What are earthquakes ?

What are earthquakes ?

Earthquakes are powerful manifestations of sudden releases of strain energy accumulated within the crust and propagated as seismic waves

Global Mosaic of earthquakes

Earthquake do not occur uniformly throughout the world

Activity mainly localised along plate boundaries-Midoceanic ridges, island arcs, orogenic belts etc.

Few earthquakes occur within plates - Intraplate earthquakes

Boundaries of major tectonic plates and distribution of
earthquakes (Judson and Richardson,1995)

Earthquake distribution and tectonic plates

Earthquakes distribution explained in the theory of plate tectonics.

The lithosphere consists of several large slabs of solid rocks called plates- Oceanic and Continental

Plates move horizontally at a speed of a few centimetres per year.

Motion of these plates is considered to be the basic cause of earthquakes.

Plate boundaries are convergent, divergent or transform

Majority of earthquakes occur along these boundaries- Interplate

Earthquakes also occur away from the plate boundaries- Intraplate and SCR

The interior of Earth

Many processes including seismic activity are ultimately related to the Earth’s internal structure

The Earth has a radius of about 6370 km

The crust which forms the upper most layer is thick under the continents but thin beneath the oceans

The layer below the crust is the mantle which extends to a depth of 2900 km

The inner portion below the mantle is the core, comprising of a liquid outer core and a solid inner core.

Interplate, intraplate and SCR earthquakes

A larger number of earthquakes will occur along the edges of interacting plates - interplate earthquakes (Example: Seismicity associated with the Himalayan seismic belt)

Earthquakes occurring within a continental or oceanic plate - intraplate earthquakes.

Very rarely, earthquakes occur in the more stable and older part of the continents (referred to as cratons) - Stable Continental Region (SCR) earthquakes (Example: The 1993 event at Latur (Killari)

The recurrence interval of interplate earthquakes is of the order of tens or hundreds of years, SCR earthquakes may recur only over tens or hundreds of thousands of years.

Due to the lack of preparation to face an earthquake, damages due to such earthquakes are generally very high

Identifying potential faults and assessing seismic hazard of SCR regions is a very challenging task.

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.

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.

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.
Fault models: Earthquakes in Japan,1996 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.
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.

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