Unit II Earthquake Disaster Mitigation
EARTHQUAKES Source: Kobe I EERI - Slide #43
Earthquakes are a natural phenomena, like drought, flood and Cyclones. Earthquakes in simple terms is a sudden trembling or shaking movement of the earth surface. Larger earthquakes usually preceded by tremors and some violent shocks and followed by smaller earthquakes of diminishing size called aftershocks.
What are Earthquakes? A sudden release of energy accumulated in deformed rocks causing the ground to tremble or shake. - Causes rupturing or brittle failure of crustal rocks. - Energy is released. - Movement of fault blocks takes place along a fault plane. Source: www.earth.leeds.ac.uk/dynamicearth
Layers of the Earth By analyzing the seismograms from many earthquakes, scientists have discovered that three main levels or shells exist within the Earth: CRUST The Earth's outermost surface is called the crust. The crust is relatively light and brittle. Most earthquakes occur within the crust. Scientists believe that below the lithosphere is a relatively narrow, mobile zone in the mantle called the asthenosphere (from asthenes, Greek for weak). Source: www.thetech.org MANTLE The region just below the crust and extending all the way down to the Earth's core is called the mantle. The mantle, a dense, hot layer of semi-solid rock approximately 2,900 km thick. COREBeneath the mantle is the Earth's core. The Earth's core consists of a fluid outer core and a solid inner core.
BASIC TERMINOLOGY Earthquake Hypocentre or focus Epicentre Focal depth Epicentral distance Origin time Foreshocks and Aftershocks Fault
CAUSES OF EARTHQUAKES • Plate Tectonics and Elastic Rebound Theory • Earthquakes due to volcanic activity
SIZE OF EARTHQUAKES Earthquake Intensity • Oldest measure of earthquake size • Qualitative description • Depends on • Distance of the site from epicenter • focal depth • Magnitude of earthquake • Soil condition • The intensity is maximum near the epicentre and decreases with increase in distance from the epicentre.
INTENSITY Intensity is a qualitative measure of the actual shaking at a location during an earthquake and it is assigned in Roman Capital Letters There are many intensity scales. Two commonly used ones are 1) Modified Mercalli Intensity (MMI) Scale. 2) MSK Scale Both scales are quite similar and range from I(less perceptive) to XII (most severe) The intensity scales are based on three features of shaking - perception by people and animals - performance of buildings - changes to surroundings
Basic Difference : Magnitude versus Intensity Reducing illumination with distance from an electric bulb
Earthquake Magnitude • It is measured on Richter Scale and is related to the logarithm (base 10) of the amount of energy released by an earthquake. • The magnitude M of an earthquake is related to the energy released at the focus of the earthquake, and is given by the approximate formula Log E ( ergs) = 11. 8 + 1.5 M
The smallest earthquake perceptible by human being corresponds to the magnitude of 2 • largest and most destructive earthquake so far known to have occurred has been assigned a magnitude 8.7. • The damage from an earthquake starts from magnitude 5 and above. • Earthquakes are often classified into different groups based on their size
SEISMIC WAVES Seismic waves are of two types - Body waves - Surface waves Body waves consist of Primary waves (P-waves) and Secondary waves (S-waves) Surface waves consist of Love waves and Rayleigh waves
Types of Waves Fastest waves Do not travel through liquid
Seismic Hazards • Ground shaking • Structural hazards • Liquefaction • Landslides • Retaining structures failure • Lifeline hazards • Tsuanami
Classification of earthquakes • Based on location • Interplate • Intraplate • Based on epicentral distance • Local earthquakes < 1º • Regional earthquakes 1 - 10 º • Teleseismic earthquakes > 10º • Based on Focal depth • Shallow depth 0-71 km • Intermediate depth 71-300 km • Deep earthquake > 300km • Based on Magnitude • Micro earthquake < 3 • Intermediate earthquake 3-4.9 • Moderate earthquake 5-5.9 • Strong earthquake 6-6.9 • Major earthquake 7-7.9 • Great earthquake > 8
Indian Subcontinent: among the world’s most disaster prone areas 65% of land vulnerable to Earthquakes 8% of land vulnerable to Cyclones 5% of land vulnerable to Floods > 1 million houses damaged annually + human, social, other losses
Seismic Zone Map of India: Year-1962
Seismic Zone Map of India: -1966
Seismic Zone Map of India: -1970
Seismic Zone Map of India: -2002 About 65 percent of the land area of India is liable to seismic hazard damage (about 26% under MSK Intensity VII, 18% under VIII and 12% under IX and higher).
Earthquake Risk • Hazard = Probability of ground motion • Site effects = Soil properties, topography • presence of Reservoirs (RIS), • Mines (MIS) • Vulnerability = Building types, Age • Risk = Hazard x Site effects xVulnerability
Earthquake Don’t Kill People but Buildings Do
VULNERABILITY • 1819 Gujarat [Kutch] 8.0 (2000 deaths) • 2001 Gujarat [Bhuj] 6.9 (13805 deaths) • Increased vulnerability in two centuries
EFFECT OF SEISMIC FORCES ON STRUCTURES • Inertia Forces in Structures • From Newton’s First Law of Motion, even though the base of the building moves with the ground, the roof has a tendency to stay in its original position
If the roof has a mass M and experiences an acceleration a, then from inertia force = Ma (direction is opposite to acc.) Clearly, more mass means higher inertia force. Therefore, lighter buildings sustain the earthquake shaking better.
Stiffness forces • The inertia force experienced by the roof is transferred to the ground via the columns, causing forces in columns. • During earthquake shaking, the columns undergo relative movement between their ends • when forced to bend, they develop internal forces • The larger is the relative horizontal displacement u between the top and bottom of the column, the larger this internal force in columns. • Also, the stiffer the columns are (i.e., bigger is the column size), larger is this force
Flow of inertial forces to foundation Structural elements and connections must be designed to transfer the inertial forces through them