Nilacharal

What is an EarthQuake?
What are Tectonic plates?
What are Tectonic Boundaries?
Indian Seismic Zones Map
What to do during an earthquake ?
EarthQuake Terminology

What is an EarthQuake?

EarthQuake is defined as a movement or trembling of the earth caused by a sudden release of stresses within the crust, usually less than 25 miles below the surface.The stresses on the crust are caused by the collision between tectonic plates. Earthquakes happen because of sudden slip on a fault, much like snapping your fingers. Before the snap, you push your fingers together and sideways. Because you are pushing them together, friction keeps them from moving to the side. When you push sideways hard enough to overcome this friction, your fingers move suddenly, releasing energy in the from of sound waves that then travels from your fingers to your ear.

What are Tectonic plates?

A tectonic plate (also called lithospheric plate) is a massive, irregularly shaped slab of solid rock. The earth's surface is broken into seven large and many small moving tectonic plates. These plates, each about 50 miles thick, move relative to one another an average of a few inches a year.The size and position of these plates change over time. The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building. The below mentioned diagram presents the seven tectonic plates and we come under the Indian plate which has convergent boundaries.

There are three main plate tectonic movements namely extensional at divergent boundaries, transform at convergent boundaries, and compressional at transform-fault boundaries.
Convergent boundaries : At convergent boundaries, plates move toward each other and collide. Where an oceanic plate collides with a continental plate, the oceanic plate tips down and slides beneath the continental plate forming a deep ocean trench (long, narrow, deep basin.) An example of this type of movement, called subduction, occurs at the boundary between the oceanic Nazca Plate and the continental South American Plate. Where continental plates collide, they form major mountain systems such as the Himalayas. At compressional boundaries, earthquakes are found in several settings ranging from the very near surface to several hundred kilometers depth, since the coldness of the subducting plate permits brittle failure down to as much as 700 km. Compressional boundaries host Earth's largest quakes, with some events on subduction zones in Alaska and Chile having exceeded magnitude 9.

Divergent boundaries : At divergent boundaries, plates move away from each other such as at the Mid-Atlantic Ridge. Where plates diverge, hot, molten rock rises and cools adding new material to the edges of the oceanic plates. This process is known as sea-floor spreading. At spreading ridges, or similar extensional boundaries, earthquakes are shallow, aligned strictly along the axis of spreading, and show an extensional mechanism. Earthquakes in extensional environments tend to be smaller than magnitude 8

Transform-fault boundaries : At transform-fault boundaries, plates move horizontally past each other. The San Andreas Fault zone is an example of this type of boundary where the Pacific Plate on which Los Angeles sits is moving slowly northwestward relative to the North American Plate on which San Francisco sits. At transforms, earthquakes are shallow, running as deep as 25 km; mechanisms indicate strike-slip motion. Transforms tend to have earthquakes smaller than magnitude 8.5.

1. If you are indoors, duck or drop down to the floor. Take cover under a sturdy desk, table or other furniture. Hold on to it and be prepared to move with it. Hold the position until the ground stops shaking and it is safe to move. Stay clear of windows, fireplaces, woodstoves, and heavy furniture or appliances that may fall over. Stay inside to avoid being injured by falling glass or building parts. If you are in a crowded area, take cover where you are. Stay calm and encourage others to do likewise.
2. If you are outside, get into the open, away from buildings and power lines.
3. If you are driving, stop if it is safe, but stay inside your car. Stay away from bridges, overpasses and tunnels. Move your car as far out of the normal traffic pattern as possible. If possible, avoid stopping under trees, light posts, power lines, or signs.
4. If you are in a mountainous area, or near unstable slopes or cliffs, be alert for falling rock and other debris that could be loosened by the earthquake.
5. If you are at the beach, move quickly to higher ground or several hundred yards inland.

Aftershocks :Large earthquakes hardly ever occur alone. When one earthquake happens, we usually see another at a nearby location. To talk about this phenomenon, seismologists coined three terms: foreshock, mainshock, and aftershock. In any cluster of earthquakes, the one with the largest magnitude is called the mainshock; anything before it is called a foreshock and anything after it is called an aftershock.

Crust :The outermost layer of the Earth is called the crust. The thinnest crust at the mid-ocean ridges is 3 to 5 miles thick and the thickest crust under the Himalayas and Tibet is as much as 60 miles thick.

Epicenter :The epicenter of an earthquake is the point on a fault where an earthquake starts.

Fault :Earthquakes always occur on faults. Faults are places in the earth where the rocks are broken and the rocks on one side have moved in some direction relative to the other. Faults are planes, not lines

Magnitude : Magnitude is the most commonly reported measure of an earthquake's size. It began as a completely empirical measure defined by Beno Gutenberg and Charles Richter in the 1930's. They wanted a quantitative way to compare earthquakes, based on instrumental recordings, independent of the location of the observer. They borrowed the idea of a magnitude scale from astronomers, who used it to classify the brightness of stars. They defined it in terms of the amplitude of ground velocity recorded on a particular seismograph, scaled by the distance from the instrument to the earthquake.

Richter Scale :The Richter scale is an idea, like the Fahrenheit scale, not an instrument. The term is sometimes used to describe the local magnitude scale, the first magnitude scale defined by Charles Richter, and sometimes used to mean any magnitude scale.

Seismograph :We record earthquakes through the use of seismographs. Seismographs are instruments that create an electrical signal when the ground moves. The ground can move and create a signal because a truck drives by, a sonic boom, wind pushing tree roots or an earthquake.