Earthquakes An earthquake is the shaking and vibration of the Earth’s crust due to plate tectonics (movement of plates) Earthquakes can happen along any type of plate boundary . They also occur along faults which are large cracks in the earth’s crust. Most faults are associated with large plate boundaries where violent earthquakes usually occur.
Causes of Earthquakes Earthquakes are caused when the tension is released from inside the crust. This happens because plates do not move smoothly - sometimes they get stuck. When this happens a great deal of pressure builds up. Eventually this pressure is released and an earthquake tends to occur. An earthquake starts deep below the earth’s surface at the focus . The focus is the point inside the Earth's crust where the pressure is released. The epicentre of an earthquake is the position on the earth’s surface directly above its focus.
EARTH INTERIOR ➢ Inner Core (radius ~1290km), ➢ the Outer Core (thickness ~2200km), ➢ the Mantle (thickness ~2900km) ➢ The Crust (thickness ~5 to 40km).
➢ The Inner Core is solid and consists of heavy metals (e.g., nickel and iron) ➢ The Outer Core is liquid in form. ➢ The Mantle has the ability to flow ➢ The Crust consists of light materials (e.g., basalts and granites). ➢ At the Core, the 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 surface of the Earth.
Plate Tectonics ➢ Earth consists of seven major tectonic plates.
➢ Convergent ➢ Divergent ➢ Transform boundaries
1. Convergent boundaries : where two plates are colliding. Subduction zones occur when one or both of the tectonic plates are composed of oceanic crust. The denser plate is subducted underneath the less dense plate. The plate being forced under is eventually melted and destroyed. i. Where oceanic crust meets ocean crust Island arcs and oceanic trenches occur when both of the plates are made of oceanic crust. Zones of active seafloor spreading can also occur behind the island arc, known as back-arc basins. These are often associated with submarine volcanoes. ii. Where oceanic crust meets continental crust The denser oceanic plate is subducted, often forming a mountain range on the continent. The Andes is an example of this type of collision. iii. Where continental crust meets continental crust Both continental crusts are too light to subduct so a continent-continent collision occurs, creating especially large mountain ranges. The most spectacular example of this is the Himalayas. 2. Divergent boundaries – where two plates are moving apart. The space created can also fill with new crustal material sourced from molten magma that forms below. Divergent boundaries can form within continents but will eventually open up and become ocean basins. i. On land Divergent boundaries within continents initially produce rifts, which produce rift valleys. ii. Under the sea The most active divergent plate boundaries are between oceanic plates and are often called mid- oceanic ridges. 3. Transform boundaries – where plates slide passed each other. The relative motion of the plates is horizontal. They can occur underwater or on land, and crust is neither destroyed nor created. Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it exceeds the threshold of the rocks, the energy is released – causing earthquakes.
Elastic Rebound – deformed rock goes back to its original shape – Rocks bends until the strength of the rock is exceeded – Rupture occurs and the rocks quickly rebound to an undeformed shape – Energy is released in waves that radiate outward from the fault This release of energy is expected to cause earthquake. When earthquake happens, slip takes place resulting in changes in positions.
The jerking movement caused by plates sticking then moving releases built-up pressure inside the Earth's crust, in the form of seismic waves . The waves spread out from the focus . The strongest waves are found near the centre of the earthquake. This means that the most severe damage caused by an earthquake will happen close to the epicentre . It is almost impossible to predict when they will occur. The effect of an earthquake depends on the depth of an earthquake as well as its magnitude. If the focus is very deep or the shockwaves have to travel through dense rock, the effect will be less.
Measuring Earthquakes The power (magnitude) of an earthquake is measured on the Richter scale , using an instrument called a seismometer.
The Richter scale is numbered 0-10 with 10 being the most powerful. The Richter scale is logarithmic – an earthquake measuring 7 is 10 times more powerful than one measuring 6 and 100 times more powerful than one measuring 5. Up until 2 on the Richter Scale only instruments will detect the earthquake. Earthquakes above 6 cause serious damage and sometimes many deaths
How are Earthquakes Measured?
The Mercalli scale measures the damage caused by an earthquake. The Mercalli scale goes from I to XII e.g. “ VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage.”
Effects of Earthquakes Primary effects occur immediately, and are all due to the shaking of the ground e.g. buildings collapsing, destruction of roads and bridges. Secondary effects happen afterwards, but can be even more devastating e.g. fire, tidal waves and disease and landslides
SEISMIC WAVES ➢ There are two types of waves, namely, – Body waves : Primary and Secondary waves – Surface waves : Raleigh and Love waves
Primary Waves (P Waves) A type of seismic wave that compresses and expands the ground The first wave to arrive at an earthquake
Secondary Waves (S Waves) A type of seismic wave that moves the ground up and down or side to side
Comparing Seismic Waves
Surface waves ➢ Move along the Earth’s surface ➢ Produces motion in the upper crust Motion can be up and down Motion can be around Motion can be back and forth ➢ Travel more slowly than S and P waves ➢ More destructive
Three Types of Faults Strike-Slip Thrust Normal
TYPICAL EARTHQUAKE GROUND MOTION ➢ It is a graph of ground motion such as acceleration, velocity or displacement of ground with time during earthquake. ➢ As P waves travel faster, they arrive early at a location. ➢ S waves and body waves arrive late and will create more violent shaking. ➢ Surface waves are generated at the surface and are most violent at this instant. This period of violent shaking is called strong motion. It is during this period, most damages take place.
➢ Hence, for a civil engineer, strong motion part of ground motion is very important as any damage should take place within this period ➢ No two earthquake ground motions are similar. Even, the ground motions at two different places under the same earthquake are different.
INSTRUMENTS FOR SEISMIC MEASUREMENTS Typical seismic instrument consists of a ➢ three directional sensor, ➢ GPS ➢ memory unit and ➢ battery backup. A seismogram is a graph of wave amplitude Vs. Time. In old seismographs, a pen drew the recording on a piece of paper. In new seismographs, the signal is recorded digitally.
Accelerographs are generally used by civil engineers. They are not sensitive, but can record very accurately the shaking parameters at a site. The graph of ground motion versus time is called accelerogram.
How Seismographs Work
CLASSIFICATION OF EARTHQUAKES Earthquakes can be broadly classified in to following subclasses. 1. Based on Focal Depth 2. Based on magnitude 3. Based on origin 4. Based on location 5. Based on Epicentral distance
1. Based on Focal Depth ➢ Shallow Focus earthquakes (<70 km) ➢ Intermediate focus earthquakes (70 to 300 km) ➢ Deep focus earthquakes (> 300 km) 2. Based on magnitude ➢ Micro earthquakes (M < 3) ➢ Intermediate earthquakes (M 3 to 5) ➢ Moderate earthquakes (M 5 to 6) ➢ Strong earthquakes (M 6 to 7) ➢ Major earthquakes (M 7 to 8) ➢ Great earthquakes (M > 8)
3. Based on origin ➢ Tectonic earthquakes ➢ Plutonic earthquakes ➢ Explosions ➢ Collapse earthquakes ➢ Volcanic earthquakes ➢ Reservoir induced earthquakes 4. Based on location ➢ Inter-plate earthquakes Intra-plate earthquakes ➢ Convergent boundaries ➢ Dip slip fault ➢ Strike slip fault ➢ Divergent boundaries ➢ Transform plane boundaries
5. Based on Epicentral distance ➢ Local shock (4 km range) ➢ Near shock (4 to 10 km range) ➢ Distant shock (10 to 20 km range) ➢ Telescopic shock (> 20 km range)
SEISMIC ZONING MAP OF INDIA ➢ India is seismically active and has experienced many earthquakes in the past ➢ More than 60 % of the country is considered to be in seismically active regions. ➢ the country is divided in to 4 zones - Zone 2 to Zone 5. ➢ Zone 2 is seismically least active and ➢ zone 5 is seismically most active. ➢ IS-1893 prescribes the earthquake resistance design
SEISMIC ZONATION OF INDIA
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