Seismology is a part of geophysics which deals about the study of earthquake waves. The waves that generate during the earthquake are called seismic waves. These waves carry huge amount of energy which are capable to shake the earth’s crust. Such phenomenon of earth’s movement is called seismicity. It is important to note that the earth’s movement due to artificially induced waves is also called seismicity or seismic activity. Earthquake seismicity is related to plate tectonics. It happens either due to continental collision or by sea floor spreading. An elongate earthquake zone especially a zone of subduction or sea floor spreading is called “Seismic Belt”. These areas are quite hazardous for settlement and construction activities.
The Himalayan was formed by collision of two continental plates the Tibetan and the Indian. So the mountain range is major global seismic belt where earthquakes of magnitude 4.5 to 5.5 occur every year. Over 600 earthquakes of magnitude 5 or above are recorded during the period of 1950 to 1990. Thus, Himalaya is the focus of the World’s greatest intra-continental earthquakes. Three great quakes Kangara(1905), Bihar- Nepal (1934) and Assam(1950) occurred within a 40 years duration and the seismic movements of these great earthquakes appear to have been the largest among intra-continental earthquakes of 20th century.
The waves that causes earthquake are called seismic waves. These waves of energy produce from the focus or hypocenter of an earthquake and radiate outward like ripples produced when a stone is thrown into pool of water. The waves that radiate gradually outward loss energy and vanish.
The seismic waves that pass through the earth is mainly due to its elastic nature. The seismic waves have been considered the output of elastic deformation of rocks. On the basis of their amplitude, wavelength and nature of vibration. The seismic waves are classified into three main types: P-wave, S-wave and L-wave.
- P-waves: These are longitudinal or compressional waves similar to sound waves. In this case rocks vibrate parallel to the direction of wave propagation. These waves travel in all media i.e solids, liquids and gases. They have short wave length and high frequency. They are called primary because they travel the fastest and arrive.
- S-waves: – These are transverse waves & move like the oscillations in a piece of string vibrated from one end. In this case, rocks vibrate perpendicular to the direction of wave propagation.
These waves are also known as ‘shear waves’ as they are capable to changing the shape of the material without changing its volume. These waves only travel on solid media. They are called secondary because they arrive second at a seismic station. They have also short wave length and high frequency. These waves move less rapidly than p-waves. Their velocity varies proportional to the density of the materials. Since transverse vibration cause the shaking of the earth’s surface, they are also known as ‘shaking wave.’
Note: Both P & S waves travel through the earth’s interior and radiate outward from the focus in all directions. They are so called ‘Body Waves’.
- L-waves: They are named by surface Rayleigh love or long waves.
Causes and Effects of Earthquakes
Earthquakes are sudden temporary vibrations caused by movement of rocks. It is a form of energy of wave motion which originates in a limited region(focus) and then spreads out in all directions from the source of disturbance. Earthquakes usually last for a few seconds to a minute. Sometimes, the vibrations are very feeble(weak) that we can’t feel them whereas the violent earthquake makes a loss in huge materials, ecosystem and human lives.
Causes: There a two types of major causes that originate quakes: non –tectonic and tectonic causes.
Non – Tectonic Causes: These cause are due various activities of geological agents (glacier, river, wind, underground water, ocean tides etc) operating on the surface of the earth, volcanic activity and subduction of earth’s surface. For example, abrupt descending of running water from the slope cause perceptively (detectable) vibration. Similarly, large landslides, rock falls, underground explosions in mines, failure of dams etc also cause vibration. Earthquakes also occur around active volcanoes due to explosive eruption. Sometimes the removal of support from below the ground surface collapses suddenly producing small scale tremors.
Tectonic Causes: Most of the Earthquakes are due to earth’s movements along existing or new faults. Such quakes are called tectonic earthquakes. Tectonic forces on rocks build up a very large storage of energy over a period of time. The energy is stored by the deformation in the rocks. When the accumulated energy exceeds the ultimate strength of rocks, the rock sequence breaks and displaces which is called as faulting. During the displacement of the rocks previously stored energy suddenly releases out in the form of seismic waves. These waves are capable to move the rocks or shack the earth surface called “Earthquake”.
Intensity and Magnitude of Earthquake:
The strength of an earthquake can be measured either by its intensity or by its magnitude. Intensity of an earthquake is a measure of degree of damage and destruction it can cause. These effects can be observed without the help of any instrument. The intensity of an earthquake diminishes outwards from the epicenter. These are two accepted scales of intensities in practice.
Magnitude of Earthquakes: It is the measure of energy released during the earthquake. This is determined on the basis of amplitude of seismic waves recorded on seismogram. Professor Charles Richter of the California purposed a scale of earthquake magnitude in 1935 to indicate the quantity of the energy released by a single earthquake. This is s numerical scale of magnitude from 0 to 9 with the higher number indicating larger earthquakes. Richter has numbered the scale in steps with each step representing an earthquake record 10 times larger than the previous steps. For example, an earthquake of magnitude 5 is 10 times larger than magnitude 4 and 100 times larger than magnitude 3 and 1000 times larger than magnitude 2 and so on. Since the Richter Scale is logarithmic, the differences between two consecutive whole numbers on the scale means an increase of 10 times in the amplitude of earth vibrations.
Rossi Forrel’s Scale: According to this scale, there exist 10 distinct and well defined intensities beginning from ‘I’ and ending with ‘X’. The scale is as follow.
|I||Imperceptible||Recorded by sensitive instrument only.|
|II||Feeble||Recorded by all seismographs and felt by experienced person only.|
|III||Very Slight||Felt by several persons at rest.|
|IV||Slight||Felt by persons in motion, a movable object disturbs.|
|V||Weak||Disturbance of furniture and beds; ringing bell.|
|VI||Moderate||General wakening of persons from sleep, visible oscillations of trees.|
|VII||Strong||Over thrown at movable objects, fall of plaster from walls without serious damages of the buildings.|
|VIII||Very Strong||Cracks in the walls of the building; fall of Chimneys.|
|IX||Severe||Partial or total destruction of buildings.|
|X||Destructive||Destruction of building, rock falls, and landslides occurs in mountainous region, river course may start to change.|
Mercalli Scale: This Scale was developed by Mercali, Italian seismologists. He made studies of the intensities and regional effect of earthquake. The Scales had at first 10 divisions but later on it was modified to a scale of 12 divisions. Higher the number of intensities greater is the damage. The Scale is as follows;
|Intensity No.||Name of shock||Effects|
|I||Instrumental||Detected by instrument only.|
|II||Very Feeble||Delicately suspended objects may swing|
|III||Feeble||Felt by several persons at rest.|
|IV||Moderate||Felt by persons in motion, windows and doors may start vibrating standing automobiles may rock noticeably.|
|V||Relatively Strong||Most sleepy wake up, unstable objects moved.|
|VI||Strong||Felt generally by all people & run outdoors, cracks developed in the walls.|
|VII||Very Strong||Slight damage to building, fissure developed in walls & chimneys broken.|
|VIII||Destructive||Buildings are significantly damaged; heavy furniture’s overturned; persons driving motor car disturbed|
|IX||Ruinous||Buildings collapses, grounds crack underground pipes broken|
|X||Disastrous||Buildings are seriously damaged or destroyed. Landslides starts on steep slopes, ground badly crack the leagues of railway tracks.|
|XI||Very Disastrous||Few buildings remain standing; bridges destroyed; great landslides occur and flood may occur|
|XII||Catastrophic||Total destruction, waves seen on ground surface, light objects in the air, river changes their courses etc.|
Effects of Earthquakes
- Loss of life and damage of property
- Fire after earthquake
- Earthquake cause Tidal Waves