Ipsa Tripathy
Bhubaneswar: When the ground begins to shake, the experience lasts only a few seconds or minutes. Yet within that short time, an earthquake can alter landscapes, damage cities and change thousands of lives. Earthquakes are among the most powerful natural events on Earth. Unlike cyclones or floods, they often strike without warning, making them particularly dangerous. However, thanks to advances in seismology, scientists today can accurately detect earthquakes within seconds, determine where they occurred, estimate their magnitude and assess the areas likely to be affected.
Behind every earthquake report lies a fascinating branch of science based on seismic waves—vibrations that travel through the Earth carrying valuable information about what happened deep beneath the surface.
Why Do Earthquakes Occur?
The Earth’s outer shell, known as the lithosphere, is divided into several massive tectonic plates. These plates are constantly moving, although only by a few centimetres each year. Sometimes the edges of these plates become locked because of friction. As they continue trying to move, enormous stress builds up within the rocks.
When this stress exceeds the strength of the rocks, they suddenly break along a fracture called a fault. The stored energy is released almost instantly, sending vibrations in every direction. These vibrations are known as seismic waves, and they are responsible for the shaking we experience during an earthquake. The point inside the Earth where the rupture begins is called the focus (or hypocentre), while the point directly above it on the Earth’s surface is known as the epicentre.
The Language of Seismic Waves
Earthquakes generate different types of seismic waves, each travelling through the Earth in its own way.
Primary Waves (P-Waves)
P-waves are the fastest seismic waves and are therefore the first to reach seismic stations. They move by compressing and expanding the rocks, much like sound waves travelling through air. Because they can travel through solids, liquids and gases, P-waves provide valuable information about the Earth’s internal structure.Although they usually cause little damage, they serve as the first indication that an earthquake has occurred.
Secondary Waves (S-Waves)
S-waves arrive after the P-waves. Unlike P-waves, they move the ground from side to side or up and down, producing stronger shaking. S-waves can travel only through solid materials and cannot pass through liquids. This property helped scientists discover that the Earth’s outer core is liquid.
Surface Waves
Once seismic waves reach the Earth’s surface, they generate surface waves, which travel along the ground. These waves move more slowly than P- and S-waves but usually produce the strongest shaking and cause the greatest destruction during large earthquakes. Most of the damage to buildings, bridges and roads is caused by these surface waves.
How Are Earthquakes Detected?
The primary instrument used to detect earthquakes is the seismometer. A seismometer is an extremely sensitive instrument capable of detecting even tiny ground movements. Modern digital seismometers continuously record ground vibrations throughout the day. During an earthquake, the instrument produces a graph called a seismogram, which records the arrival times and amplitudes of different seismic waves.
Since P-waves travel faster than S-waves, scientists calculate the difference in their arrival times at multiple seismic stations. Using this information, they determine the location of the earthquake’s epicentre through a process called triangulation. Today, thousands of seismic stations operate across the world, allowing earthquakes to be detected within minutes, often much sooner.
Measuring the Strength of an Earthquake
Many people confuse an earthquake’s magnitude with its intensity, but they are not the same. Magnitude measures the total amount of energy released at the earthquake’s source. Today, scientists primarily use the Moment Magnitude Scale (Mw) because it provides accurate measurements for both small and large earthquakes. It has largely replaced the older Richter Scale for scientific purposes. Since the scale is logarithmic, an earthquake of magnitude 7 releases about 32 times more energy than one of magnitude 6.
Intensity, on the other hand, describes how strongly the earthquake is felt at a particular location and the extent of damage it causes. Scientists commonly use the Modified Mercalli Intensity (MMI) Scale, which ranges from I (not felt) to XII (total destruction). An earthquake therefore has one magnitude but can produce different intensities in different places depending on distance, geology and building construction.
Can Earthquakes Be Predicted?
Despite major scientific advances, no reliable method currently exists to predict the exact time, location and magnitude of an earthquake. Scientists can identify earthquake-prone regions and estimate long-term probabilities based on fault activity and historical records, but precise short-term prediction remains beyond current scientific capability.
However, earthquake early warning systems have become increasingly effective. These systems detect the faster-moving P-waves and immediately send alerts before the slower, more destructive S-waves and surface waves arrive. Although the warning may last only a few seconds to tens of seconds, it can be enough to stop trains, shut down industrial systems and allow people to take protective action. Countries such as Japan, Mexico and the United States have developed advanced earthquake early warning networks.
Which Areas Are Most Prone to Earthquakes?
Earthquakes occur mainly along tectonic plate boundaries. The world’s most active seismic region is the Pacific Ring of Fire, a horseshoe-shaped zone surrounding the Pacific Ocean. It experiences about 90% of the world’s earthquakes and includes countries such as Japan, Indonesia, Chile, New Zealand and the western United States. Another highly active region is the Alpine-Himalayan Belt, which stretches from the Mediterranean through Turkey, Iran, Pakistan and the Himalayas.
India is particularly vulnerable because the Indian Plate continues to collide with the Eurasian Plate, slowly uplifting the Himalayan mountain range. According to the Bureau of Indian Standards, the most earthquake-prone regions of India include Jammu and Kashmir, Himachal Pradesh, Uttarakhand, northeastern states, parts of Bihar, Gujarat and the Andaman and Nicobar Islands. These regions fall within the country’s highest seismic hazard zones.
What Should You Do During an Earthquake?
Earthquake safety depends on quick and calm action.
If you are indoors:
- Drop, Cover and Hold On.
- Take shelter under a sturdy table or desk.
- Protect your head and neck with your arms.
- Stay away from windows, mirrors and heavy furniture.
- Do not use elevators.
If you are outdoors:
- Move to an open area away from buildings, trees, streetlights and power lines.
- Remain there until the shaking stops.
If you are driving:
- Pull over safely away from bridges, flyovers and power lines.
- Stay inside the vehicle until the shaking ends.
After the earthquake:
- Check yourself and others for injuries.
- Be prepared for aftershocks.
- Turn off gas or electricity if damage is suspected.
- Follow official instructions and emergency advisories.
Understanding the Earth’s Movements
Every earthquake reminds us that the Earth is far from static. Beneath our feet, tectonic plates are constantly reshaping the planet, releasing enormous amounts of energy through seismic waves. Although earthquakes cannot yet be predicted with precision, science has made remarkable progress in detecting them, measuring their strength and providing early warnings that save lives.
The most effective protection remains a combination of scientific monitoring, earthquake-resistant infrastructure, public awareness and disaster preparedness. The ground may shake without notice, but informed communities are always better prepared to withstand its impact.