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KNOWING EARTHQUAKES
Earthquakes: Distribution and types Knowing Earthquakes Earthquakes in India Earthquake Research in India Earthquake studies in CESS 

 

Earthquake terminologies
How to locate an Earthquake?
What is a fault?

Fault types and movements

 

 

 

 

 

 

 

 

 

 

 

 

 


Earthquake terminologies

Propagation of seismic waves through the earth. P waves move particles along their path; S waves displace particles at right angle to the line of travel (Bolt,1993)

 Mainshock,foreshocks and aftershocks
A large earthquake is generally preceded and followed by many smaller shocks. The largest earthquake is called the main shock. The smaller ones that precede the main shock are called foreshocks and the subsequent shocks are called aftershocks.
Earthquake swarms
The earthquake swarms are groups of earthquakes which are concentrated in a certain region, but none of them is significantly larger than the others.
Seismograph
Seismograph is the instrument for recording motions of the earth’s surface caused by seismic waves, as a function of time. The simplest earthquake recording system consists of a sensor and an analog or digital recorder. The record is known as a seismogram. Location and magnitude of an earthquake are calculated from seismograms.

 

 

 

 

 

 

 

 

 

 

 

 

Hypocentre, epicentre and hypocentral region of a earthquake (Earthquake in Japan, 1996)

 Seismic waves
 In earthquake studies, we generally deal with P, S and surface waves. P wave is the primary or the fastest wave travelling away from an earthquake source, consisting of a train of compressions and dilatations parallel to the direction of travel of the wave. S wave is the secondary wave, travelling more slowly than the P wave and consisting of elastic vibrations transverse to the direction of travel. Earthquakes also generate surface waves that follow the Earth’s surface only, with a speed less than the S waves. Propagation of surface waves also causes considerable damage during an earthquake. The difference in arrival time between the P wave and the S wave can suggest the approximate distance from the epicentre. From records of earthquakes at several places, the epicentre can be determined by reading the time differences.
Intensity
Intensity is description of the effects of an earthquake at a particular place, based on observations of damage, using a descriptive scale like the Modified Mercalli Scale. A map showing intensities at individual locations may be contoured based on isoseismals, which are lines of equal intensity. An isoseismal map provides a representation of broad variations of shaking over the region surrounding the earthquake.
Magnitude
Magnitude is a measure of the size of the earthquake, calculated from the amplitude of the seismic waves and is closely related to the energy released by the earthquake. If the magnitude increases by 1, then the energy is about 30 times larger; if it increases by 2, then the energy is about 900 times. Richter magnitude, surface-wave and body-wave magnitudes are commonly used to indicate this measure. Duration or coda- magnitude based on the duration of the seismic signal is also in use.
Hypocentre and epicentre
The earthquake occurs as a result of the motion of a fault, that is, by rupture of rocks. The point where the rupture originates is called the hypocentre or the focus and the point directly above this on the ground is called the epicentre. Depth to the hypocentre is known as the focal depth.

 

 Recurrence interval
Recurrence interval is the average time interval between two strong earthquakes of similar magnitude in a given location. Recurrence interval for large earthquakes in the active regions like the Himalaya is considered to be of the order of a few hundreds of years. In contrast, the recurrence interval for such earthquakes in the peninsular India may be much longer.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

How to locate an earthquake?



P and S waves leave the focus of an earthquake at precisely the same time

P waves are faster than the S waves and will reach the distant stations first

The farther they travel, the greater is the difference in the arrival times of P and S waves

The delay at different stations can be graphically used to determine how far the source is

Using observations from three stations, the epicentre can be located.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fault types and movements
The plane of the fault that cuts the horizontal surface of the ground along a line whose direction is measured from the north is called the strike of the fault. The fault plane itself is usually not vertical but dips at an angle. The block resting on the fault plane is called the hanging wall and that beneath is called the footwall. Faults can be classified as dip-slip and strike-slip, depending on the direction of movement of the blocks.
A dip-slip fault involves movement along the sloping direction. When the hanging wall moves down with respect to the footwall, it is called a normal fault.
Fault models: Earthquakes in Japan,1996
When the hanging wall moves up relative to the footwall, it is called a reverse fault. Thrust fault is a special type of reverse fault in which the dip is very small.
A strike-slip fault involves movement of blocks parallel to the fault plane. Right-lateral and left-lateral strike-slip faults are defined on the basis of the sense of movement.
 
If you stand on oneside of a fault and the other side slips to the right, then it is called a right-lateral fault. In a left-lateral fault, movement occurs to the left.
Movement on strike-slip faults generates much of the world’s seismic activity each year. This is partly because strike-slip faults are commonly very long, thus providing widespread sites for earthquakes. Strike-slip faults may extend for thousands of kilometres and several hundred kilometres of movement may have occurred along them in the past.
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

What is a fault?
A fault is a fracture having appreciable movement parallel to the plane of the fracture. Faults are of practical importance because they generate earthquakes. It is important to understand faults for designing the long-term stability of dams, bridges, buildings, power plants etc. We need to understand the basic anatomy of faults, to appreciate their behaviour. The most obvious feature related to faulting is the displacement of marker layers along the actual movement surface called the fault plane.