Earthquake in-depth

AMHERST, Mass. (Mass Appeal) - The earthquake in Japan occurred when two of the Earth's tectonic plates collided. Does that sound like jibberish to you? Tekla Harms, a professor of Geology at Amherst College helps translate.

The earth is rock from the surface down to the core. The outermost part of that rock is exposed to space and it is cold. Cold rocks are brittle. They break like plates would break. Not very far down, 100 kilometers down the Earth is warm enough with the mantle, flow in the solid state like silly putty. That outer rhine, that outer crust is thin like a dinner plate and it breaks and it moves.

They are already broken and moving. It is when the edges of two plates rub side by side. That causes the earthquakes.

The mantle is flowing, in order driven by the heat that the Earth has. It gives the energy to flow. Convecting like boiling water on the stove. Hot mantle rises from below. Comes up towards the surface where it cools and sinks back down. That is moving the plates around the surface sort of as passive parts of the system. The plates move about one or two inches a year. They are pretty much constantly in motion.

For the most part, most earthquakes, occur along plate boundaries. Those that do he occur, occur around plate boundaries, where they are stuck. Two plates will slide along the edges constantly and consistently and quietly. There are places where the two plates are frozen or stuck, friction makes them stuck. In that case, it is easy to explain with a rubber band that the rock in between the two moving plates that is stuck gets stretched like the rubber band is stretching.

At a certain point, it reaches a threshold and the rubber band will move. The rocks will break. That releases the energy that has been stored by stretching the bonds of the rock. Just like the rubber band, the rock snaps back to initial shape and it release a great deal of energy as it does so. That is the earthquake.

In Japan, there are two plates that, instead of sliding side by side, one plate is diving under the other as the mantle returns, the cold mantle returns back down. So the one plate that was going below the other is stuck in a certain point. As that reached a threshold and finally broke, the overriding plate lurches forward, down going plate lurched down. That releases the energy that is the earthquake and also makes the floor of the ocean physically move and that generates the wave.

Three plate boundaries. Plates move apart. Push together in Japan. Slide side by side. Places where plates move apart are the least seismic. Produce earthquakes but quite small. Places where plates come together in zone, one dives below the other.

In Chile, those are known to generate the largest of earthquakes. I don't want to minimize the size of earthquakes, but side by side fault can produce. That is what generated the Haitian earthquake like the san and res, two plates moving side by side. They can produce earthquakes that are destructive in their own right. The largest ones that the eighth makes are in seduction zones.

There are earthquakes in New England. They are not historically as sizable as the one in Japan or the ones on the san and res. That is partly because we are not near a plate boundary. These are the few kinds of earthquakes that are generated in ways other than plate boundaries than earthquakes. There is a locust of earthquakes that happens under the reservoir because of the weight of the water. They are typically around 4 on the richter scale which is quite a bit smaller than the san and res produces.

They are felt fairly strongly because the rocks under New England are harder than the rocks under California.

California, a bit more of a rolling experience. But despite the fact that it is a jolt, it won't be a jolt as big as what the San Andreas can produce.

The plates move as large bodies, with a great deal of inertia. What is variable about it, where the plate boundaries, where it is stuck.

Earthquakes are difficult to predict. Which is why we have these terrible earthquakes with no warning.

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