The measurement of earthquakes is a fundamental aspect of seismology, the scientific study of earthquakes and their effects. Earthquakes are typically measured using two primary units: magnitude and intensity. Each serves a distinct purpose and provides different insights into the earthquake’s characteristics.
Magnitude of Earthquakes
1. Definition and Scale
Magnitude is a measure of the energy released during an earthquake. It is a logarithmic scale, meaning each whole number increase on the magnitude scale represents a tenfold increase in the amplitude of the seismic waves and roughly 31.6 times more energy release. The most commonly used magnitude scale is the Richter scale, developed by Charles F. Richter in 1935.
2. Richter Scale
The Richter scale quantifies the size of an earthquake by measuring the amplitude of seismic waves recorded by seismographs. It is primarily used for smaller to medium-sized earthquakes. However, its effectiveness diminishes for very large or distant earthquakes. The scale ranges from minor (less than 2.0) to great (8.0 and above) earthquakes. For example, a magnitude 5.0 earthquake can be described as moderate, whereas a magnitude 7.0 earthquake is classified as major.
3. Moment Magnitude Scale (Mw)
To address the limitations of the Richter scale for larger and more distant earthquakes, seismologists use the Moment Magnitude Scale (Mw). This scale, developed in the late 20th century, provides a more accurate measure of the earthquake’s size by accounting for the seismic moment, which is a function of the area of the fault that slipped, the average amount of slip on the fault, and the rigidity of the rocks involved. The Moment Magnitude Scale has become the standard for measuring large earthquakes due to its ability to provide consistent results across various earthquake sizes and distances.
Intensity of Earthquakes
1. Definition and Scales
Intensity measures the effects of an earthquake at specific locations, including the amount of shaking and the damage caused. Unlike magnitude, which is a measure of the earthquake’s energy, intensity reflects the earthquake’s impact on people, buildings, and the environment. Several intensity scales are used globally, with the most well-known being the Modified Mercalli Intensity (MMI) scale.
2. Modified Mercalli Intensity Scale
Developed by Giuseppe Mercalli in 1902 and later modified to the MMI scale, this scale assesses earthquake intensity based on observations of the earthquake’s effects. It ranges from I (not felt) to XII (total destruction). The MMI scale considers factors such as the earthquake’s impact on people, buildings, and natural features. For instance, an MMI of VII may describe significant damage to buildings and a noticeable shaking felt by most people, while an MMI of X indicates severe damage and widespread destruction.
3. European Macroseismic Scale (EMS)
Another important intensity scale is the European Macroseismic Scale (EMS), used primarily in Europe. The EMS is similar to the MMI scale but tailored to European conditions. It provides detailed descriptions of the effects of earthquakes on buildings, infrastructure, and the natural environment, helping to assess the overall impact and guide post-earthquake response efforts.
Instrumental Measurements
1. Seismographs
Seismographs are the primary instruments used to measure and record seismic waves generated by earthquakes. They consist of a mass suspended on a spring and a pen or sensor that records the motion of the mass relative to the ground. When an earthquake occurs, the seismograph detects the ground movement and produces a seismogram, a graphical representation of the seismic waves’ amplitude and frequency.
2. Accelerometers
Accelerometers measure the acceleration of the ground during an earthquake. They are often used in conjunction with seismographs to provide more detailed information about the shaking intensity and the potential for structural damage. Accelerometers are crucial for earthquake engineering and assessing the seismic performance of buildings and infrastructure.
Conclusion
The measurement of earthquakes through magnitude and intensity provides valuable information for understanding and responding to seismic events. Magnitude scales, such as the Richter and Moment Magnitude Scales, quantify the energy released during an earthquake, while intensity scales, such as the Modified Mercalli Intensity and European Macroseismic Scales, describe the effects and impact of the earthquake on people and structures. By combining these measurements with instrumental data from seismographs and accelerometers, seismologists can better assess the nature of earthquakes and develop strategies for mitigating their effects.