20.2. Tectonic Earthquakes

Tectonic earthquakes are the most common and destructive type, caused by the movement of the Earth's lithospheric plates. The theory of plate tectonics explains that the lithosphere is divided into several major and minor plates that float over the semi-fluid asthenosphere. These plates interact at their boundaries, leading to stress accumulation and eventual release as seismic energy.

Tectonic earthquakes occur due to the movement of large sections of the Earth's outer shell, known as the lithosphere. Imagine the Earth as a giant puzzle; each piece represents a tectonic plate. These plates float on a layer called the asthenosphere, which is semi-fluid and allows the plates to shift. At the edges of these plates, where they meet, stress builds up as the plates push against or pull apart from each other. When the stress is too high, it is released suddenly, producing an earthquake.

Think of trying to push two heavy furniture pieces together in a room. As you push, you can feel the resistance building; if you keep pushing too hard, eventually, the pieces will crash together. In the Earth, this crash is what we feel as an earthquake.

• Convergent Boundaries: Plates collide, causing compression. Example: Himalayas. • Divergent Boundaries: Plates move apart, causing tension. Example: Mid-Atlantic Ridge. • Transform Boundaries: Plates slide past each other. Example: San Andreas Fault.

There are three main types of boundaries where tectonic plates interact. At convergent boundaries, plates collide, leading to the formation of mountains or deep ocean trenches due to compression. The Himalayas were formed this way when the Indian plate collided with the Eurasian plate. Divergent boundaries occur where plates move apart, creating new crust, like the Mid-Atlantic Ridge. Here, magma rises and forms new ocean floor. Finally, at transform boundaries, like the San Andreas Fault, plates slide horizontally past one another, which can cause earthquakes.

Think of a conveyor belt. At the convergent parts (like a pinch), items get squished together and pile up (forming mountains), while at divergent sections, new items pop out as the belt pulls apart (creating new land). At transform boundaries, items just slide past each other, which can sometimes cause them to wobble or shake.

A fault is a fracture in the Earth’s crust where blocks of rock move relative to each other. The Elastic Rebound Theory explains how stress builds up in rocks until it exceeds their strength, causing a sudden rupture and release of energy—an earthquake.

Faults are essentially cracks in the Earth's crust where there’s movement. Over time, stress accumulates at these faults as tectonic plates shift. According to the Elastic Rebound Theory, rocks behave somewhat like rubber bands: they can stretch and store energy until they can't anymore. When the stress exceeds the strength of the rocks, they snap back to their original shape, leading to an earthquake as the stored energy is released.

Imagine stretching a rubber band. As you pull it, energy builds until you reach a point where it snaps back. This sudden release of energy is similar to what happens at a fault during an earthquake. It’s that snap that we can feel as shaking.

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