Earthquakes

Earthquakes are the Earth’s natural means of releasing stress. When the Earth’s plates move against each other, stress is put on the lithosphere. When this stress is great enough, the lithosphere breaks or shifts. Imagine holding a pencil horizontally. If you were to apply a force to both ends of the pencil by pushing down on them, you would see the pencil bend. After enough force was applied, the pencil would break in the middle, releasing the stress you have put on it. The Earth’s crust acts in the same way. As the plates move they put forces on themselves and each other. When the force is large enough, the crust is forced to break.

The surface of the earth is divided like a jigsaw puzzle into giant pieces called tectonic or crustal plates. These giant pieces move slowly over partially melted rock known as the mantle. As they move, they slide along each other, move into each other, move away from each other, or one slips under another. On these active plate boundaries about 95% of all the world’s earthquakes occur. California, Alaska, Japan, South America, and the Philippines are all on plate boundaries. Only 5% are in areas of the plates far away from the boundaries. These are called mid-plate or intra-plate earthquakes and are, as yet, poorly understood.

When a large earthquake occurs, the earthquakes in the days prior are called foreshocks. The small earthquakes that occur after a large earthquake are called aftershocks. Little earthquakes don’t necessarily have to be either, but scientists may cluster earthquake activity before and after a large earthquake in the hopes of better understanding how and why earthquakes occur and under what circumstances they are most likely. Still using the theory that little earthquakes are precursors to big earthquakes is not sound science. Only some of them are. It’s more accurate to suggest that active fault lines that produce these tiny unfelt shakers are likely at some point to produce larger earthquakes.

The Richter Scale, named after Dr. Charles F. Richter of the California Institute of Technology, is the best known scale for measuring the magnitude of earthquakes. The scale is logarithmic so that a recording of 7, for example, indicates a disturbance with ground motion 10 times as large as a recording of 6. A quake of magnitude 2 is the smallest quake normally felt by people. Earthquakes with a Richter value of 6 or more are commonly considered major; great earthquakes have magnitude of 8 or more on the Richter scale.

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