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How Are Earthquakes Rated?

Earthquakes result from the buildup of seismic forces created by rocks and tectonic plates moving against each other. This seismic energy continues to increase until the rocks can no longer restrain the pressure and suddenly move, creating seismic waves. These tectonic forces occur with plate movement and alter the surface of the Earth as the solid masses within the Earth strain and shift.

Magnitude and Intensity
- Two terms used to describe the severity of an earthquake are the magnitude and the intensity. The magnitude reflects the amount of seismic energy released from the epicenter of the earthquake. As an objective measurement, it varies according to the location and depth of the epicenter and is based on the amplitude of seismic waves recorded on calibrated equipment at different sites near the quake. The intensity is a more subjective measurement based on the observed effects of the earthquake -- the ground shaking effects on people and on structures such as buildings and bridges. The most damage is likely to be seen closer to the epicenter and to human population centers.
Seismograph
- The vibrations from an earthquake are the seismic waves that travel through the Earth away from the epicenter of an earthquake. A seismograph is an instrument that records the varying amplitude or motion of the oscillations as the waves move beneath the Earth's surface. A pen suspended above a roll of paper, which is slowly pulled across the table by a motor, records the amplitude of the tremors over time.
  
  The sensitivity of seismographs magnifies the ground motions. These instruments can detect strong earthquakes around the planet. The time, location and magnitude of the disturbance can be calculated from the data recorded by seismic stations around the globe.
Richter Magnitude Scale
- The Richter magnitude scale is a logarithmic scale that functions as an objective standard against which the relative sizes of earthquakes are compared. The numbers on the scale measure factors of 10, so that an earthquake measuring 5.0 on the Richter scale is 10 times larger than one coming in at 4.0 on the scale. Its mathematical formula determines the magnitude of an earthquake with factors to compensate for the distance between the seismograph and the location of the epicenter.
  
  Anything less than 2.0 on the Richter scale is a minor Earth shiver that the average person cannot detect; geologists refer to these as micro-quakes. Earthquakes on the moderate scale measure below 6.0; those measuring more than that are significant seismic events that can cause great damage depending on the location of the epicenter.
The Mercalli Intensity Scale
- The Mercalli seismic scale subjectively measures the intensity or the effects of an earthquake on humans, structures and other objects. Its scale ranges from Level I, where observers do not feel any tremors, up to Level XII, which reflects total destruction. The highest intensities occur near the epicentral area and depend on the distance to the epicenter. Observers who experienced the quake supply the data that determine the intensity value for their locations.
  
  Disturbances measured at Level V are felt by most residents in the affected area as dishes and windows break. At Level X, the damage intensifies with the destruction of wooden structures, and most masonry structures and their foundations are completely destroyed. Level XI leaves few structures standing, and all bridges and railroads are bent and destroyed.

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