Earthquakes are sudden movements occurring in the Earth caused by the release of accumulated energy. Over time, energy accumulates as tectonic plates in the Earth's crust push up against each other. Eventually, the amount of energy stored becomes more than what the Earth can withstand. When this happens, the energy releases suddenly and violently as an earthquake. In most cases, earthquakes happen along fault lines, which are breaks in the Earth's crust. Fault lines can be less than an inch long, or they can be thousands of miles long. At these fault lines, the plates push against each other, sometimes also rubbing back and forth due to the pent-up energy. The back and forth movements constitute earthquakes. The tremors continue until the plates catch on each other, which causes them to get stuck and stop moving again. The point where rocks in the plates break is known as the earthquake focus. The point directly over the focus has the most energy, and it is called the epicenter of the earthquake. The energy moves out from the epicenter in waves. While earthquakes are often caused by sudden movements along fault lines, this is not the only cause of earthquakes. Underground explosions associated with construction work or mining can also cause earthquakes.
Earthquake prediction includes not only the place and the time, but also the magnitude of the tremor. Scientists know that earthquakes occur along fault lines, so this makes it possible to predict the places where they will occur. The size of the fault lines also makes it possible to have a rough idea about the intensity of earthquakes that will occur in specific areas. However, predicting the times these events might occur has not been possible to date. 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 1930s 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 7.0 on the Richter scale would be 10 times larger than an earthquake that measures 6.0. Very large earthquakes that measure more than 7.0 on the Richter scale often cannot be measured precisely using this method because the energy released has a lower frequency but a broader range of power. 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. Intensity measured by the modified Mercalli scale is assigned numbers between I and XII based on the amount of damage and reports received from eye witnesses. At level I, vibrations are extremely minor, possibly felt by only a few people. Level II involves vibrations on the upper floors of buildings with suspended objects swinging slightly. At level V, everyone feels vibrations. Objects fall off shelves and are overturned. Level VIII involves significant damage of even strong and well-designed structures. Brick buildings are often destroyed, and heavy objects become overturned. At the highest XII level, most structures are destroyed and objects become airborne.
By: Steven Moore