Now, instruments are carefully calibrated with respect to each other. Thus, magnitude can be computed from the record of any calibrated seismograph. The Richter Scale has no upper limit. Recently, another scale called the moment magnitude scale MMS has been devised for more precise study of great earthquakes.
Tectonic earthquakes can range in size from magnitudes less than zero, resulting from fault slippage of a few centimetres, to the largest events magnitude greater than 9 , where fault displacements are on the order of many metres.
The size of an earthquake is not only a function of the amount of displacement but also the area of the fault plane that ruptures.
Hence the larger the rupture area, the larger is the earthquake. A magnitude 7 earthquake ruptures a fault area of about km2 or about 50 km long and 20 km wide. Also depth is an important factor influencing earthquake severity. We know that earthquakes can originate at various depths within the Earth's solid core.
The deeper the earthquake, the more powerful it is, but it is also far less likely to reach the surface. That's why shallow earthquakes are more common and more dangerous, because the shallower an earthquake, the more damage to surface structures it can cause.
In , Charles F Richter — a scientist at the California Institute of Technology — developed the scale as a way to measure earthquakes.
To measure historical earthquakes that occurred before the introduction of the Richter scale, seismologists can use contemporary damage reports. Looking at accounts of damage to buildings, the radius in which tremors were felt and even reported changes in soil can tell a lot about the scale of an earthquake. The scale is used to rate the magnitude, or strength, of an earthquake. This is measured by the level of energy it releases. That level of energy is measured by a seismograph — scientific instruments securely mounted to the ground.
As the case moves, a suspended instrument inside it remains stable. This is the seismometer and measures the magnitude of an earthquake. Another factor that contributes to the exact location of earthquakes is the way that energy tends to travels along a fault line, which can cause the overall size of an earthquake to be very large if the stress triggers tectonic plate movement along a long expanse of a fault line.
The Richter scale was developed in the s and is most effective for large-scale earthquakes that are of moderate intensity. The Richter scale measures the maximum amplitude of seismic waves as they reach seismographs.
This scale is expressed with a logarithmic scale. Thus, an earthquake measuring 7. Very large earthquakes that measure more than 7. For this reason, scientists now utilize more precise and advanced measurement methods. Earthquakes are now measured in two separate ways based on the amount of energy released at the epicenter and the intensity of the earth shaking that occurs in specific locations. The moment magnitude scale measures the overall magnitude or size of the earthquake, and the modified Mercalli scale measures earthquake intensity.
For the moment magnitude scale, scientists use a formula to convert the energy released, as measured by a seismometer. Generally, you won't see much damage from earthquakes that register below 4 on the Richter scale. Richter ratings only give you a rough idea of the actual impact of an earthquake, though. As we've seen, an earthquake's destructive power varies depending on the composition of the ground in an area and the design and placement of man-made structures.
The extent of damage is rated on the Mercalli scale. Mercalli ratings, which are given as Roman numerals , are based on largely subjective interpretations. A low intensity earthquake, one in which only some people feel the vibration and there is no significant property damage, is rated as a II. The highest rating, a XII, is applied to earthquakes in which structures are destroyed, the ground is cracked and other natural disasters, such as landslides or tsunamis , are initiated.
Richter scale ratings are determined soon after an earthquake, once scientists can compare the data from different seismograph stations. Mercalli ratings, on the other hand, can't be determined until investigators have had time to talk to many eyewitnesses to find out what occurred during the earthquake. Once they have a good idea of the range of damage, they use the Mercalli criteria to decide on an appropriate rating.
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