The earthquake that struck Delhi, despite having a magnitude of only 4.0, felt unusually strong to many residents. Earthquakes are a complex natural phenomenon, and their effects can vary significantly depending on various factors, such as depth, distance from the epicenter, geological conditions, and human perception. Although a magnitude of 4.0 is generally considered minor on the Richter scale, the intensity of the shaking and the perception of its severity can be amplified due to a combination of these factors. In this essay, we will explore why a relatively low-magnitude earthquake like the one in Delhi could still feel intense.
1. Magnitude vs. Intensity
Before diving into the reasons behind the strong perception of this earthquake, it’s important to clarify the difference between magnitude and intensity.
- Magnitude is a measurement of the energy released at the source of the earthquake. The Richter scale or other similar scales (such as the Moment Magnitude Scale) are used to quantify this. A magnitude of 4.0, as mentioned, is considered to be on the lower end of the scale, and typically would cause minimal structural damage.
- Intensity refers to the strength of the shaking experienced at specific locations and is influenced by various factors, including the distance from the epicenter, the local geology, and building structures. Intensity is measured on scales such as the Modified Mercalli Intensity (MMI) scale, which ranges from I (not felt) to XII (total destruction).
Thus, an earthquake’s magnitude may not necessarily correlate directly with how intense or damaging the shaking is in a given area.
2. Shallow Depth
One of the primary reasons why a low-magnitude earthquake can feel much stronger is its depth. The depth of an earthquake plays a critical role in determining its intensity. Earthquakes that occur at shallower depths tend to produce stronger shaking at the surface compared to deeper earthquakes, even if their magnitudes are the same.
For example, if an earthquake with a magnitude of 4.0 occurs at a depth of just a few kilometers, the shaking will be felt more strongly at the surface than if the same earthquake had occurred 700 kilometers deep in the Earth. In the case of Delhi’s recent earthquake, if it occurred at a shallow depth, this could have contributed significantly to the intensity felt in the city.
Shallow earthquakes release their energy closer to the Earth’s surface, which means the seismic waves have less distance to travel, causing them to lose less energy and resulting in stronger shaking. This could make an otherwise moderate earthquake feel more severe, especially in urban areas where buildings amplify the effects.
3. Proximity to the Epicenter
Delhi, being located in a seismically active region, is close to several fault lines, including those in the Himalayan belt. The closer a city is to the epicenter of an earthquake, the stronger the shaking will be felt. Even though a magnitude of 4.0 is generally low, being near the epicenter can cause it to be felt much more intensely.
If the earthquake’s epicenter was near or within the region surrounding Delhi, it would cause strong shaking in the city, even though the magnitude would be low. For example, a magnitude of 4.0 that originates near the surface in the Delhi region can create vibrations that travel less distance to reach the city, thus causing it to be felt more intensely than if the same magnitude earthquake occurred far away from the city.
4. Building Infrastructure and Urban Environment
Delhi, being a densely populated urban metropolis with a mix of old and new buildings, is particularly vulnerable to the effects of earthquakes. The way buildings are designed, constructed, and maintained can significantly influence how an earthquake is felt by the population.
- Old Buildings: Many older buildings, particularly those constructed before modern seismic codes were implemented, are more susceptible to shaking. These buildings may not have been designed to withstand seismic forces, leading to heightened perceptions of an earthquake’s strength. In areas with poor construction quality, even a low-magnitude earthquake can cause visible shaking, rattling, or minor damage, which can make people feel that the earthquake was stronger than it really was.
- High-Rise Buildings: In a high-rise environment, the motion of a low-magnitude earthquake is often amplified. Tall buildings can sway or shake in response to seismic waves, and this movement can be unsettling to residents. Even though the earthquake’s magnitude is only 4.0, the way seismic waves interact with tall structures can make the shaking feel stronger, especially on higher floors. This is often more pronounced in dense, urban areas where the population is concentrated in high-rise buildings.
- Building Materials: The construction materials used in buildings also play a role. Concrete, steel, and brick structures behave differently during an earthquake, and the resonance between the frequency of the seismic waves and the frequency of vibration of buildings can amplify the shaking. If the frequency of the earthquake’s waves matches that of the structure, the shaking can feel more intense.
5. Soil and Geological Conditions
The local geological conditions and soil types at the surface can significantly influence how strong an earthquake feels. The phenomenon known as site amplification can cause the effects of an earthquake to be stronger in certain areas due to the underlying geology and soil composition.
- Soft Soils: Areas with soft soils or loose sediments, such as riverbeds or flood plains, tend to experience stronger shaking during an earthquake. This is because soft soils amplify seismic waves, while hard rock tends to absorb or dissipate them. Delhi, being built on a mixture of riverbeds and alluvial deposits, is susceptible to this effect. In such regions, seismic waves are amplified, making an otherwise mild earthquake feel much more intense.
- Ground Shaking and Liquefaction: In some cases, the shaking of soft soils can lead to liquefaction, where the ground temporarily loses its solid structure. Although this is more common in larger earthquakes, even moderate events can trigger localized effects in vulnerable areas. Liquefaction can cause significant disruption, leading to a heightened sense of the earthquake’s strength.
6. Psychological Factors and Human Perception
Finally, human perception and psychological factors play a significant role in how an earthquake is felt. In areas with high population density and where people may be unaware of seismic risks, even a small earthquake can be perceived as much stronger than it actually is. When people are not accustomed to earthquakes, the shock and suddenness of the tremor can cause a heightened sense of fear and anxiety, making the event seem more intense.
Additionally, the collective response of the population (such as mass panic, emergency alerts, or news reports) can influence the way people perceive the earthquake. In the case of Delhi, given its history of significant seismic events in the broader region, even a small earthquake could trigger memories of larger, more destructive quakes, further magnifying the perceived intensity of the event.
Conclusion
Although a magnitude 4.0 earthquake is typically classified as minor, its perceived strength can be influenced by various factors. In the case of Delhi, the shallow depth of the earthquake, proximity to the epicenter, urban infrastructure, and geological conditions likely contributed to the heightened perception of the tremor. Additionally, human factors such as fear, the behavior of buildings, and the amplification of seismic waves in soft soils further enhanced the experience of this earthquake. Understanding the complex interplay of these factors helps explain why even a relatively minor earthquake can leave a lasting impression on the people who feel it.
