Faults and earthquake

Subject: Geography

Context:

Geologists have discovered a series of faults at the foot of the Himalayas.

Concept:

• An earthquake is shaking or trembling of the earth’s surface, caused by the seismic waves or earthquake waves that are generated due to a sudden movement (sudden release of energy) in the earth’s crust (shallow-focus earthquakes) or upper mantle (some shallow-focus and all intermediate and deep-focus earthquakes).
• The point where the energy is released is called the focus or the hypocentre of an earthquake.
• The point on the surface directly above the focus is called epicentre (first surface point to experience the earthquake waves).
• A line connecting all points on the surface where the intensity is the same is called an isoseismic line.

Causes:

• Fault Zones
• Plate tectonics
• Volcanic activity
• Human Induced Earthquakes

Waves:

• There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body wavesand surface waves.
• Body waves can travel through the earth’s inner layers, but surface waves can only move along the surface of the planet like ripples on water
• The first kind of body wave is the P waveor primary wave. This is the fastest kind of seismic wave, and, consequently, the first to ‘arrive’ at a seismic station. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air.
• The second type of body wave is the S waveor secondary wave, which is the second wave felt in an earthquake. An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium. It is this property of S waves that led seismologists to conclude that the Earth’s outer core is a liquid.
• S waves move rock particles up and down, or side-to-side–perpendicular to the direction that the wave is traveling in (the direction of wave propagation)

Distribution of Earthquakes

Measuring earthquake: Mercallivs Richter

• The Mercalli scale bases its measurement on the observed effects of the earthquake and describes its intensity. It is a linear measurement.
• On the other hand, the Richter scale measures the seismic waves, or the energy released, causing the earthquake and describes the quake’s magnitude. It is a logarithmic

Faults:

• A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other.
• This movement may occur rapidly, in the form of an earthquake – or may occur slowly, in the form of creep.
• Faults may range in length from a few millimeters to thousands of kilometers.
• Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other.
• The fault surface can be horizontal or vertical or some arbitrary angle in between.
• Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault to classify faults.
• Faults which move along the direction of the dip plane are dip-slip faults and described as either normal or reverse (thrust), depending on their motion.
• Faults which move horizontally are known as strike-slip faults and are classified as either right-lateral or left-lateral.
• Faults which show both dip-slip and strike-slip motion are known as oblique-slip faults.