Earthquakes are among the greatest of natural disasters and they can cause tremendous devastation over vast areas. Yet even people who have lived through earthquakes often don’t know what causes them. Part of this is because several phenomenon can cause earthquakes.
It needs to be explained that we are really talking about any large vibration in the earth when we talk about earthquakes. This means that many quakes are man-caused. Detonating explosives on or in the ground cause quakes, though usually not over a large area. In fact, scientists who study earthquakes often use explosives in their research.
This said, explosives wouldn’t be considered to be a natural cause. Other causes also won’t be discussed at length, because the effect is localized. For instance, if a slab of concrete was dropped nearby, you’d feel the tremble in the earth as the weight slammed onto the ground. However, someone standing a mile away probably wouldn’t feel a thing. Thus, we will focus on quakes that have longer ranging effects.
Rockslides and volcanos
Rockslides and mudslides can cause quakes that can be felt for miles, if the slide is large enough. The same is true of snow avalanches. In both cases, this is because of the total weight that shifts.
Volcanoes also cause quakes, but for a different reason. Prior to an eruption and as an eruption occurs, molten rock moves into a magma chamber under the volcano and then from the chamber to or toward the ground surface, where it often breaks through as lava, dust, ash and superheated gasses. The movement of magma and the eruption causes the ground to tremble and depending on the quantity of material that is moving, along with how explosive the volcano is, the quake can be often felt over a large area.
In some cases, the eruption can even be heard over a much larger area than it can be felt. This isn’t surprising if you consider that sound is merely vibration, usually of air, just as an earthquake is a vibration of the earth.
The explosive eruption of Krakatoa is an example. The eruption was so great that the volcano virtually blew itself apart. It caused an enormous amount of damage in the area, primarily because of tsunamis, but the explosion was actually heard over 3,000 miles away because of the sound waves the explosion produced.
The main cause of the largest earthquakes, though, is mostly due to plate tectonics. A cross section of the earth would show it divided into the inner core, outer core, mantle and crust. Most of the volume is in the mantle, which is molten rock that has somewhat the consistency of taffee or thick syrup. The land masses and oceans lay on the crust.
The important thing to know about the crust is that it isn’t one solid piece. It is in sections, known as tectonic plates. Because the mantle is so hot and constantly in motion, the plates are moving in respect to each other. Two plates may move away from each other, toward each other or they might slide past each other. Sometimes the movement is slow and sometimes it isn’t. In many places, the plates are moving more than four inches per year in respect to the next plate.
The place where plates meet is called a fault or convergence zone and while the plates are in motion, the rock that is above the fault is solid rock. This means that the rock ‘sticks’ on each side of the fault, even though the plates are moving. When the stress in the rock along the fault line gets too great for the rocks to bear, the rocks fracture or break. This sudden release of energy is felt as an earthquake.
One of the best examples of this is the San Andreas fault, part of which reaches through part of California. From the air, the fault line can be clearly seen in places. This is also why there are so many earthquakes in an area reaching from Los Angeles to beyond San Francisco.
There can also be numerous earthquakes in one location, because the crust can be ten to twenty miles thick and it doesn’t fracture all at once, top to bottom. This is a good thing, since the energy that could be released if it all let loose at once would be far more devastating than what we normally see during a quake.
When plates collide, one of two things usually happens. Either the surface is forced upward, creating mountains like the Himalayas or one plate is forced beneath the other. This second action is called subduction. Both types of plate collisions cause earthquakes, however in subduction, the plate edge that is being forced beneath another plate is melted due to friction. Since the molten rock is lighter than solid rock, it tries to move upward. When and if it succeeds and breaks through the surface, it is a volcanic eruption. This is the cause of the ‘ring of fire’, a loop of active volcanoes that reaches from the South Pacific, up past Asia and Russia, then around the Aleutian Islands of Alaska and down the west coast of North and South America. The San Andreas fault previously mentioned is part of the ring of fire.
It should be noted, too, that the deepest known points under the oceans are all in subduction zones. Most of the Philippines lays just west of a large subduction zone. The same is true of Japan. In fact, the deepest point known under the ocean is called the Challenger Deep and it is located in the Marianas Trench, which is east of the Philippines, south of Japan, north of Papua New Guinea and just south of Guam.
Earthquakes can occur far from plate boundaries, however most happen very close to where plates meet. The stresses between plates is easily the main natural cause of earthquakes. This also explains why some areas, like California, Puget Sound in Washington, the Philippines, Japan and the Aleutian Islands of Alaska are so prone to earthquakes.