What are earthquakes and what causes them?

Earthquake

An earthquake is the brittle, sudden failure of the earth's crust or mantle. Earthquakes are caused by several factors however the common element is that stress builds in rocks until the yield strength of the rock is exceeded, at which point rupture occurs. The relative movement between the major tectonic plates is responsible for the stress build-up that causes the vast majority of earthquakes.

To understand the earthquake process some simple physical quantities should be defined first.

Stress is the force per unit area.
Elastic deformation: A material changes shape when stressed but after the stress is removed, it returns to its original shape. The bonds between the molecules and atoms of an elastically behaving material, when stressed, stretch and bend, but retain memory of their original configuration. Once the stress is released, the stored energy is released and the material returns to its original shape examples are rubber band or super ball and rocks
Brittle deformation: Rupture occurs in response when stress that is exerted on the material exceeds that materials yield strength example are glass, ceramic and rocks
Plastic deformation: Flow occurs in response to stress and material does not return to original shape after applied stress is removed. Just like a Play Dough which can be shaped as liked. Some rocks (rock salt or halite) and other evaporites flow when subjected to stress.

How does elastic rocks break?. 

It can be best understood by the following example of a plywood. When the strip of plywood is subjected to a bending force, it deforms elastically at first. If the bending force is released, the wood returns to its original unbent shape. However, if the bending exceeds the yield strength of the weakest part of the wood, that part will rupture. Once the rupture occurs, we all hear the cracking noise - this represents the propagation of acoustic energy through the air due to the rupture. In other words, the rupture releases energy into the surrounding medium and this energy spreads away from the point of rupture. The farther that one is located from the point of the rupture, the softer the rupture noise is because that finite amount of energy released by the cracking wood is being spread over a larger volume as it moves away from the source.

Fault breaks which results in earthquake

So how does earthquake occurs in the crust?

In the earth's crust and in particular, within the fault zones that accommodate the motion between the rigid interiors of plates, the crust deforms elastically between earthquakes. The faults have geometric irregularities (bends) that prevent the crust on either side of the fault from slipping smoothly (creeping) in response to the steady state motion of the plates on either side of the fault. Because friction prevents steady state slip along a fault, rocks near the fault deform elastically in response to plate motion far from the fault. Once the amount of elastically stored energy exceeds the strength of the weakest area of rocks along a fault, that patch of the fault ruptures. At the point of rupture, rocks on either side of the fault slip to their new location and in the process, release lots of stored energy that propagates away from the point of rupture. A small rupture in one area of a fault can place a sudden strain on a nearby, more strongly locked section of the fault and cause that section of the fault to rupture, too. Thus, one earthquake can trigger another. Faults often have bends; the rocks on the fault face can have different frictional and elastic properties; fluids may lubricate the fault; and other nearby faults may change the local stresses.

Once an earthquake has occurred along a section of a fault, much of the stress on those rocks is relieved. However, since steady state plate motion is still occurring, stress immediately begins to build again, leading to the earthquake cycle in which repeat earthquakes occur along sections of a fault. The frequency and strength of earthquakes along a given fault depends on how quickly the stress builds, how weakly or strongly the fault is locked in a particular region, and interactions with other nearby faults that are also responding to the stress build up. This makes it difficult to to model the earthquake cycle.

Rupture and propagation of seismic energy

To understand the propagation of seismic waves one can demonstrate it by throwing a stone into a pond. Well ripples on the pond carries energy away from the point of impact. Some of the energy is also carried down into the pond as sound, which we could hear if we were beneath water when the stone was thrown. In a similar fashion, during an earthquake rupture, two broad categories of seismic waves are generated.

• Body waves, which carry seismic energy through the interior of the earth
• Surface waves, which carry seismic energy along the surface.
• Body waves can be further sub-divided as follows:
• P wave (primary) is Compressional. Particles displaced in direction of energy propagation
• S wave (secondary) is Shearing. Particles displaced perpendicular to direction of energy propagation.

Surface waves, which cause the earth's surface to roll as they pass by are often responsible for the majority of earthquake damage. Surface wave amplitudes can reach several meters, meaning that during a large earthquake, one end of your house could be in the trough of a surface wave several meters beneath the other end of your house which could be surfing on the crest of a surface wave. Surface waves travel slowly often take several minutes or longer to travel tens of miles. Body waves arrive within seconds but aren't as likely to cause major shaking.

Why are seismic waves useful?

Seismic waves are useful for locating earthquakes, determining the amount of energy that was released, and determining what type of fault slip occurred. Seismologists routinely exploit this information using a global network of seismographs that continuously feed their readings into several analysis centres. Earthquake locations (epicentres) and magnitudes are typically available less than an hour after an earthquake. 

To find the location, three things required to completely describe the location of an earthquake 

• Its latitude 
• Its longitude 
• Its depth. 

These three together describe the earthquake focal point, which is the point within the earth where an earthquake started to rupture a fault. The point on the earth's surface directly above the focus is called the epicentre.
Magnitude: The magnitude of an earthquake measures the total amount of ground shaking produced near the epicentre. There are many scales to measure the magnitude of an earthquake but the most used one is Richter scale Richter magnitudes vary from 1 to about 9, with 1 being very small and 9 being enormous. In general, an increase of 1 point in the magnitude represents a 10X increase in the amount of ground motion and a 31X increase in the amount of energy release.
Intensity: An alternative way to measure the size of an earthquake is by its effect on humans and surface features such as buildings. This technique has shortcomings because it depends on the often subjective observations of individuals. However, for earthquakes that occurred before regular instrumental recording made it possibly to routinely estimate earthquake magnitudes, estimates of intensity are the only way to locate epicentres and determine how large the earthquake was. 

Earthquake Risk factors

• Fault movement: direct breakage of structure built on fault trace.
• Ground Shaking: ground vibration caused by seismic waves travelling away from focus.
• Landslide: ground shaking can induce failure of weak slopes.
• Liquefaction: ground shaking of wet soil can induce creep of soil.
• Tsunami: disturbance of sea floor causing seismic sea wave.
• Fire: rupture of gas lines etc.

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