Model Answer
0 min readIntroduction
Earthquakes, sudden releases of energy in the Earth’s lithosphere, are among the most devastating natural hazards. Understanding the characteristics of earthquakes is crucial for hazard assessment and mitigation. Two fundamental parameters used to describe the size of an earthquake are magnitude and intensity. While both relate to the earthquake’s power, they represent different aspects – magnitude quantifying the energy released at the source, and intensity describing the effects experienced at a particular location. This answer will explain these concepts, their measurement, and the damage caused by earthquakes, considering various influencing factors.
Magnitude: Measuring Earthquake Size
Magnitude is a quantitative measure of the energy released during an earthquake. It is a single number assigned to an earthquake based on the amplitude of seismic waves recorded by seismographs. Several magnitude scales exist, each with its strengths and limitations:
- Richter Scale (ML): Developed by Charles Richter in 1935, it’s based on the logarithm of the amplitude of the largest seismic wave recorded on a specific type of seismograph. It’s most accurate for local earthquakes and underestimates the magnitude of large, distant earthquakes.
- Moment Magnitude Scale (Mw): Currently the most widely used scale, especially for larger earthquakes. It’s based on the seismic moment, which is related to the physical size of the fault rupture, the amount of slip, and the rigidity of the rocks. It provides a more accurate estimate of earthquake size across a wider range of magnitudes.
- Body Wave Magnitude (Mb) & Surface Wave Magnitude (Ms): These scales use amplitudes of body and surface waves respectively, and are useful for determining magnitudes of earthquakes at greater distances.
The magnitude scale is logarithmic; meaning an increase of one unit represents approximately a 32-fold increase in energy released. For example, a magnitude 6 earthquake releases about 32 times more energy than a magnitude 5 earthquake.
Intensity: Measuring Earthquake Effects
Intensity, unlike magnitude, is a qualitative measure of the effects of an earthquake at a specific location. It describes how strongly the earthquake was felt by people, and the extent of damage to structures. The most commonly used intensity scale is the Modified Mercalli Intensity Scale (MMI).
- Modified Mercalli Intensity Scale (MMI): This scale uses Roman numerals (I to XII) to describe the intensity based on observed effects. Intensity I represents imperceptible shaking, while Intensity XII represents total destruction. Factors like distance from the epicenter, local geological conditions, and building construction influence the intensity.
Intensity values vary from place to place for the same earthquake. An earthquake might have a high magnitude but low intensity in areas far from the epicenter, and vice versa.
Damage Caused by Earthquakes
The damage caused by an earthquake is a complex function of several factors:
- Magnitude and Intensity: Higher magnitude and intensity generally lead to more widespread and severe damage.
- Distance from Epicenter: Damage decreases with increasing distance from the epicenter.
- Geological Conditions: Liquefaction (where saturated soil loses strength and behaves like a liquid) and landslides are common earthquake-induced hazards, particularly in areas with loose sediments or steep slopes.
- Building Construction: Structures built to withstand seismic forces (earthquake-resistant buildings) suffer less damage than those that are not. Building materials, design, and adherence to building codes are crucial.
- Population Density: Higher population density in affected areas leads to a greater number of casualties and economic losses.
- Depth of Focus: Shallow-focus earthquakes (less than 70 km deep) generally cause more damage than deep-focus earthquakes because the energy has less distance to travel and is more concentrated.
Earthquake-induced tsunamis are another significant source of damage, particularly in coastal areas. The 2004 Indian Ocean tsunami, triggered by a magnitude 9.1 earthquake off the coast of Sumatra, caused widespread devastation and over 230,000 fatalities.
| Parameter | Magnitude | Intensity |
|---|---|---|
| Definition | Quantitative measure of energy released at the source. | Qualitative measure of effects at a specific location. |
| Scale | Richter, Moment Magnitude (Mw), Body Wave (Mb), Surface Wave (Ms) | Modified Mercalli Intensity (MMI) |
| Measurement | Seismograph recordings of wave amplitude. | Observed effects on people, structures, and the environment. |
| Variation | Single value for an earthquake. | Varies from place to place for the same earthquake. |
Conclusion
In conclusion, magnitude and intensity are complementary measures of earthquake size and impact. Magnitude quantifies the energy released, while intensity describes the experienced effects. Understanding both parameters, along with the factors influencing earthquake damage, is vital for effective earthquake hazard assessment, mitigation planning, and building resilient communities. Continued research into earthquake prediction and improved building codes are crucial steps towards reducing the devastating consequences of these natural disasters.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.