Earthquake

An earthquake occurs when there are movements deep inside the earth. These movements release a lot of energy, which creates seismic waves. These waves cause the Earth's surface to tremble, shake, and sometimes even crack. The shaking or vibrations that occur on the earth’s crust are called earthquakes.

• Movements inside the Earth cause a release of energy, which travels in the form of seismic waves. These waves reach the earth's surface, causing the ground to shake or tremble.
• Earthquakes can occur naturally, but sometimes human activities like mining or building large dams can also lead to earthquakes.

When an earthquake happens, there are two essential terms to know

1. Focus: The focus is the exact point inside the earth where the earthquake starts. It's the source of the earthquake, deep below the ground.
2. Epicentre: The epicentre is the point on the Earth’s surface directly above the focus. Because it is closest to the focus, the strongest shaking is usually felt at the epicentre. The strongest shaking occurs at or near the epicentre, often causing the most damage to buildings and infrastructure.

Focal point and epicentre of earthquake

Earthquake shocks can range from very mild to extremely strong.

• Mild earthquakes are those that cause little to no damage and are often barely felt by people. These mild earthquakes occur frequently around the world.
• Strong earthquakes, on the other hand, are powerful enough to cause significant destruction. They are less common but are more dangerous.

Every day, many small earthquakes are recorded around the world. According to the National Earthquake Information Centre, between 12,400 and 14,000 earthquakes occur each year worldwide. This means that the earth is always experiencing some level of vibration.

During an earthquake, seismic waves travel through the Earth. There are three main types of seismic waves:

1. Primary or ‘P’ Waves: P waves are the fastest seismic waves and the first detected during an earthquake. They move through the Earth’s interior in a push-pull motion, similar to sound waves. These waves can travel through both solid and liquid layers of the earth.
2. Secondary or ‘S’ Waves: S waves are slower than P waves and arrive second. They move in a side-to-side or up-and-down motion. S waves can only travel through solid layers, not liquids.
3. Surface or ‘L’ Waves: L waves are the slowest and last waves to be felt. These waves travel along the Earth’s surface and cause the most damage because they create strong shaking on the ground. L waves move in a rolling motion, which can cause buildings and roads to crack or collapse.

1. Seismometer

A seismograph (or seismometer) is a device used to detect and record earth movements. It has three main parts: a heavy weight, a pen, and a base. The base is fixed to the ground and moves during an earthquake, while the suspended weight stays mostly still. This causes the pen to draw lines on a rotating drum, creating a seismogram.

Seismometer

• It provides a detailed record of ground shaking. And helps scientists determine the earthquake’s intensity and duration.
• Data from multiple seismographs can locate the earthquake’s origin.

2. Richter Scale

The Richter Scale measures the magnitude (strength) of an earthquake, indicating how much energy is released at its source. Developed in 1935 by Charles F. Richter, it uses a logarithmic scale, meaning each whole number increase represents 10 times greater wave amplitude and about 32 times more energy released.

• Magnitude is calculated using the height of seismic waves recorded by a seismograph.
• An earthquake with a magnitude of 5.0 is 10 times stronger in wave amplitude than one with a magnitude of 4.0 and releases 32 times more energy.

Magnitude Categories,

1. If You're at Home

• Stay calm and don’t run. Stand still or sit on the floor.
• Hide under sturdy furniture like a table or bed and protect yourself until the shaking stops.
• If there’s no furniture, sit in a corner and cover your head with your hands.

2. If You're Outside or in a Vehicle

• Park your vehicle in a safe spot and stay inside.
• Stay away from tall buildings, trees, and electric poles.

3. Other Precautions

• Don’t use lifts; use the stairs instead, especially in tall buildings.
• Avoid staying still for too long; move your body to stay comfortable and alert.
• Turn off the main electric supply carefully to prevent fires.
• Use a flashlight or battery instead of candles, lanterns, or matches.

Earthquake-resistant buildings are specially designed to withstand seismic forces, minimising damage and ensuring safety. They are built to handle ground shaking up to a certain intensity, reducing the risk of collapse.

Design and Construction Standards:

1. Building Codes

• IS 456: Guidelines for strong and durable reinforced concrete structures.
• IS 1893: Principles for designing buildings to endure earthquakes.
• IS 13920: Reinforcement guidelines to keep buildings flexible and ductile, allowing them to bend without breaking.

2. Advanced Techniques

• Base Isolation: Using flexible bearings to separate buildings from foundations, reducing seismic force impact.
• Energy Dissipation Devices: Absorb and reduce earthquake energy.
• Reinforced Frames & Flexible Joints: Allow controlled movement during earthquakes.

1. Laser Ranging & VLBI: Measure ground shifts with precision.
2. Geiger Counters: Detect underground activity related to earthquakes.
3. Creep Meters & Strain Meters: Track ground movement and crust deformation.
4. Tide Gauges & Tilt Meters: Monitor sea level changes and ground tilt, signalling possible seismic events.
5. Volumetric Strain Gauges: Measure changes in volume within the earth's crust to detect seismic stress.