Basic of seismic engineering
- 1. BASICS OF EARTHQUAKE By : Asst. Prof. M.M.Parekh Civil Engineering Department 0 AITS-Civil 1
- 2. Interior of Earth 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). The Inner Core is solid and consists of heavy metals while the Crust consists of light materials (e.g., basalts and granites). The Outer Core is liquid in form and the Mantle has the ability to flow. 0 AITS-Civil 2
- 3. Plate Tectonics 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 takes place in pieces called Tectonic Plates. The surface of the Earth consists of seven major tectonic plates 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 another (and rifts are created). In another case, two 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 boundaries 0 AITS-Civil 3
- 4. How Earthquake Strikes Tectonic plates are made of elastic but brittle rocky material. And so, elastic strain energy is stored in them during the relative deformations that occur due to the gigantic tectonic plate actions taking place in the Earth. But, when the rocky material along the interface of the plates in the Earth’s Crust reaches its strength, it fractures and a sudden movement takes place there . the interface between the plates where the movement has taken place (called the fault) suddenly slips and releases the large elastic strain energy. 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!! The sudden slip at the fault causes the earthquake… 0 AITS-Civil 4
- 5. Types of Earthquakes and Faults Most earthquakes in the world occur along the boundaries of the tectonic plates as described above and are called Inter-plate Earthquakes (e.g., 1897 Assam (India) earthquake). A number of earthquakes also occur within the plate itself but away from the plate boundaries these are called Intra-plate Earthquakes. Here, a tectonic plate breaks in between. In both types of earthquakes, the slip generated at the fault during earthquakes is along both vertical and horizontal directions (called Dip Slip) and lateral directions (called Strike Slip) with one of them dominating sometimes. 0 AITS-Civil 5
- 6. Seismic Waves Large strain energy released during an earthquake 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. Body waves consist of Primary Waves (P-waves) and Secondary Waves. Surface waves consist of Love waves and Rayleigh waves. 0 AITS-Civil 6
- 7. Terminology The point on the fault where slip starts is the Focus or Hypocenter, and the point vertically above this on the surface of the Earth is the Epicenter. The depth of focus from the epicenter, called as Focal Depth, is an important parameter in determining the damaging potential of an earthquake. Most of the damaging earthquakes have shallow focus with focal depths less than about 70km. Distance from epicenter to any point of interest is called epicentral distance. A number of smaller size earthquakes take place before and after a big earthquake (i.e., the Main Shock). Those occurring before the big one are called Foreshocks, and the ones after are called Aftershocks. 0 AITS-Civil 7
- 8. Magnitude Magnitude is a quantitative measure of the actual size of the earthquake. Professor Charles Richter noticed that , (a) at the same distance, seismograms of larger earthquakes have bigger wave amplitude than those of smaller earthquakes; and (b) for a given earthquake, seismograms at farther distances have smaller wave amplitude than those at close distances. These prompted him to propose the now commonly used magnitude scale, the Richter Scale. An increase in magnitude (M) by 1.0 implies 10 times higher waveform amplitude and about 31 times higher energy released. For instance, energy released in a M7.7 earthquake is about 31 times that released in a M6.7 earthquake, and is about 1000 (≈31×31) times that released in a M5.7 earthquake.0 AITS-Civil 8
- 9. Basic Difference: Magnitude versus Intensity 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 the other hand, intensity is an indicator of the severity of shaking generated at a given location. Clearly, the severity of shaking is much higher near the epicenter than farther away. Thus, during the same earthquake of a certain magnitude, different locations experience different levels of intensity. Here, the size of the bulb (100-Watt) is like the magnitude of an earthquake, and the illumination at a location like the intensity of shaking at that location. 0 AITS-Civil 9
- 10. Magnitude and Intensity in Seismic Design One often asks: Can my building withstand a magnitude 7.0 earthquake? But, the M7.0 earthquake causes different shaking intensities at different locations, and the damage induced in buildings at these locations is different. Thus, indeed it is particular levels of intensity of shaking that buildings and structures are designed to resist, and not so much the magnitude. The peak ground acceleration (PGA), i.e., maximum acceleration experienced by the ground during shaking, is one way of quantifying the severity of the ground shaking. 0 AITS-Civil 10
- 11. Seismic Zones of India The varying geology at different locations in the country implies that the likelihood of damaging earthquakes taking place at different locations is different. Thus, a seismic zone map is required to identify these regions. Based on the levels of intensities sustained during damaging past earthquakes, the 1970 version of the zone map subdivided India into five zones – I, II, III, IV and V 0 AITS-Civil 11
- 12. Inertia Forces in Structures 0 AITS-Civil 12
- 13. End of Part – 1 To be Continued… 0 AITS-Civil 13