The size, or the magnitude, of an earthquake is recorded using an instrument called a seismometer. Using very heavy weights that remain still while the room it is in is shaking, the machine records the amount of movement on a rotating drum of paper. This type of record is measured on the Richter scale. The physical and visible effects of a quake are measured using the Vertical Modified Mercalli scale (see below).
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 size of an earthquake depends on the size of the fault and the amount of slip on the fault, but that’s not something scientists can simply measure with a measuring tape since faults are many kilometers deep beneath the earth’s surface. So how do they measure an earthquake? They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was (figure 5). 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.
The size of the earthquake is called its magnitude. There is one magnitude for each earthquake. Scientists also talk about theintensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.
The Modified Mercalli scale:
1 Only detected by instruments. Doors begin to swing.
2 Some people inside high buildings may feel a tremor.
3 Rapid vibrations possibly felt indoors.
4 Stationary cars rock; windows shake; people indoors feel something.
5 Effects felt outdoors; small objects fall over; some buildings shake.
6 Trees begin to shake; crockery broken; everyone in the area feels it.
7 People alarmed; chimneys begin to crack; windows break.
8 Cars crash; buildings and trees damaged.
9 Many people panic; cracks in the ground; buildings fall down.
I0 Buildings destroyed; underground services disrupted; rivers affected.
II Bridges collapse; landslides happen; railways affected.
12 Widespread devastation; landscape changed.
Picture Credit : Google