What are 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. When the break occurs, the stress is released as energy which moves through the Earth in the form of waves, which we feel and call an earthquake.

Types of earthquakes

There are many different types of earthquakes: tectonic, volcanic, and explosion. The type of earthquake depends on the region where it occurs and the geological make-up of that region. The most common are tectonic earthquakes. These occur when rocks in the earth's crust break due to geological forces created by movement of tectonic plates. Another type,volcanic earthquakes, occur in conjunction with volcanic activity. Collapse earthquakes are small earthquakes in underground caverns and mines, and explosion earthquakes result from the explosion of nuclear and chemical devices. We can measure motion from large tectonic earthquakes using GPS because rocks on either side of a fault are offset during this type of earthquake.

Forces

A force can be thought of as a push or pull. Force has both magnitude and direction, therefore it is a vector. From physics and Newton's 2nd law, we know that force is equal to a change in an object's momentum (mass x velocity) which describes the quantity of motion. Often, in the discussion of geology and earthquakes we use terms that describe force and the result of force on the Earth. When a force is applied to an object, the object is said to be under stress. Stress is the deforming force per area. Stress producesstrain, the actual deformation. Stress and strain are related, so it is easy to determine one from the other if you know the value of proportionality, a constant value that relates strain to stress, of the substance that is being deformed (different for each individual material.)

What causes stress?

So far we understand that there are different types of earthquakes, caused by forces under the Earth's crust that change the shape of the material they are acting on, and produce a variety of waves which we feel. But what are these forces? Where do they occur? What causes them?

The explanation for the majority of earthquakes in recent years falls under the category of plate tectonics. When two plates interact at their boundaries they put forces on each other. These forces of reaction cause physical and chemical changes at their boundaries. Plates move side to side, up and down, and also interact head on. Earthquakes also occur in these areas where new plates are being created and old plates are being subducted into the Earth's interior. Earthquakes which are due to the interaction of plates are called interplate earthquakes. But what about intraplate earthquakes, which occur across one plate? Less common than earthquakes that occur at plate boundaries, these earthquakes are due to local systems of forces, such as lack of strength or changes in temperature below the Earth's crust. Most often they are due to movement on pre-existing faults.

Elasticity

In an earlier example, we described what happened to a pencil when force was applied to both of its ends. We said that the first sign of the force on the pencil was seen when the pencil bent slightly. The ability of the pencil to bend shows that it has elastic properties. This means that the pencil is allowed to be deformed, or have its shape changed, but returns to its original shape when the force on it is released. Like the pencil, or a rubber band, rocks have elastic properties. This means that when forces are applied to rocks, such as pulling, pushing, twisting, or compression, they change their shape. Rocks, like all other materials with elastic properties, have an elastic limit, a point at which any additional force will permanently deform the object's shape. Sometimes there is plastic deformation, which means that the shape of an object can be changed an additional amount beyond it's elastic limit before it breaks; other times, if the substance is brittle, it breaks at its elastic limit before any plastic deformation occurs. As we know from the pencil and rubber band, when a substance with elastic properties breaks there is some displacement or total change in position. There is also elastic rebound, in which the objects return to their original shape after they have been broken apart. During an earthquake, seismic waves are generated as a result of this type of rebound.

In the animation below, we see the fence undergo elastic deformation until it reaches the elastic limit, and then finally breaks during an earthquake.

Waves

There are three types of waves that are created when stress is released as energy in earthquakes: P, S, and surface waves. The P wave, or primary wave, is the fastest of the three waves and the first detected by seismographs. They are able to move through both liquid and solid rock. P waves, like sound waves, are compressional waves, which means that they compress and expand matter as they move through it. S waves, or secondary waves, are the waves directly following the P waves. As they move, S waves shear, or cut the rock they travel through sideways at right angles to the direction of motion. S waves cannot travel through liquid because, while liquid can be compressed, it can't shear. S waves are the more dangerous type of waves because they are larger than P waves and produce vertical and horizontal motion in the ground surface. Both P and S waves are called body-waves because they move within the Earth's interior. Their speeds vary depending on the density and the elastic properties of the material they pass through, and they are amplified as they reach the surface. The third type of wave, and the slowest, is the surface wave. These waves move close to or on the outside surface of the ground. There are two types of surface waves: Love waves, that move like S waves but only horizontally, and Rayleigh waves, that move both horizontally and vertically in a vertical plane pointed in the direction of travel.

Detection and recording

Earthquakes vary in size. Those that do the most damage are extremely large, but some are so small they are almost undetectable. So, how are these measurements recorded? And how is their size determined?

Geologists use seismographs to record the surface and body waves. Inside a seismograph designed to measure horizontal motion, a weight is freely suspended. As waves from earthquakes reach the seismograph the mass stays in relatively the same place, while the ground and the support move around it. This movement is recorded on magnetic tape by a pen attached to the mass. In a seismograph designed to measure vertical motion, the mass is connected to a spring, so as the ground and support move up and down, the pen on the mass measures the vertical motion. The metal tape which the motion is recorded on is marked with lines that correspond to one minute intervals. When motion is recorded a seismogram is created, which tells about the waves--how big they were and how long they lasted. P waves are recorded first, followed by S waves and then surface waves. While surface waves are the last to reach the seismograph, they last the longest time.

Using the information from the seismogram, the epicenter and focus of the earthquake can be determined. The focus is the point on the fault at which the first movement or break occurred. The epicenter is the point on the surface directly above the focus. Once several seismograph stations have determined their distance from the epicenter, the actual epicenter can be located, using triangulation, on a map.