An earthquake is the vibration of Earthproduced by the rapid release ofaccumulated energy in elastically strainedrocks Energy released radiates in all directions from its source, the focus . Energy propagates in the form of seismic waves . Sensitive instruments around the world record the event .
Crust: Continental crust (25-40 km) Oceanic crust (~6 km) Mantle Upper mantle (650 km) Lower mantle (2235 km) Core Outer core: liquid (2270 km) Inner core Values in brackets represent the approximate thickness of each layer
The theory of Plate tectonics was proposed in1960s based on the theory of continental drift. This is the Unifying theory that explains theformation and deformation of the Earth’s surface. According to this theory, continents are carriedalong on huge slabs (plates) on the Earth’s outermostlayer (Lithosphere). Earth’s outermost layer is divided into 12 majorTectonic Plates (~80 km deep). These plates moverelative to each other a few centimeters per year.
Divergent plate boundaries: where plates move apart Convergent Plate boundaries: where plates come together Transform plate boundaries: where plates slide past each other
Movement of Tectonic PlatesEarth is divided into sections called Tectonicplates that float on the fluid-like interior of the Earth. Earthquakes are usually caused by sudden movement of earth plates Rupture of rocks along a faultFaults are localized areas of weakness in the surface of the Earth, sometimes the plate boundary itself
The point withinEarth where faulting begins is the focus, or hypocenter The point directly above the focus on the surface is the epicenter
Explains how energy is stored in rocks: Rocks bend 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.
When a earthquakeoccurs, two types of seismicwaves are generated:- Body waves, that travelthrough the interior of theearth. They are of two types- P-wave- primary waves or push-pull waves or compression waves. S-waves- shear waves, secondary waves or traverse waves. Surface waves, that travelalong the surface layers of theearth. They are of two types- Love wavesRayleigh waves
• INTENSITY • MAGNITUDE Qualitative measure of an Quantitative measure of the earthquake which describes the earthquake which is related to effect of an earthquake at a the amount of energy released particular location. to cause the earthquake. Isoseismal lines identify areas of It is measured by Richter scale equal intensity. and Seismograph. It is measured by modified- Magnitude is same for a mercalli intensity scale. particular earthquake. Intensity changes with distance from epicenter of earthquake.
PEAK GROUND MOTION DURATION OF GROUND MOTION FREQUENCY CONTENT
They are the instrumentsused to record strong groundmotion i.e. motion largeenough to be potentiallydamaging.These instrument recordacceleration of the ground asa function of time.Triaxial acceleographs areable to measure groundmotion in two perpendicularhorizontal directions and inthe vertical directions.
EARTHQUAKE MAGNITUDE EPICENTRAL DISTANCE LOCAL SOIL CONDITION CHARACTERISTIC OF LOCAL GEOLOGY ALONG THE PROPAGATION PATH SOURCE MECHANISM OF THE EARTHQUAKE
INTERNAL FORCES DUE TO LARGE PEAK GROUND ACCELERATIONS CAUSESSTRUCTURAL FAILURE, LANDSLIDES, RETAINING WALL FAILURE. LIQUEFACTION OF SATURATED SOIL CAUSES STRUCTURAL FAILURES, LARGELATERAL DISPLACEMENT OF SLOPES, INCREASED LATERAL PRESSURE ON RETAININGWALLEARTHQUAKE INDUCED FIRE CAUSES LOSS OF PROPERTYEARTHQUAKE INDUCED WATER WAVES KNOWN AS TSUNAMIS CAUSESUBMERGENCE OF SHORE AREAS CAUSING DEATH AND LOSS OF PROPERTYRARELY THERE MIGHT BE A PERMANENT CHANGE IN GROUND ELEVATION
Strange Animal Behavior stress in the rocks causes tiny hairline fractures to form, the cracking of the rocks evidently emits high pitched sounds and minute vibrations imperceptible to humans but noticeable by many animals. Foreshocks unusual increase in the frequency of small earthquakes before the main shock Changes in water level porosity increases or decreases with changes in strain Seismic Gaps based of the chronological distribution of major earthquakes
Prepare a Seismic Risk Map for the globe which identifies rocktypes, liquefaction potential, landslide potential.Extensive geologic surveying has to be done to identify all activefaults, including hidden faults.Earthquake Resistant Design of Structures Enact building codes to design and build earthquake-resistant structures in high seismic risk areas. wood, steel and reinforced concrete are preferred as they tend to move with the shaking ground (unreinforced concrete and heavy masonry tend to move independently and in opposition to the shaking, battering one another until the structure collapses)Critical facilities such as nuclear power plants and dams shouldbe built on stable ground and as far as possible from active faults.