Earthquake EngineeringIntroduction Whenever the earth is disturbed, vibrations are produced which is in all directions from the place of their origin. Whenever this vibrations travel, an earthquake is said to have taken place. Earthquake Engineering These vibrations are more intense near their source, and as the distance increases, these become feeble and slowly die out. Some earthquakes can be very destructive and result in collapse of the structures, and resulting in heavy loss of life and damage to the buildings. 2 Thus it is necessary to study the earthquake in detail and take every precautionary measure and protection to minimize the loss of life and property. Earthquake is defined as the shaking of earth’s surface due to any reason which, results in release of large amount of energy. The size and severity of an earthquake is estimated by two important parameters-Magnitude and Intensity. Hence Seismology or Earthquake Engineering is defined as the branch of science which deals with the study of earthquakes. Structure of Earth / Earth Interior • The earth is conceived to be composed of a sequence of shells or layers called geosphere. The various geospheres that constitute the earth are: 3 4 BARYSPEHERE: Know as the CORE, the inner most part of the earth Compose of the inner and outer cores Is mainly liquid zone, and has Nickel and Iron The inner core, 1221 km in radius, is mainly composed of Nickel and Iron Its density is 1600 kg/m3 and behave like a solid The outer core surrounding the inner core is 2259 km thick and is composed of Nickel and Iron alloyed with silica The outer core exists as a liquid of density 12000 kg/m3 The temperature at the core is about 2500 °C and the pressure is about 4*106 atm. 6 5 Thus. The relative motion of crustal plates gives rise to three kinds of plate boundaries or marginal zones. These currents result in circulation of the earth’s mass from crust to core and vice-versa. large amount of material masses joined together to from the earth. This sliding of the earth’s mass takes place in portions called Tectonic plates. 8 The upper part formed is called as crust and the inner part is called as core. resulting in rising and sinking of the continents ie formation of mountains and valleys. These plates move in different directions and at different speeds. 11 12 . 9 10 Plate Tectonic Theory The surface of earth consists of seven major tectonic plates and may small plates. As the earth cooled down. Is in semi-solid state and the approximate temperature inside the mantle is 3000 °C with density of 5000-6000 kg/m3 LITHOSPHERE: Known as the CRUST thinnest outer solid shell 100 km thick with density of 1500 kg/m3 The temperature of the crust is about 25 °C and the pressure within it is 1 atm 7 Convection Currents The earth crust consists of a number of large rigid blocks called crustal plates. The high temperature and pressure difference between the crust and the core results in the convection currents. According to this theory.Earthquake Engineering ASTHENOSPHERE: Known as MANTLE 2685 km thick surrounding the core The outer mantle extend upto 660 km and inner mantle upto 2900 km. Large amount of heat generated during this fusion. the heavier material sank to the center and lighter ones towards the top. a long time ago. Continents are preferentially preserved in this manner relative to oceanic crust. This is accomplished at convergent plate boundaries. Instead. 13 14 Zones of Convergence (Destructive Margin) Given that Earth is constant in volume. also known as destructive plate boundaries. warmer one. where one plate descends at an angle—that is. the greater buoyancy of continental crust prevents it from sinking. 15 16 Zones of Convergence (Destructive Margin) Oceanic Crust – Continental Crust Features Subduction Zone Trench Volcanic Mountain Arc 17 18 . the older. such as the Himalayas. Where two plates carrying continental crust collide. This molten material. known as magma. As a result. is subducted—beneath the other. it wells up from below and cools close to the surface to generate new crust.Earthquake Engineering Zones of Divergence (Constructive Margin) As plates move apart at a divergent plate boundary. which is continuously recycled into the mantle. Where one of the plate margins is oceanic and the other is continental. Because new crust is formed. the release of pressure produces partial melting of the underlying mantle. are created. the continuous formation of new crust produces an excess that must be balanced by destruction of crust elsewhere. Where two oceanic plates meet. and the oceanic plate is preferentially subducted. denser plate is preferentially subducted beneath the younger. divergent margins are also called constructive margins. towering mountain ranges. neither is subducted. It is for this reason that the oceanic crust is much younger than the continental crust which is not recycled. Trenches are the deepest parts of the ocean and remain largely unexplored. The subducting plate is bent down into the mantle to form a deep depression in the seafloor called a trench. The oceanic crust descends into the mantle at a rate of centimetres per year. sinks below the continental crust. This is called a Subduction Zone.Earthquake Engineering Subduction zone: At a convergent boundary where continental crust pushes against oceanic crust. because they are dense. Eventually the subducting slab sinks down into the mantle to be recycled. it dehydrates and releases water into the overlying mantle wedge The addition of water into the mantle wedge changes the melting point of the molten material there forming new melt which rises up into the overlying continental crust forming volcanoes. This oceanic crust is called the “Subducting Slab” 19 When the subducting slab reaches a depth of around 100 kilometres. one runs over the top of the other causing it to sink into the mantle and a subduction zone is formed. 20 Trenches: When two oceanic plates converge. Subduction is a way of recycling the oceanic crust. the oceanic crust which is thinner and more dense than the continental crust. 21 22 Continental Crust – Continental Crust Features Folded Mountains 23 24 . known as a volcanic arc. 29 30 . pushing up (and down into the mantle) high mountain ranges. This boundary is also called as a Parallel or transform Fault Boundary. creating tension along the boundaries . The lithosphere plate slides past each other without any creation or destruction.e. Melting in the mantle wedge produces magma.Earthquake Engineering When continental crust pushes against continental crust both sides of the convergent boundary have the same properties (i. thereby stimulating partial melting of mantle in the plate above the subduction zone. If both plates are oceanic. For example. This magma rises to the surface and gives birth to a line of volcanoes in the overriding plate. the volcanoes form a curved line of islands. 27 28 Fractures Zones (Conservative Margin) Are also known as transformed faults. The edges of the two plates scrapes each other closely. Oceanic Crust – Oceanic Crust Features Subduction Zone Trench Island Arc 25 26 Island Arc: When the downgoing slab reaches a depth of about 100 km (60 miles). and as a result the two plates push against each other and the crust buckles and cracks. the European Alps and Himalayas formed this way. Neither side of the boundary wants to sink beneath the other side. such as in the modern western Pacific Ocean. it gets sufficiently warm to drive off its most volatile components. known as an island arc. thick and buoyant). Rocks bend elastically until the strength of the rock is exceeded Rupture occurs and the rocks quickly rebound to nearly an undeformed shape 31 32 Faults A fault is a break in the Earth’s crust along which the blocks of the crust slide relative to one another Earthquakes occur along faults because of the sliding Energy is released in several types of waves that radiate outward from the fault at different speeds 33 34 Terminology of Faults Fault Plane: The plane splitting the rock into two blocks along which movement occurs. Hanging Wall and Foot Wall: in the case of an inclined fault plane. 35 36 . the block which rests over the other is called as hanging wall and the underlying block is called as foot wall. This is known as the elastic rebound theory. It may be horizontal or vertical.Earthquake Engineering Elastic Rebound Theory Stress builds up on a fault till it breaks. Slip of the Fault: The relative displacement of two points which where initially against each other is know as slip of a fault. Primary. Oblique Slip Faults Those in which slip is neither along dip nor along strike of the fault.Earthquake Engineering Types of Faults The sudden slip of fault produces vibrations in the earth’s crust causing earthquakes. Shear. there are three types of faults: 1. Secondary. rather it is oblique. Transverse 41 . On the basis of slip of fault. S-waves do not travel through fluids. S waves travel slower than P waves in a solid and. Alternating transverse motions (perpendicular to the direction of propagation. The displacement along the dip of the fault is called as dip slip. Compressional. P waves in a liquid or gas are pressure waves. and the raypath). perpendicular to the wavefront. Other Characteristics P motion travels fastest in materials. therefore. Suggest both normal faulting and strike slip faulting. including sound waves. 39 40 Seismic Waves Wave Type (and names) P. Dip Slip Faults 2. Oblique Slip Faults 37 Dip Slip Faults: A fault on which the movement is in the direction of the dip of the fault. commonly approximately polarized such that particle motion is in vertical or horizontal planes. arrive after the P wave. Generally smaller and higher frequency than the S and Surfacewaves. Longitudinal Seismic Body Waves Particle Motion Alternating compressions (“pushes”) and dilations (“pulls”) which are directed in the same direction as the wave is propagating (along the raypath). so do not exist in Earth’s outer core (inferred to be primarily liquid iron) or in air or water or molten rock (magma). S. Strike Slip Faults 3. and therefore. It is caused by combination of shearing and tension of compression forces. so the P-wave is the firstarriving energy on a seismogram. 38 Strike Slip Faults Fault along lateral direction is known as strike slip fault. with lower frequencies penetrating to greater depth. Amplitude decreases with depth. Material returns to its original shape after wave passes. Amplitude decreases with depth. Material returns to its original shape after wave passes. Earth’s layers tend to cause mostly vertical (SV. that is. Seismic Surface Waves Wave Type (and names) L. Appearance and particle motion are similar to water waves. Rayleigh waves travel slightly slower than Love waves. Material returns to its original shape after wave passes. Generally. Particle motion is horizontal and perpendicular to the direction of propagation (transverse). Love Wave (L-Wave) Animation R. Intensity is determined from effects on people. Long waves Particle Motion Transverse horizontal motion. Surface waves. Love waves are dispersive. Particle motion consists of alternating compression and dilation. Depth of penetration of the Rayleigh waves is also dependent on frequency. Intensity Intensity measures the strength of shaking produced by the earthquake at a certain location. Particle motion consists of alternating transverse motions. Deformation propagates. and “phased” so that the motion is generally elliptical – either prograde or retrograde. the wave velocity is dependent on frequency. Surface waves. Magnitude is determined from measurements on seismographs. and the natural environment. Rayleigh Wave (R-Wave) Animation Magnitude Magnitude measures the energy released at the source of the earthquake. in the vertical plane) or horizontal (SH) shear motions. Ground roll Motion is both in the direction of propagation and perpendicular (in a vertical plane). 48 . Deformation propagates. Other Characteristics Love waves exist because of the Earth’s surface. To aid in seeing that the particle motion is purely horizontal. generally with low frequencies propagating at higher velocity. Material returns to its original shape after wave passes. Particle motion is perpendicular to the direction of propagation (transverse). Particle motion consists of alternating transverse motion. However. perpendicular to the direction of propagation and generally parallel to the Earth’s surface.Earthquake Engineering Compressional Wave (P-Wave) Animation Shear Wave (S-Wave) Animation Deformation propagates. Love. Deformation propagates. Transverse particle motion shown here is vertical but can be in any direction. Rayleigh waves are also dispersive and the amplitudes generally decrease with depth in the Earth. Long waves. Particle motion is parallel to the direction of propagation (longitudinal). human structures. focus on the Y axis (red line) as the wave propagates through it. Rayleigh. Particle motion consists of elliptical motions (generally retrograde elliptical) in the vertical plane and parallel to the direction of propagation. They are largest at the surface and decrease in amplitude with depth. the stiffness force in a column is the column stiffness times the relative displacement between its ends. Also. larger is this force. the larger this internal force in columns. bigger is the column size).e.. 52 Torsion (IS 1893:2002) Due to Eccentricity in the mass and stiffness distribution Due to accidental causes. the stiffer the columns are (i. these internal forces in the columns are called stiffness forces. uncertainties in dead load due to variation in workmanship and materials 53 . For this reason. In fact. including the rotational component of ground motion about a vertical axis Difference between assumed and actual stiffness and mass Uncertain live load distribution. How????? 51 Effect of Deformations in Structures The larger is the relative horizontal displacement u between the top and bottom of the column.Earthquake Engineering Seismic Zones in India Seismograph The instrument that measures earthquake shaking. It has three components(i) Sensor (ii) Recorder (iii) Timer Earthquake tips No: 4 49 50 Seismic Effects of Structures Inertia Forces in Structures Que: lighter buildings sustain the earthquake shaking better.