EARTHQUAKE

An earthquake is what happens when two blocks of earth suddenly slip past one another. The surface where they slip is called the fault or fault plane. The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter.

Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. The largest, main earthquake is called the mainshock. Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock.

The earth has four major layers: the inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet.

But this skin is not all in one piece – it is made up of many pieces like a puzzle covering the surface of the earth. Not only that, but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle pieces tectonic plates, and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake. The tectonic plates divide the Earth's crust into distinct "plates" that are always slowly moving. Earthquakes are concentrated along these plate boundaries.

While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us!

Earthquakes are recorded by instruments called seismographs.

The recording they make is called a seismogram. The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead, the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.

The seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was. A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.

Seismograms come in handy for locating earthquakes too, and being able to see the P wave and the S wave is important. P waves are also faster than S waves, and this fact is what allows us to tell where an earthquake was.

By looking at the amount of time between the P and S wave on a seismogram recorded on a seismograph, scientists can tell how far away the earthquake was from that location.

Scientists then use a method called triangulation to determine exactly where the earthquake was.

It is called triangulation because a triangle has three sides, and it takes three seismographs to locate an earthquake. If you draw a circle on a map around three different seismographs where the radius of each is the distance from that station to the earthquake, the intersection of those three circles is the epicenter!

Ductility - It is the capacity of a structure or its members to undergo large inelastic deformations without significant loss of strength or stiffness.

Response Spectrum - It is the representation of maximum responses of a spectrum of idealized single degree freedom systems of different natural periods but having the same damping, under the action of the same earthquake ground motion at their bases. The response referred to here can be maximum relative velocity, or maximum relative displacement.

Seismic Mass of a Structure- It is the seismic weight of a structure divided by acceleration due to gravity.

Time History Analysis - It is an analysis of the dynamic response of the structure at each instant of time, when its base is subjected to a specific ground motion time history.

Design Seismic Base Share - It is the horizontal lateral force in the considered direction of earthquake shaking that the structure shall be designed for.

Lateral Force Resisting System: It is part of structural system and consists of all structural members that resist lateral inertia forces induced in the building during earthquake shaking.

Weak Storey - It is one in which the Storey lateral strength is less than that in the Storey above.

Storey Drift: It is the relative displacement between the floors above and/or below the Storey under consideration.

Ground Motion - The characteristics (intensity, duration, frequency content, etc.) of seismic ground vibrations expected at any site depend on magnitude of earthquake, its focal depth, epicentral distance, characteristics of the path through which the seismic waves travel, and soil strata on which the structure is founded.

Design of buildings as per earthquake requirements is important. This is achieved by implementing the standards and following regulations.

Thank you for devoting time??

Refer & Source SP7, USGS

Regards Er. P. D. Sathe? ? ? Happy Learning????????