Seismic Zones of India

Seismic zones of IndiaOn a seismic map, the country is divided into several zones in terms of severity of expected ground motion. The zoning map is reviewed and revised periodically based on additional data available. A brief account of significant features of the seismic zoning maps in the past has been presented by Jai Krishna (1992).The first zoning map (1962) was prepared on the basis of historical data available regarding the occurrence of earthquakes all over the country and their severity (magnitude).

Seismic zones of IndiaThis was based primarily on the epicentral map and on the data of isoseismal maps published by the GSI. For some strong earthquakes and the earliest zoning map brought out by GSI in 1935.Epicenters of earthquakes with magnitude 5 and above were plotted. Seven zones were indicated. Zone I is least severe and the zone VI is the most severe. The entire peninsular India except for few areas was shown as zone-0 and considered a seismic (very stable region free from earthquakes).

Seismic zones of IndiaSubsequently the zoning map was revised in 1966. Zone-0 was narrowed in its extent. Koyna earthquakes in 1967 has confirmed that Peninsular India is no longer a seismic. In the revised map of 1970, zone-0 was accordingly removed. Many of the areas in zones V and VI were merged into one zone because of their high risk.

Seismic zones of IndiaSo the number of zones got reduced from seven to five. In the zone map brought out in the year 2000, the earlier zone II and I were merged. Additional regions in south have been included in zone III. Zone IV includes areas of high risk. Entire NE region, parts of Uttaranchal, Rann of Kutch (Gujarath) and Srinagar are included in zone V.

SeismologyEarth science which studies seismic waves and earthquakes.Divided into : Local seismicity, which deals with the probability of an earthquake occurring in a given portion of the earths crust andRegional seismicity, which deals with the probabilities of an earthquakes of a given intensity shaking a given region of the earths surface.

SeismologyThe study of local seismicity basically involves a knowledge of the geotectonic features and seismic history at micro-and macro levels, regional seismic waves, local geological information and statistical data on intensities of past earthquakes to establish an estimate of seismic risk.The historical data for a site is seldom sufficient to permit a statistical estimate of seismic risk. Seismic risk is therefore assessed through deductive probabilities models using the available data.

TerminologyFocus: of hypocenter is the point in the earths crust where the original seismic waves originate. From the focus seismic vibrations spreads in other directions.Epicenter: is the vertical projection of the focus on the earths surface. The location of a site in relation to an earthquake may be specified by focal distance (R) or by epicentral distance (X) and focal depth (f): R=(x2 + H2)1/2

TerminologyIntensity (I) : is a measure of earthquake local destructiveness. It is measured on a subjective scale, such as the modified Mercalli (MM) scale.Magnitude : is a measure of the seismic earthquake in terms of the energy released. There are several definitions of magnitude. The original definition of Richter (1958) defined magnitude as the common logarithm of the travel amplitude in micron or a standard seismograph located 100 km from the epicenter.

Size of an EarthquakesDefined by its intensity and magnitude. Intensity is expressed based on the degree of destruction caused and hence varies from place to place. It is maximum around the epicentreal area. At each place, the intensity is determined based on the modified Mercalli scale.Once the intensity values are assigned to different locations of the region on a map, equal intensity contours are drawn. These are known as isoseismals. The intensity at a place depends on several factors such as distance from the epicenter, depth of focus, geological formation and type of construction of a structure.

Modified Mercalli scale of earthquake intensities

IntensityDesignationEffects producedIInstrumentalDetected only by very sensitive instruments.IIVery feebleFelt by few persons at rest, particularly on upper floor of buildingsIIIFeebleFelt by persons indoors, standing automobiles disturbed slightly, vibration similar to a moving truck IVModerateFelt by persons indoors, also be few outdoors, windows and doors rattle; loose objects disturbedVRelatively strongFelt by almost everyone; bells ring; breaking of dishes; pendulum stop; wall plaster breaks.VIStrongFelt by all; falling of plaster; walls crack; heavy furniture disturbedVIIVery StrongPersons run outdoors; tremors felt by persons in moving Vehicles, slight to moderate damage in well-built structures.VIIIDestructiveConsiderable damage to ordinary buildings; falling of walls; ejection of sand and mud in small quantities; changes in well water.IXRuinousGround cracks, breaking of underground pipes; considerable damage to building; building shifted off foundations.XDisastrousBending of rails; most of the structures damaged, occurrence of Landslides, ground badly cracked. XIVery disastrousBuilding destroyed; broad open ground fissures develop, rails badly twisted.XIICatastrophic Total destruction; surface displacements; objects thrown into air.

Size of an EarthquakesSince the intensity value is fixed on the basis of collection of data in the field from several sources including experiences of persons, quit often it is more subjective.On the contrary, magnitude of an earthquake does not vary from place to place. Magnitude is a function of the energy released in an earthquake and is commonly expressed as Richters magnitude.

Size of an EarthquakesThe relation between magnitude (M) and energy (E) is expressed asLog10 E = 4.4 + 2.14 M 0.054 M2E is obtained from the expression: E = c(a/h).(d2 + h2)E= Total energy released (ergs), c = constant (taken as 0.625), a = ground acceleration, d= distance (km) of the recording station from the epicenter and h = depth of focus (km)

Size of an EarthquakesRichters scale has magnitude numbers up to 10. but the maximum known magnitude is around 9.6 only. Beyond this value, the earthquake energy is so high that there is total destruction.To provide an idea of the energy released in an earthquake, a quake of 6.0M involves energy of around 2.5x1020 ergs (equal to that of an ordinary atom bomb), for 7.0M earthquakes, it is around 80x1020 ergs (equivalent to that of a H-bomb), for 8.0M, it is around 2500x1020 ergs. The energy increases by a factor of 30 for each increment in the magnitude. In Richters scale, the amplitude increases on a logarithmic scale with the increase in magnitude.

Important earthquakes in different parts of the world.

PlaceYearMagnitudeNorth BoliviaJune 9, 19948.2Kuril IslandsOct. 4, 19948.3N. Chile coastJuly 30, 19958.0Jalisco coast, MexicoOct. 9, 19958.0Irian Jaya, IndonesiaFeb. 17, 19968.2Balleny IslandsMarch 25, 19988.1New Islands Region, P.N.GNov. 16, 20008.0Peru CoastJune 23, 20018.4Hokkaido, JapanSept. 25, 20038.3N. Of MacQuairie IslandDec. 23, 20048.1Off W. Coast, N. SumatraDec. 26, 20049.0

Important earthquakes in India

PlaceYearMagnitudeShillongJune 12, 18978.7KangraApril 4, 19058.0North BiharJan. 15, 19348.3AssamAug. 15, 19508.6KoynaDec. 11, 19676.4UttarkashiOct. 20, 19916.5Killari (Latur)Sept. 30, 19936.5JabalpurMay 22, 19976.0GujaratJan. 26, 20017.5AndamansDec. 26, 20049.0

The Great Sumatra Earthquake and Indian Ocean Tsunami of December 26, 2004 The great mega thrust M 9 Sumatra earthquake on 26 December 2004 at 06:28:53 am IST created the most devastating tsunami in the known history. The deadly tsunami waves lashed low-lying towns adjoining the coastline of eleven countries, including Indonesia, Thailand, Malaysia, India and Sri Lanka, causing more than 150,000 deaths. Closest Indian landmasses to the epicentre are Andaman and Nicobar Islands over a narrow arc of about 800 km in the Bay of Bengal. The maximum intensity of shaking (on the MSK scale) along the AndamanNicobar Islands may be placed at VII and that along the mainland Indian coast at V. It resulted in the death of over 10,000 persons in India with over 5600 persons missing. Extensive devastation of the built environment occurred across the populated AndamanNicobar Islands and the coastal states of Andhra Pradesh, Tamil Nadu and Kerala along the mainland coastline of India.

The Republic Day, Bhuj Earthquake of 26 January 2001 The powerful earthquake that struck the Kutch area in Gujarat at 8:46 am on 26 January 2001 has been the most damaging earthquake in the last five decades in India. The M7.9 quake caused a large loss of life and property. Over 18,600 persons are reported to be dead and over 167,000 injured; the number of deaths is expected to rise with more information coming in. The estimated economic loss due to this quake is placed at around Rs.22,000 Crores (~US$5 billions).

The Republic Day, Bhuj Earthquake of 26 January 2001 The earthquake was felt in most parts of the country and a large area sustained damages. About 20 districts in the state of Gujarat sustained damage. The entire Kutch region of Gujarat, enclosed on three sides by the Great Runn of Kutch, the Little Runn of Kutch and the Arabian Sea, sustained highest damage with maximum intensity of shaking as high as X on the MSK intensity scale. Several towns and large villages, like Bhuj, Anjaar, Vondh and Bhachau sustained widespread destruction.

The Republic Day, Bhuj Earthquake of 26 January 2001 The other prominent failures in the Kutch region include extensive liquefaction, failure of several earth dams of up to about 20m height, damage to masonry arch and RC bridges, and failure of railroad and highway embankments. Numerous recently-built multistorey RC frame buildings collapsed in Gandhidham and Bhuj in the Kutch region, and in the more distant towns of Morbi ( ~125km east of Bhuj), Rajkot (~150km southeast of Bhuj) and Ahmedabad (~300km east of Bhuj). At least one multistorey building at Surat (~375km southeast of Bhuj) collapsed killing a large number of people.

Seismic wavesAn earthquake occurs when deformed rocks under stress can no longer resist fracturing. Due to the sudden yielding of stresses, energy waves are generated and forced out from the earth. These waves of energy are known as Seismic Waves. These waves are generated due to elastic deformation of rocks on the basis of amplitude, nature of vibration and wavelength. Seismic waves are broadly classified into three types:Primary waves,Secondary waves andLong waves.

Seismic wavesPrimary or p-waves :These are the similar to sound waves. Primary waves are longitudinal in character; rocks vibrate parallel to the wave direction. Primary waves travel in all media, i.e., in solids, liquids and gases.P-waves travels faster than secondary waves and hence are known as primary waves. They move towards the epicenter.The velocity of primary waves depends on the density and rigidity of the medium through which they travel.

Seismic wavesSecondary or shear S waves :These are transverse waves and move like light waves in which the particles vibrate at a right angle to the direction of propagation.These waves are capable of changing its volume. Shear waves travel in solid media.Shear waves travel with less velocity than primary waves.Shear waves are also known as Shaking waves. Due to transverse vibrations they cause shaking of the earths surface.

Seismic wavesLong waves, surface waves or L waves :These waves are of the most destructive nature and cause much damage to life and property during an earthquake. Surface waves are far more transverse in nature and travel on the earths surface away from the epicenter. These waves are also called Raleigh waves. They have a greater wave length than P and S waves. The intensity of these waves decreases with depth and they travel at a lower velocity compared to P and S waves.

Earthquakes and Civil Engineering An earthquake is a vibratory motion having components in all directions. The vertical component is more dominant near the epicenteral tracts and the horizontal components away from these tracts. Hence strong structures have to withstand bigger forces near the epicenter and soft, and flexible structures are safer; away from the epicenter flexible structures suffer severe damage while hard structures are safer

Earthquakes and Civil Engineering Building: Steel-framed tall building in which the frame supports all wall and floor loads usually behave well during earthquakes.Reinforce concrete buildings may develop cracks in walls and piers.Houses with roofs, walls and foundations tied into one strong unit behave safely during earthquakes.Houses built with wood and flexible materials of construction absorb earthquake shocks.

Earthquakes and Civil Engineering In our country modern methods are increasingly being adopted and reinforce brick buildings are built against earthquake forces.This method increases the construction cost 2 to 5% but simultaneously saves buildings and lives.Even more recently by a new direction in research has developed isolators for absorbing energy transmitted by ground motion to reduce damage to structures.

The Earthquake ProblemSeverity of ground shaking at a given location during an earthquake can be minor, moderate and strong. Relatively speaking, minor shaking occurs frequently, moderate shaking occasionally and strong shaking rarely. For instance, on average annually about 800 earthquakes of magnitude 5.0-5.9 occur in the world while the number is only about 18 for magnitude range 7.0-7.9 (see Table 1 of IITK-BMTPC Earthquake Tip 03 at www.nicee.org).

So, should we design and construct a building to resist that rare earthquake shaking that may come only once in 500years or even once in 2000 years at the chosen project site, even though the life of the building itself may be only 50 or 100 years? Since it costs money to provide additional earthquake safety in buildings, a conflict arises: Should we do away with the design of buildings for earthquake effects? Or should we design the buildings to be earthquake proof wherein there is no damage during the strong but rare earthquake shaking? Clearly, the former approach can lead to a major disaster, and the second approach is too expensive. Hence, the design philosophy should lie somewhere in between these two extremes.

Earthquake-Resistant BuildingsThe engineers do not attempt to make earthquake proof buildings that will not get damaged even during the rare but strong earthquake; such buildings will be too robust and also too expensive. Instead, the engineering intention is to make buildings earthquake resistant; such buildings resist the effects of ground shaking, although they may get damaged severely but would not collapse during the strong earthquake. Thus, safety of people and contents is assured in earthquake-resistant buildings, and thereby a disaster is avoided. This is a major objective of seismic design codes throughout the world.

Earthquake Design Philosophy(a) Under minor but frequent shaking, the main members of the building that carry vertical and horizontal forces should not be damaged; however building parts that do not carry load may sustain repairable damage. (b) Under moderate but occasional shaking, the main members may sustain repairable damage, while the other parts of the building may be damaged such that they may even have to be replaced after the earthquake; and (c) Under strong but rare shaking, the main members may sustain severe (even) irreparable damage, but the building should not collapse.Figure 2: Performance objectives under different intensities of earthquake shaking seeking low repairable damage under minor shaking and collapse-prevention under strong shaking.

Earthquake Design PhilosophyThus, after minor shaking, the building will be fully operational within a short time and the repair costs will be small. And, after moderate shaking, the building will be operational once the repair and strengthening of the damaged main members is completed. But, after a strong earthquake, the building may become dysfunctional for further use, but will stand so that people can be evacuated and property recovered. Figure 2: Performance objectives under different intensities of earthquake shaking seeking low repairable damage under minor shaking and collapse-prevention under strong shaking.

Earthquake Design PhilosophyThe consequences of damage have to be kept in view in the design philosophy. For example, important buildings, like hospitals and fire stations, play a critical role in post-earthquake activities and must remain functional immediately after the earthquake. These structures must sustain very little damage and should be designed for a higher level of earthquake protection. Collapse of dams during earthquakes can cause flooding in the downstream reaches, which itself can be a secondary disaster. Therefore, dams (and similarly, nuclear power plants) should be designed for still higher level of earthquake motion.Figure 2: Performance objectives under different intensities of earthquake shaking seeking low repairable damage under minor shaking and collapse-prevention under strong shaking.

Damage in Buildings: UnavoidableDesign of buildings to resist earthquakes involves controlling the damage to acceptable levels at a reasonable cost. Contrary to the common thinking that any crack in the building after an earthquake means the building is unsafe for habitation, engineers designing earthquake-resistant buildings recognize that some damage is unavoidable. Different types of damage (mainly visualized though cracks; especially so in concrete and masonry buildings) occur in buildings during earthquakes. Some of these cracks are acceptable (in terms of both their size and location), while others are not. Figure3:Diagonalcracks in columns jeopardize vertical load capacity of building - unacceptable damage

Damage in Buildings: UnavoidableFor instance, in a reinforced concrete frame building with masonry filler walls between columns, the cracks between vertical columns and masonry filler walls are acceptable, but diagonal cracks running through the columns are not (Figure 3). In general, qualified technical professionals are knowledgeable of the causes and severity of damage in earthquake-resistant buildings. Earthquake-resistant design is therefore concerned about ensuring that the damages in buildings during earthquakes are of the acceptable variety, and also that they occur at the right places and in right amounts. Figure3:Diagonalcracks in columns jeopardize vertical load capacity of building - unacceptable damage

Damage in Buildings: UnavoidableThis approach of earthquake-resistant design is much like the use of electrical fuses in houses: to protect the entire electrical wiring and appliances in the house, you sacrifice some small parts of the electrical circuit, called fuses; these fuses are easily replaced after the electrical over-current. Likewise, to save the building from collapsing, you need to allow some pre-determined parts to undergo the acceptable type and level of damage.Figure3:Diagonalcracks in columns jeopardize vertical load capacity of building - unacceptable damage

Acceptable Damage:Ductility (Figure3):Diagonalcracks in columns jeopardize vertical load capacity of building - unacceptable damage So, the task now is to identify acceptable forms of damage and desirable building behaviour during earthquakes. To do this, let us first understand how different materials behave. 4(b) Brittle failure of a reinforced column.4(a) building performances during earthquakes: Two extremes-the ductile and the brittleFigure 4: Ductile and Brittle structures seismic design attempts toavoid structures of latter kind.

Acceptable Damage:Consider white chalk used to write on blackboards and steel pins with solid heads used to hold sheets of paper together. Yes a chalk breaks easily!! On the contrary, a steel pin allows it to be bent back-and-forth. Engineers define the property that allows steel pins to bend back-and-forth by large amounts, as ductility; chalk is a brittle material.4(b) Brittle failure of a reinforced column.4(a) building performances during earthquakes: Two extremes-the ductile and the brittleFigure 4: Ductile and Brittle structures seismic design attempts toavoid structures of latter kind.

Acceptable Damage:Earthquake-resistant buildings, particularly their main elements, need to be built with ductility in them. Such buildings have the ability to sway back-and-forth during an earthquake, and to withstand earthquake effects with some damage, but without collapse (Figure 4).Ductility is one of the most important factors affecting the building performance.4(b) Brittle failure of a reinforced column.4(a) building performances during earthquakes: Two extremes-the ductile and the brittleFigure 4: Ductile and Brittle structures seismic design attempts toavoid structures of latter kind.

Thus, earthquake-resistant design strives to predetermine the locations where damage takes place and then to provide good detailing at these locations to ensure ductile behaviour of the building.