6 February 2023

Turkey’s earthquake: What causes an earthquake and why can’t it be predicted?

GS-1: Important Geophysical phenomena such as earthquakes, Tsunami, Volcanic activity, cyclone etc.

A 7.8 magnitude earthquake hit South-Central Turkey and Northwest Syria early Monday, causing over 1,500 deaths and hundreds of injuries. Search and rescue operations are ongoing with hundreds believed to be trapped under debris. 

The earthquake was centered 18 km deep and 33 km from Gaziantep and felt across West Asia, Northern Africa, and South Eastern Europe. 

India is among the 45 countries offering assistance to Turkey and is sending search and rescue teams and medical teams with relief material.

What is an earthquake?

• An earthquake is a phenomenon where the ground experiences intense shaking due to movement below the earth's surface. 
• It occurs when two blocks of the earth suddenly move past each other, releasing stored energy in the form of seismic waves which spread through the earth causing the shaking.

What is the cause of earthquakes?

• Earthquakes are caused by the movement of tectonic plates on the earth's surface. 
• The tectonic plates, which make up the earth's crust, are fragmented and their edges are known as plate boundaries, consisting of faults. These plates move at a slow pace, sliding and bumping into each other.
• When the rough edges of the plates get stuck, pressure builds up until the plates unstick on a fault and an earthquake occurs. 
• The location below the earth's surface where the earthquake originates is referred to as the hypocenter, while the location directly above it on the surface of the earth is known as the epicenter.

Earthquake waves

• Earthquakes occur in the lithosphere, which is the portion of earth's depth up to 200 km from its surface.
• Seismographs record the earthquake waves reaching the surface, resulting in a wave pattern with three distinct sections.
• There are two types of earthquake waves: body waves and surface waves.
• Body waves, generated by energy release at the focus, travel through the earth's body in all directions. They create surface waves by interacting with surface rocks.

• Body waves are further classified into two types: P and S-waves. 

• P-waves are the first to arrive and are similar to sound waves. They move parallel to the direction of the wave, causing pressure on the material they pass through and leading to stretching and squeezing. 
• S-waves arrive later and can only travel through solid materials.They vibrate perpendicular to the wave direction in the vertical plane, causing troughs and crests in the material. 
• Reflection and refraction of these waves help to understand the earth's interior structure.

• Surface waves are the last to be recorded on seismographs and are more destructive, causing displacement of rocks and collapse of structures.

Shadow zone

• Seismographs record earthquake waves at far-off locations, but there are specific areas where waves are not recorded, called the shadow zone.
• For each earthquake, there is a unique shadow zone.
• Seismographs within 105° of the epicenter record both P and S-waves, while those beyond 145° from the epicenteronly record P-waves.
• The zone between105° and 145° from the epicenter is the shadow zone for both types of waves.
• The shadow zone of S-waves is larger, covering over 40% of the earth's surface.
• The shadow zone for any earthquake can be determined with the knowledge of the epicenter's location.

Quantifying the Magnitude and Intensity of Earthquakes

• The earthquake events are scaled either according to the magnitude or intensity of the shock. 
• The magnitude scale is known as the Richter scale. The magnitude, which is expressed in numbers 0-10, relates to the energy released during the quake. 
• The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.

Types of Earthquakes

1. Tectonic earthquakes are the most common and occur due to sliding of rocks along a fault plane.
2. Volcanic earthquakes are a type of tectonic earthquakes confined to areas of active volcanoes.
3. Collapse earthquakes occur due to roof collapse in underground mines in areas of intense mining activity.
4. Explosion earthquakes occur due to the explosion of chemical or nuclear devices.
5. Reservoir induced earthquakes occur in areas with large reservoirs.

Immediate Effects of an Earthquake

• Tsunamis, though not an earthquake themselves, are waves generated by the shaking and can cause extensive damage if the earthquake magnitude is greater than 5 on the Richter scale.

Earthquake Occurrence and Distribution Pattern

• Earthquakes are a natural hazard and high magnitude earthquakes can cause significant damage to life and property.
• High magnitude earthquakes (8+ on the Richter scale) are rare, occurring once in 1-2 years whereas tiny earthquakes occur almost every minute.
• Not all parts of the world experience severe earthquakes.
• The distribution of seismic activity and volcanoes is shown in Figure below. 

• A line of dots is seen in the central Atlantic Ocean and extends into the Indian Ocean, bifurcating into two branches near the Indian subcontinent. This line of dots coincides with the mid-oceanic ridges and earthquakes in this area are shallow. 
• The shaded belt, another area of concentration, coincides with the Alpine-Himalayan system and the rim of the Pacific Ocean, where earthquakes are deep-seated.
• The map of volcanoes also follows a similar pattern with the rim of the Pacific known as the "Rim of Fire" due to the presence of active volcanoes.

Is it possible to predict earthquakes?

• No, currently there is no way to accurately predict earthquakes as there is no reliable precursor signal that can be detected before a large earthquake occurs.

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Fact File

This Word Means: North star The term "North Star" refers to a bright star in the Northern Hemisphere that is used for navigation due to its steady position in the sky.  It is also known as Polaris, and its significance in guiding people towards the north has been recognized for centuries. Polaris appears stationary in the northern sky due to its close proximity (less than 1°) to the north celestial pole, which aligns with the Earth's rotational axis. This makes it a useful tool for navigation as all other stars appear to rotate around it. It was first charted by Roman mathematician and astronomer Ptolemy in the 2ndcentury BC. During the Age of Exploration, it became a crucial tool for navigation and was used by Columbus on his first trans-Atlantic voyage.  The North Star has also been mentioned in literature, including Shakespeare's Julius Caesar.  However, "North Star" is a title that changes as the Earth's axis of rotation wobbles and the celestial pole moves in a slow circle, pointing towards different stars over time.  Currently, Polaris is the North Star, but it will eventually be replaced by Vega in about 12,000 years. [Ref- IE]