Why and what earthquake is? Many people living around the world have at one time or another experienced an earthquake. Most earthquakes occur as the result of slowly accumulating pressure that causes the ground to slip abruptly along a geological fault plane on or near a plate boundary. The resulting waves of vibration within the earth create ground motion at the surface that vibrates in a very complex manner. In other words earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. They don’t just slide smoothly; the rocks catch on each other. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that’s built up. When the rocks break, the earthquake occurs. For example this is recent Chile’s earthquake explaination
During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter of the earthquake.
You can try this little experiment:
Break a block of foam rubber in half.
Put the pieces on a smooth table.
Put the rough edges of the foam rubber pieces together.
While pushing the two pieces together lightly, push one piece away from you along the table top while pulling the other piece toward you. See how they stick?
Keep pushing and pulling smoothly.
Soon a little bit of foam rubber along the crack (the fault) will break and the two pieces will suddenly slip past each other. That sudden breaking of the foam rubber is the earthquake. That’s just what happens along a strike-slip fault.
Earthquake-like seismic waves can also be caused by explosions underground. These explosions may be set off to break rock while making tunnels for roads, railroads, subways, or mines. These explosions, however, don’t cause very strong seismic waves. You may not even feel them. Sometimes seismic waves occur when the roof or walls of a mine collapse. These can sometimes be felt by people near the mine. The largest underground explosions, from tests of nuclear warheads (bombs), can create seismic waves very much like large earthquakes. This fact has been exploited as a means to enforce the global nuclear test ban, because no nuclear warhead can be detonated on earth without producing such seismic waves.
No wonder that creating airplanes man watched birds trying to find the secret of their fly. As the wings move through the air (blue lines), the special airfoil shape of the wings causes the air pressure above the wings to be lower than the pressure underneath. The difference in pressure is lift, a force that acts roughly perpendicular to the wing surface and keeps the bird from falling.
Flapping flight uses the same principle, but the movement of the wings is more complicated. There are three important motions in addition to the bird’s forward motion:
By flapping its wings down, together with the forward motion of the body, a bird can tilt the lift of its wings forward for propulsion. Why don’t birds simply move their wings up and down, without twisting and folding? Notice that the outer part of the wing moves down much farther than the inner part close to the body. Twisting allows each part of the wing to keep the necessary angle relative to the airflow. If part of the wing is angled lower than the airflow, there might not be enough lift. If part of the wing is pointed too high, there could be a lot of drag. The wings are flexible, so they twist automatically.
Wing folding isn’t essential – ornithopters fly without it – but it helps birds fly with less effort. To see why it is helpful, think about what happens during the upstroke. Because the wing is going up, the lift vector points backward, especially in the outer portion of the wing. The upstroke actually slows the bird down! By folding its wings (decreasing the wingspan) a bird can reduce drag during the upstroke.
In addition to the three basic movements described here, birds can do a lot of other things with their wings to allow them to maneuver in the air. Instead of using their tails for flight control, they move their wings forward and backward for balance. To make a turn, they can twist the wings or apply more power on one side. For slow flight, birds can flap their wings almost forward and backward instead of vertically; the upstroke and downstroke produce lift without forward body motion.
Since flapping wings are subject to unsteady flows – they not only move but accelerate through the air – they can produce more lift than fixed wings and are resistant to stalling.