20.1. Classification of Earthquakes Based on Causes

Earthquakes are classified based on their origin or causative factors. These include:

This introductory statement explains that the focus of the section is on how earthquakes are categorized according to their causes. It indicates that various factors contribute to the occurrence of earthquakes, setting the stage for a detailed exploration of each type.

Think of classifying books in a library. Just as books can be sorted by genre, author, or publication date, earthquakes can be categorized based on their causes. This helps scientists understand and study them better.

1. Tectonic Earthquakes

Tectonic earthquakes are caused by the movement of the Earth's lithospheric plates. These movements happen due to the Earth's internal heat and lead to stress build-up at the edges of these plates. When the stress exceeds the strength of the rocks, it results in an earthquake. Tectonic earthquakes are the most common type and can be very destructive.

Imagine two cars bumping against each other at a traffic light; the pressure builds as they push against one another until one moves. Similarly, tectonic plates push against each other, leading to seismic activity when stress is released.

2. Volcanic Earthquakes

Volcanic earthquakes occur in areas with volcanic activity. They can result from the movement of magma beneath the Earth's surface, sudden fracturing of rocks due to pressure from magma, or the collapse of volcanic structures. While often localized, these events can indicate impending volcanic eruptions.

Think about how bubbles form in a boiling pot of water. When the pressure builds up from the heat, bubbles burst to the surface, similar to how volcanic activity can create tremors as magma moves and pressure builds.

3. Collapse Earthquakes

Collapse earthquakes are generally minor and occur when underground caves or mine tunnels collapse. They can also happen due to sudden failures of rock strata or roof falls in mines. Although these quakes usually have low magnitudes, they can still impact local structures and communities.

Picture a sandcastle that suddenly collapses when too much water erodes its base. Just as sand collapses, so too can underground structures lead to minor seismic events when they give way.

4. Explosion-induced Earthquakes

These earthquakes are man-made and result from activities like nuclear detonations or large chemical explosions. Construction activities using heavy blasting can also induce such earthquakes. They typically last for a short duration and have a distinct seismic signature that differentiates them from natural earthquakes.

Think of throwing a rock into a pond. The ripples created are temporary and localized, similar to the way explosion-induced earthquakes behave — quick and distinct without causing widespread effects.

5. Reservoir-induced Earthquakes

These earthquakes occur when large reservoirs behind dams are filled. The added weight of the water increase stress on underlying faults and can lead to slippage. Changes in pore pressure from water infiltration can also reduce friction on faults, causing earthquakes.

Imagine sitting on a seesaw: if a friend jumps on one side suddenly, it can flip. Similarly, when water is added to a reservoir, it increases stress on fault lines, which can result in minor earthquakes.

6. Isostatic Adjustment Earthquakes

These earthquakes are connected to isostatic rebound, where land masses rise or fall due to the removal (like melting glaciers) or addition of surface loads (like sediment deposits). This adjustment of crustal weight can trigger seismic activity.

Think of a sponge that is compressed when squeezed; once you let go, it slowly expands back to its original shape. As the land adjusts to changes over time, it may also 'release' energy in the form of small earthquakes.

7. Induced Seismicity due to Human Activities

This type of seismicity arises from activities such as deep well injection, hydraulic fracturing, mining, and geothermal extraction. These actions alter stress and pore pressure in the Earth's crust, potentially triggering earthquakes.

Picture a tightly packed crowd in a concert; when a few people push towards the exit, it can set off a chain reaction of movement in the crowd. Similarly, human interventions can disrupt stress balance in the Earth, leading to earthquakes.

--