates continually grind, energy is released in the form of an earthquake (snap your fingers — the “snap” occurs due to the same types of forces that occur when tectonic plates move against each other). The results of this subduction are the Kuril Islands and the Kurile Trench, some 34,000ft deep. Additional subducting of the Filipino Sea plate beneath the Eurasian plate has resulted in the Nankai Trough. Seismologists believe the Nankai Trough is actively deforming (in part due to intense sedimentary filling).

The three types of boundaries: Transform, Convergent, Divergent. Image: Wikipedia

What lies in store for Japan? Tectonically, more of the same. The Pacific and Eurasian plates are not going anywhere soon but they are, indeed, going. And that’s the problem.

Read what ImpactWeather StormWatch manager and expert geologist Fred Schmude has written earlier today here.

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