19.1. Causes of Earthquakes

• Caused due to relative motion between tectonic plates. • Most earthquakes are generated by the sudden release of energy accumulated due to plate movements. • Boundaries: divergent, convergent, and transform faults.

Tectonic movements are the primary cause of most earthquakes. The Earth's outer layer, known as the lithosphere, is divided into several large and small pieces called tectonic plates. These plates move relative to one another due to the heat from the Earth's interior. When these plates interact, they can either move apart (divergent boundaries), push together (convergent boundaries), or slide past each other (transform faults).

Over time, stress builds up along these plate boundaries as they move. When the stress exceeds the strength of the rocks composing the plates, it results in a sudden release of energy in the form of seismic waves, which we feel as an earthquake. This explains why areas near tectonic plate boundaries are particularly earthquake-prone.

Imagine two cars moving toward each other and suddenly crashing into each other, creating a shockwave. Similarly, when tectonic plates collide or rub against each other, they build up tension until it releases, causing an earthquake. The boundaries between these plates can be thought of as the 'road' where these 'cars' interact.

• Earthquakes that occur due to volcanic eruptions. • Less frequent but can be highly destructive locally.

Volcanic activity can also cause earthquakes, although these are generally less frequent than those caused by tectonic movements. When magma rises to the Earth's surface, it can cause the surrounding rock to fracture, leading to earthquakes. These earthquakes often occur before, during, or after a volcanic eruption. While they may not happen as often as tectonic earthquakes, they can be particularly destructive because they occur in populated areas near volcanoes.

Think of a balloon being inflated. As it fills up, the skin stretches and can create small ruptures. Similarly, as magma fills a volcanic chamber, it can cause the surrounding rocks to crack and lead to earthquakes before the actual eruption happens.

• Result of human activities such as: – Reservoir-induced seismicity. – Mining explosions. – Deep well injections and hydraulic fracturing (fracking).

Induced seismicity refers to earthquakes that are caused by human activities rather than natural processes. This can happen in several ways. For example, when large reservoirs are created by damming rivers, the pressure from the water can trigger earthquakes in the surrounding area. Similarly, mining activities can cause explosions that release energy, leading to smaller earthquakes. Another significant contributor is hydraulic fracturing (fracking), a process used to extract oil or gas where high-pressure fluids are injected into the ground, which can also trigger seismic events.

Imagine shaking a soda bottle and then suddenly releasing the cap. Just as the built-up pressure causes a burst of bubbles, human activities can create pressure changes in the Earth that result in earthquakes. Every time we apply stress to the Earth through activities like fracking or building reservoirs, we risk causing these minor tremors.

• Caused by underground cave-ins or mine collapses. • Usually of minor magnitude.

Collapse earthquakes are the result of underground structures failing, such as caves collapsing or mine shafts giving way. These types of earthquakes generally have a minor magnitude compared to tectonic earthquakes. They occur when the ground underneath a surface structure loses support, causing it to cave in, which can send shockwaves through the surrounding earth. Though smaller, they can still be dangerous, particularly in populated or industrial areas.

Think about when you build a sandcastle on the beach. If you dig too deeply or remove too much sand from the base, the structure may collapse. This is similar to how the ground can cave in and create minor earthquakes when underground support structures fail.

--