Earthquakes are recorded by a seismographic network. Each seismic station in the network measures the movement of the ground at that site. The slip of one block of rock over another in an earthquake releases energy that makes the ground vibrate.
That vibration pushes the adjoining piece of ground and causes it to vibrate, and thus the energy travels out from the earthquake hypocenter in a wave.
The vibrations produced by earthquakes are detected, recorded, and measured by instruments call seismographs.
The zig-zag line made by a seismograph, called a “seismogram,” reflects the changing intensity of the vibrations by responding to the motion of the ground surface beneath the instrument.
From the data expressed in seismograms, scientists can determine the time, the epicenter, the focal depth, and the type of faulting of an earthquake and can estimate how much energy was released.
Earthquake recording instrument, 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.
Understanding the size of the earthquake
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.
They use 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.
Earthquake intensity and magnitude
The severity of an earthquake can be expressed in terms of both intensity and magnitude. However, the two terms are quite different, and they are often confused.
Intensity: Intensity measurement varies from location’s nearness to the epicenter. The intensity of shaking from an earthquake varies depending on where you are during the earthquake. It manifests the degree of damage, which gets diminished as we go away from the main shock source zone and the reverse is also true.
Magnitude: Magnitude is related to the amount of seismic energy released at the hypocenter of the earthquake whereas intensity is based on the observed effects of ground shaking on people, buildings, and natural features. The magnitude is a number that characterizes the relative size of an earthquake.
Magnitude is based on measurement of the maximum motion recorded by a seismograph. Several scales have been defined, but the most commonly used are local magnitude (ML), commonly referred to as ‘Richter magnitude‘:
- 3-3.9-magnitude – Minor earthquake that may be felt
- -4.9-magnitude – Light tremors that are likely felt
- 5-5.9-magnitude – Moderate earthquake where minor damaged might occur
- 6-6.9-magnitude – Strong earthquake where damage is most likely to occur
- 7-7.9-magnitude – Major earthquake where major damage and loss of lives are expected
- 8-magnitude or larger – Significant damage is expected with major loss of lives
What is Richter Scale
The Richter magnitude scale was developed in 1935 by Charles F Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs.
Adjustments are included in the magnitude formula to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude of 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3.
Read more via US Geological Survey