Formation of Earthquakes

Formation of Earthquakes

Faults

A fault is a fracture or a break that can be found in the Earth’s crust where earthquake is most likely to occur repeatedly. It is formed when the rocks of the crust are compressed or stretched because of the movement of the plates. Because these rocks are brittle, they do not bend when they are compressed or stretched. Instead, they break along areas where they are weak. These breaks are called as the fault zones if there are many faults spread over a wide area.

Classification of Faults

• Classification based on level of activity

• 1. Active Faults - Known to have recently generated earthquakes within the last 10000 years, which may still continue to generate earthquakes 

• 2. Inactive Faults - Do not show signs of ever having generated earthquake in the last 10000 years but may possibly still generate an earthquake in the future. 

Multiple Choice

Your science teacher asks you to classify a certain fault based on its level of activity. If the fault has a record of

movement and generated earthquakes 100 years ago, what is then the classification of the fault ?

• Active Fault

• Inactive Fault

• Deactivate Fault

• Activate Faults

Classification of Fault

*Based on the Fault Movements

· Force applied to rocks is called as stress.

There are three types of stress:

(1) Tensional Stress – Rocks are stretched away from each other; (2)Compressional Stress – Rocks are pushed towards each other; and (3) Shear Stress – Rocks are pushed toward each other but not in the same axis.

When rocks are stretched or compressed along the horizon, they break and form a fault plane at an angle to the horizon called as dip. Since the faults moves or “slips” along the dip, these type of faults is called dip-slip faults.

o  Types of Dip Slip Faults

(i)  Normal Faults – formed when the rocks are stretched.

-         The block that sits on the fault plane, called as the hanging wall, has moved downward with respect to the foot wall along the dip direction. 

(ii)  Reverse Faults – formed when the rocks are compressed.

-         The hanging wall has moves up with respect to the foot wall, causing the length of the rock of the body shorten. 

Multiple Select

Which of the following stresses is responsible for creating Normal Fault?

• Tensional Stress

• Compressional Stress

• Shear Stress

• Force Stress

When the rocks are forced to move in parallel but opposite directions they are doing the special type of movements called as shear. When rocks are made to shear, they break along a fault plane that forms in the direction of the shear.

o  The type of fault that will be formed is called as strike-slip fault. The strike-slip fault slips in the direction of the strike, which is the line of intersection between the horizon and a planar surface. 

Faults Generates Earthquake

The movement of the fault will likely cause an earthquake. As the rocks of the plates stretched or compressed, and elastic energy is stored in the rocks as this happens. When the rocks breaks along a fault, the stored elastic energy is released as a special type of kinetic energy called as seismic energy. This energy passes through rocks as seismic waves, and spread from their point of origin towards the Earth’s surface. Most people do not realize that when they experience earthquakes, they are actually feeling the arrival of seismic waves from deep within Earth.  

Intensity and Magnitude

When an earthquake happens, we will hear its strength being reported using two different numbers: Intensity and Magnitude. Intensity tells how much a certain area was shaken when an earthquake reached that area, whereas magnitude describes the total amount of an energy that was released by the earthquake at the focus. The amount of shaking is usually stronger when an area is near the epicenter, and becomes weaker farther away. So an earthquake may be described with more than one value of intensity but only one value of magnitude. For example, PHILVOCS reported that the February 6,2012 Negros earthquake had a magnitude of 6.9, but had intensities of VII in Tayasan, Negros Oriental, VI in Cebu City, and II in Boracay.

Open Ended

What is the difference between the Intensity and Magnitude?