EQTip01_merged.pdf - Earthquake Tip 1: What Causes Earthquakes PDF

Learning 1 Earthquake Design Earthquake Tip and...

Learning 1 Earthquake Design Earthquake Tip and Construction What causes Earthquakes? The Earth and its Interior Long time ago, a large collection of material masses coalesced and formed the Earth. Large amount of heat was generated by this fusion, and slowly as the Earth cooled, the heavier and denser materials sank to the center and the lighter ones rose to the top. The differentiated Earth consists of the Inner Core (radius ~1290km), the Outer Core (thickness ~2200km), the Mantle (thickness ~2900km) and the Crust (thickness ~5 to 40km). Figure 1 shows these layers. The Inner Core is solid and consists of heavy metals (e.g., nickel and iron), while the Crust consists of light materials (e.g., basalts and granites). The Outer Core is liquid in form Figure 2: and the Mantle has the ability to flow. At the Core, the Local Convective Currents in the Mantle temperature is estimated to be ~2500°C, the pressure ~4 million atmospheres and density ~13.5 gm/cc; this is in contrast to ~25°C, 1 atmosphere and 1.5 gm/cc on the Plate Tectonics surface of the Earth. The convective flows of Mantle material cause the Crust and some portion of the Mantle, to slide on the hot molten outer core. This sliding of Earth’s mass Crust takes place in pieces called Tectonic Plates. The surface Mantle of the Earth consists of seven major tectonic plates and many smaller ones (Figure 3). These plates move in Outer Core different directions and at different speeds from those Inner Core of the neighbouring ones. Sometimes, the plate in the front is slower; then, the plate behind it comes and collides (and mountains are formed). On the other hand, sometimes two plates move away from one Figure 1: another (and rifts are created). In another case, two Inside the Earth plates move side-by-side, along the same direction or in opposite directions. These three types of inter-plate interactions are the convergent, divergent and transform The Circulations boundaries (Figure 4), respectively. The convergent boundary has a peculiarity (like at the Himalayas) that Convection currents develop in the viscous sometimes neither of the colliding plates wants to sink. Mantle, because of prevailing high temperature and The relative movement of these plate boundaries pressure gradients between the Crust and the Core, varies across the Earth; on an average, it is of the order like the convective flow of water when heated in a of a couple to tens of centimeters per year. beaker (Figure 2). The energy for the above circulations is derived from the heat produced from the incessant decay of radioactive elements in the Eurasian North American rocks throughout the Earth’s interior. These convection Plate Plate currents result in a circulation of the earth’s mass; hot molten lava comes out and the cold rock mass goes Pacific into the Earth. The mass absorbed eventually melts Plate South under high temperature and pressure and becomes a Indo- African American Plate part of the Mantle, only to come out again from Australian Plate Plate another location, someday. Many such local circulations are taking place at different regions Antarctic Plate underneath the Earth’s surface, leading to different Figure 3: portions of the Earth undergoing different directions Major Tectonic Plates on the Earth’s surface of movements along the surface. 1 IITK-BMTPC Earthquake Tip 1 What causes Earthquakes? page 2 The sudden slip at the fault causes the earthquake… a violent shaking of the Earth during which large elastic strain energy released spreads out in the form of seismic waves that travel through the body and along the surface of the Earth. And, after the earthquake is over, the process of strain build-up at this modified interface between the tectonic plates Convergent Boundary starts all over again (Figure 6). Earth scientists know this as the Elastic Rebound Theory. The collection of material points at the fault over which slip occurs Transform Boundary usually constitutes an oblong three-dimensional volume, with its long dimension often running into Figure 4: Types of tens of kilometers in case of significant earthquakes. Divergent Boundary Inter-Plate Boundaries Types of Earthquakes and Faults Most earthquakes in the world occur along the The Earthquake boundaries of the tectonic plates as described above Tectonic plates are made of elastic but brittle rocky and are called Inter-plate Earthquakes (e.g., 1897 Assam material. And so, elastic strain energy is stored in them (India) earthquake). A number of earthquakes also during the relative deformations that occur due to the occur within the plate itself but away from the plate gigantic tectonic plate actions taking place in the Earth. boundaries (e.g., 1993 Latur (India) earthquake); these But, when the rocky material along the interface of the are called Intra-plate Earthquakes. Here, a tectonic plate plates in the Earth’s Crust reaches its strength, it breaks in between. In both types of earthquakes, the fractures and a sudden movement takes place there slip generated at the fault during earthquakes is along (Figure 5); the interface between the plates where the both vertical and horizontal directions (called Dip Slip) movement has taken place (called the fault) suddenly and lateral directions (called Strike Slip) (Figure 7), slips and releases the large elastic strain energy stored with one of them dominating sometimes. in the rocks at the interface. For example, the energy released during the 2001 Bhuj (India) earthquake is about 400 times (or more) that released by the 1945 Atom Bomb dropped on Hiroshima!! Dip Slip Faults Strike Slip Faults Stage A Stage B Slip Figure 5: Elastic Strain Build-Up Stage C and Brittle Rupture Figure 7: Type of Faults Reading Material Bolt,B.A., (1999), Earthquakes, Fourth Edition, W. H. Freeman and EQ Company, New York, USA EQ http://earthquake.usgs.gov/faq/ Cumulative Slip EQ C http://neic.usgs.gov/neis/general/handouts/ Slip general_seismicity.html A B http://www.fema.gov/kids/quake.htm A Authored by: C Time C.V.R.Murty (years) Indian Institute of Technology Kanpur Elastic Stress Energy Strength Build-Up Kanpur, India B Energy Sponsored by: Release Building Materials and Technology Promotion Council, New Delhi, India A C Time (years) This release is a property of IIT Kanpur and BMTPC New Delhi. It may be reproduced without changing its contents Figure 6: Elastic Rebound Theory and with due acknowledgement. Suggestions/comments may be sent to: [email protected]. Visit www.nicee.org or www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips. 2  Learning 2 Earthquake Design Earthquake Tip and Construction How the ground shakes? Seismic Waves Large strain energy released during an earthquake P-Waves Push and pull travels as seismic waves in all directions through the Earth’s layers, reflecting and refracting at each interface. These waves are of two types - body waves and surface waves; the latter are restricted to near the Earth’s surface (Figure 1). Body waves consist of Extension Compression Primary Waves (P-waves) and Secondary Waves (S- S-Waves waves), and surface waves consist of Love waves and Up and down Rayleigh waves. Under P-waves, material particles undergo extensional and compressional strains along direction of energy transmission, but under S-waves, oscillate at right angles to it (Figure 2). Love waves cause surface motions similar to that by S-waves, but Side to side with no vertical component. Rayleigh wave makes a Direction of material particle oscillate in an elliptic path in the Energy Transmission vertical plane (with horizontal motion along direction of energy transmission). Love Waves Sideways in horizontal plane Structure Surface Waves Rayleigh Waves Elliptic in vertical plane Soil Body Waves Fault EQ Rupture Geologic Strata Figure 2: Motions caused by Body and Surface Waves (Adapted from FEMA 99, Non-Technical Figure 1: Arrival of Seismic Waves at a Site Explanation of the NEHRP Recommended Provisions) P-waves are fastest, followed in sequence by S-, Love and Rayleigh waves. For example, in granites, P- Measuring Instruments and S-waves have speeds ~4.8 km/sec and The instrument that measures earthquake shaking, ~3.0km/sec, respectively. S-waves do not travel a seismograph, has three components – the sensor, the through liquids. S-waves in association with effects of recorder and the timer. The principle on which it works Love waves cause maximum damage to structures by is simple and is explicitly reflected in the early their racking motion on the surface in both vertical seismograph (Figure 3) – a pen attached at the tip of an and horizontal directions. When P- and S-waves reach oscillating simple pendulum (a mass hung by a string the Earth's surface, most of their energy is reflected from a support) marks on a chart paper that is held on back. Some of this energy is returned back to the a drum rotating at a constant speed. A magnet around surface by reflections at different layers of soil and the string provides required damping to control the rock. Shaking is more severe (about twice as much) at amplitude of oscillations. The pendulum mass, string, the Earth's surface than at substantial depths. This is magnet and support together constitute the sensor; the often the basis for designing structures buried drum, pen and chart paper constitute the recorder; and underground for smaller levels of acceleration than the motor that rotates the drum at constant speed those above the ground. forms the timer. 3 IITK-BMTPC Earthquake Tip 2 How the ground shakes? page 2 local soil (Figure 1). They carry distinct information regarding ground shaking; peak amplitude, duration of strong shaking, frequency content (e.g., amplitude of String shaking associated with each frequency) and energy Magnet content (i.e., energy carried by ground shaking at each frequency) are often used to distinguish them. Pendulum Bob Peak amplitude (peak ground acceleration, PGA) is Pen physically intuitive. For instance, a horizontal PGA Rotating value of 0.6g (= 0.6 times the acceleration due to Support Drum gravity) suggests that the movement of the ground can cause a maximum horizontal force on a rigid structure equal to 60% of its weight. In a rigid structure, all Chart Paper Direction of points in it move with the ground by the same Ground Shaking Recorded amount, and hence experience the same maximum acceleration of PGA. Horizontal PGA values greater than 1.0g were recorded during the 1994 Northridge Figure 3: Schematic of Early Seismograph Earthquake in USA. Usually, strong ground motions One such instrument is required in each of the two carry significant energy associated with shaking of orthogonal horizontal directions. Of course, for frequencies in the range 0.03-30Hz (i.e., cycles per sec). measuring vertical oscillations, the string pendulum 1985 Mexico Earthquake (SCT 1A; N90E) (Figure 3) is replaced with a spring pendulum oscillating about a fulcrum. Some instruments do not 1940 Imperial Valley Earthquake (El Centro; S00E) have a timer device (i.e., the drum holding the chart paper does not rotate). Such instruments provide only 1971 San Fernando Earthquake (Pacoima Dam; N76W) the maximum extent (or scope) of motion during the earthquake; for this reason they are called seismoscopes. 0 10 20 30 40 50 60 Time (sec) The analog instruments have evolved over time, but today, digital instruments using modern computer 0.5g 1991 Uttarkashi Earthquake (Uttarkashi, N75E) technology are more commonly used. The digital instrument records the ground motion on the memory Figure 4:: of the microprocessor that is in-built in the instrument. Some typical recorded accelerograms Strong Ground Motions Shaking of ground on the Earth’s surface is a net Generally, the maximum amplitudes of horizontal consequence of motions caused by seismic waves motions in the two orthogonal directions are about the generated by energy release at each material point same. However, the maximum amplitude in the within the three-dimensional volume that ruptures at vertical direction is usually less than that in the the fault. These waves arrive at various instants of horizontal direction. In design codes, the vertical time, have different amplitudes and carry different design acceleration is taken as 1 2 to 2 3 of the levels of energy. Thus, the motion at any site on ground is random in nature with its amplitude and horizontal design acceleration. In contrast, the direction varying randomly with time. maximum horizontal and vertical ground accelerations Large earthquakes at great distances can produce in the vicinity of the fault rupture do not seem to have weak motions that may not damage structures or even such a correlation. be felt by humans. But, sensitive instruments can Reading Material record these. This makes it possible to locate distant Bolt,B.A., (1999), Earthquakes, Fourth Edition, W. H. Freeman and earthquakes. However, from engineering viewpoint, Company, New York, USA strong motions that can possibly damage structures are of interest. This can happen with earthquakes in Authored by: the vicinity or even with large earthquakes at C.V.R.Murty reasonable medium to large distances. Indian Institute of Technology Kanpur Characteristics of Strong Ground Motions Kanpur, India The motion of the ground can be described in Sponsored by: terms of displacement, velocity or acceleration. The Building Materials and Technology Promotion variation of ground acceleration with time recorded at Council, New Delhi, India a point on ground during an earthquake is called an accelerogram. The nature of accelerograms may vary This release is a property of IIT Kanpur and BMTPC New Delhi. It may be reproduced without changing its contents (Figure 4) depending on energy released at source, and with due acknowledgement. Suggestions/comments type of slip at fault rupture, geology along the travel may be sent to: [email protected]. Visit www.nicee.org or path from fault rupture to the Earth’s surface, and www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips. 4  Learning 3 Earthquake Design Earthquake Tip and Construction What are Magnitude and Intensity? Terminology released goes into heat and fracturing the rocks, and The point on the fault where slip starts is the Focus only a small fraction of it (fortunately) goes into the or Hypocenter, and the point vertically above this on seismic waves that travel to large distances causing the surface of the Earth is the Epicenter (Figure 1). The shaking of the ground en-route and hence damage to depth of focus from the epicenter, called as Focal Depth, structures. (Did you know? The energy released by a is an important parameter in determining the M6.3 earthquake is equivalent to that released by the damaging potential of an earthquake. Most of the 1945 Atom Bomb dropped on Hiroshima!!) damaging earthquakes have shallow focus with focal Earthquakes are often classified into different depths less than about 70km. Distance from epicenter groups based on their size (Table 1). Annual average to any point of interest is called epicentral distance. number of earthquakes across the Earth in each of these groups is also shown in the table; it indicates that Epicentral Distance on an average one Great Earthquake occurs each year. Epicenter Place of Table 1: Global occurrence of earthquakes Interest Group Magnitude Annual Average Number Great 8 and higher 1 Focal Major 7 – 7.9 18 Fault Depth Rupture Strong 6 – 6.9 120 Moderate 5 – 5.9 800 Light 4 – 4.9 6,200 (estimated) Focus Minor 3 – 3.9 49,000 (estimated) Very Minor < 3.0 M2-3: ~1,000/day; M1-2: ~8,000/day Source: http::/neic.usgs.gov/neis/eqlists/eqstats.html Figure 1: Basic terminology Intensity A number of smaller size earthquakes take place Intensity is a qualitative measure of the actual before and after a big earthquake (i.e., the Main Shock). shaking at a location during an earthquake, and is Those occurring before the big one are called assigned as Roman Capital Numerals. There are many Foreshocks, and the ones after are called Aftershocks. intensity scales. Two commonly used ones are the Modified Mercalli Intensity (MMI) Scale and the MSK Magnitude Scale. Both scales are quite similar and range from I Magnitude is a quantitative measure of the actual (least perceptive) to XII (most severe). The intensity size of the earthquake. Professor Charles Richter scales are based on three features of shaking – noticed that (a) at the same distance, seismograms perception by people and animals, performance of (records of earthquake ground vibration) of larger buildings, and changes to natural surroundings. Table earthquakes have bigger wave amplitude than those of 2 gives the description of Intensity VIII on MSK Scale. smaller earthquakes; and (b) for a given earthquake, The distribution of intensity at different places seismograms at farther distances have smaller wave during an earthquake is shown graphically using amplitude than those at close distances. These isoseismals, lines joining places with equal seismic prompted him to propose the now commonly used intensity (Figure 2). magnitude scale, the Richter Scale. It is obtained from the seismograms and accounts for the dependence of waveform amplitude on epicentral distance. This scale is also called Local Magnitude scale. There are other magnitude scales, like the Body Wave Magnitude, Surface Wave Magnitude and Wave Energy Magnitude. X These numerical magnitude scales have no upper and IX lower limits; the magnitude of a very small earthquake can be zero or even negative. VIII An increase in magnitude (M) by 1.0 implies 10 times higher waveform amplitude and about 31 times VII higher energy released. For instance, energy released Figure 2: Isoseismal Map of the 2001 Bhuj (India) in a M7.7 earthquake is about 31 times that released in Earthquake (MSK Intensity) a M6.7 earthquake, and is about 1000 (≈31×31) times Source: that released in a M5.7 earthquake. Most of the energy http::/www.nicee.org/nicee/EQReports/Bhuj/isoseismal.html 5 IITK-BMTPC Earthquake Tip 3 What are Magnitude and Intensity? page 2 Table 2: Description of shaking intensity VIII as per enclosed by the isoseismal VIII (Figure 2) may have MSK scale experienced a PGA of about 0.25-0.30g. However, now Intensity VIII - Destruction of Buildings strong ground motion records from seismic (a) Fright and panic. Also, persons driving motorcars are instruments are relied upon to quantify destructive disturbed. Here and there branches of trees break off. Even ground shaking. These are critical for cost-effective heavy furniture moves and partly overturns. Hanging lamps are damaged in part. earthquake-resistant design. (b) Most buildings of Type C suffer damage of Grade 2, and Table 3: PGAs during shaking of different intensities few of Grade 3. Most buildings of Type B suffer damage of MMI V VI VII VIII IX X Grade 3, and most buildings of Type A suffer damage of Grade 4. Occasional breaking of pipe seams occurs. PGA 0.03-0.04 0.06-0.07 0.10-0.15 0.25-0.30 0.50-0.55 >0.60 Memorials and monuments move and twist. Tombstones (g) overturn. Stonewalls collapse. Source: B.A.Bolt, Earthquakes, W.H.Freeman and Co., New York, 1993 (c) Small landslips occur in hollows and on banked roads on Based on data from past earthquakes, scientists steep slopes; cracks develop in ground up to widths of several centimeters. Water in lakes becomes turbid. New Gutenberg and Richter in 1956 provided an reservoirs come into existence. Dry wells refill and existing approximate correlation between the Local Magnitude wells become dry. In many cases, changes in flow and level ML of an earthquake with the intensity I0 sustained in of water are observed. the epicentral area as: ML ≈ 2 3 I0 + 1. (For using this Note: Type A structures - rural constructions; Type B - ordinary equation, the Roman numbers of intensity are replaced masonry constructions; Type C - Well-built structures with the corresponding Arabic numerals, e.g., intensity Single, Few – about 5%; Many – about 50%; Most – about 75% IX with 9.0). There are several different relations Grade 1 Damage – Slight damage; Grade 2 – Moderate damage; Grade 3 – Heavy damage; Grade 4 – Destruction; proposed by other scientists. Grade 5 – Total damage Basic Difference: Magnitude versus Intensity 100 Watt Bulb Magnitude of an earthquake is a measure of its size. For instance, one can measure the size of an earthquake by the amount of strain energy released by the fault rupture. This means that the magnitude of the earthquake is a single value for a given earthquake. On Near the other hand, intensity is an indicator of the severity of shaking generated at a given location. Clearly, the Bright severity of shaking is much higher near the epicenter (100 lumens) than farther away. Thus, during the same earthquake of a certain magnitude, different locations experience different levels of intensity. Normal To elaborate this distinction, consider the analogy (50 lumens) of an electric bulb (Figure 3). The illumination at a Far location near a 100-Watt bulb is higher than that farther away from it. While the bulb releases 100 Watts Dull of energy, the intensity of light (or illumination, (20 lumens) measured in lumens) at a location depends on the Figure 3: Reducing illumination with distance wattage of the bulb and its distance from the bulb. from an electric bulb Here, the size of the bulb (100-Watt) is like the magnitude of an earthquake, and the illumination at a Reading Material location like the intensity of shaking at that location. Richter,C.F., (1958), Elementary Seismology, W. H. Freeman and Company Inc, USA (Indian Reprint in 1969 by Eurasia Publishing Magnitude and Intensity in Seismic Design House Private Limited, New Delhi) One often asks: Can my building withstand a http://neic.usgs.gov/neis/general/handouts/magnitude_intensity. magnitude 7.0 earthquake? But, the M7.0 earthquake html causes different shaking intensities at different locations, and the damage induced in buildings at Authored by: these locations is different. Thus, indeed it is particular C.V.R.Murty levels of intensity of shaking that buildings and Indian Institute of Technology Kanpur structures are designed to resist, and not so much the Kanpur, India magnitude. The peak ground acceleration (PGA), i.e., Sponsored by: maximum acceleration experienced by the ground Building Materials and Technology Promotion during shaking, is one way of quantifying the severity Council, New Delhi, India of the ground shaking. Approximate empirical This release is a property of IIT Kanpur and BMTPC New correlations are available between the MM intensities Delhi. It may be reproduced without changing its contents and with due acknowledgement. Suggestions/comments and the PGA that may be experienced (e.g., Table 3). may be sent to: [email protected]. Visit www.nicee.org or For instance, during the 2001 Bhuj earthquake, the area www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips. 6  Learning 4 Earthquake Design Earthquake Tip and Construction Where are the Seismic Zones in India? Basic Geography and Tectonic Features across the central part of peninsular India leaving India lies at the northwestern end of the Indo- layers of basalt rock. Coastal areas like Kachchh show Australian Plate, which encompasses India, Australia, a marine deposits testifying to submergence under the major portion of the Indian Ocean and other smaller sea millions of years ago. countries. This plate is colliding against the huge Prominent Past Earthquakes in India Eurasian Plate (Figure 1) and going under the Eurasian A number of significant earthquakes occurred in Plate; this process of one tectonic plate getting under and around India over the past century (Figure 2). another is called subduction. A sea, Tethys, separated Some of these occurred in populated and urbanized these plates before they collided. Part of the areas and hence caused great damage. Many went lithosphere, the Earth’s Crust, is covered by oceans unnoticed, as they occurred deep under the Earth’s and the rest by the continents. The former can undergo surface or in relatively un-inhabited places. Some of subduction at great depths when it converges against the damaging and recent earthquakes are listed in another plate, but the latter is buoyant and so tends to Table 1. Most earthquakes occur along the Himalayan remain close to the surface. When continents converge, plate boundary (these are inter-plate earthquakes), but large amounts of shortening and thickening takes a number of earthquakes have also occurred in the place, like at the Himalayas and the Tibet. peninsular region (these are intra-plate earthquakes). Eurasian Plate Himalayas Indo- Gangetic Plains Narmada Plains Mahanadi Plains Deccan Godavari Shield Plains