Why do Earthquakes Happen?

A song by Carol King from the 1960s included the line, “I feel the earth move under my feet.” She was singing about how some people feel when they are in love, though I don’t believe that love causes the earth to move. But we will definitely feel movement during an earthquake.

Our planet seems very solid and stable to us, but the truth is the surface of our Earth and the material beneath it are anything but solid and stable. The outer surface of the planet, called the crust, is made of separate plates that float on liquid deeper down. (This interior part that is made of molten/liquid rock is called the mantle.)

Movement in this liquid layer sometimes causes the plates to move. At the locations where two plate edges come together the movement can cause what we experience as earthquakes. The plates grind against one another or move above or below each other. The ground shakes and trembles and in extreme cases can move in waves, heave up or even split apart.

Scientists measure this massive energy release in levels of magnitude, using the Richter scale to take the measurements. For example, a low number such as 3 is mildly noticeable but when that number doubles to 6 the earthquake is not twice as strong but thousands of times stronger. Terms associated with these measurements include seismic and seismograph, the latter being a method of recording earthquake intensity.

Earthquakes don’t just happen at the conjunction of two tectonic plates however. Other movement or disturbances in the molten rock of the mantle or movement of rock within the earth can trigger minor earthquakes as well. If pieces of a plate break off the movement of these smaller sections can lead to trembling and shuddering that might be disturbing but won’t cause major damage.

In fact, scientists who specialize in the study of earthquakes (seismologists) have different categories for earthquakes. They are able to separate the quakes that occur in various ways at the junction of two plates. If the plate ends strike each other or the ends slip against each other we usually feel the shaking of a more minor earthquake. When the one plate edge slides above or below another the surface of the Earth will often move upwards, as in a more serious event.

A few years ago, people in Indonesia, India and many island countries experience a serious and life-threatening tsunami. When an earthquake occurs under the ocean the sea water moves in massive waves because it is more susceptible to the energy of plate movement. These giant waves can smash into the land and in some cases overwhelm the houses and people living there. In this recent earthquake event, thousands of people lost their lives.

Detailed studies of the Earth’s structure, the tectonic plates and mantle show that the majority of the earthquakes occur in an area of the Pacific Ocean. This part of the globe is often referred to as the “Ring of Fire” because of the active and relatively new volcano and earthquake activity. But some interior sections of the Earth are also susceptible to earthquake, such as the New Madrid Fault near the Mississippi River in the United States.