A.1. Earthquakes
An earthquake is a natural phenomenon characterized by the shaking and trembling of the Earth’s surface. It occurs when there is a sudden release of energy in the Earth’s crust, resulting in seismic waves that propagate through the Earth. Earthquakes can vary in magnitude, from minor tremors that go unnoticed to major events that cause widespread destruction.
The primary cause of earthquakes is the movement of tectonic plates, which make up the Earth’s rigid outer shell. The Earth’s lithosphere is divided into several large plates that float on the semi-fluid asthenosphere below. These plates are in constant motion, albeit very slowly, driven by the convective currents within the Earth’s mantle.
Most earthquakes occur along plate boundaries, where the edges of two plates interact.
There are three main types of plate boundaries:
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Divergent Boundaries
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- At divergent boundaries, plates move apart from each other. This creates tensional forces that cause the Earth’s crust to stretch and thin, leading to the formation of faults. As the plates separate, magma rises to fill the gap, creating a new crust. Earthquakes at divergent boundaries are generally less intense and more localized.
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Convergent Boundaries
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- Convergent boundaries occur when two plates collide. There are three types of convergent boundaries: oceanic-oceanic, oceanic-continental, and continental-continental. In these collisions, one plate is usually forced beneath the other in a process called subduction. The intense forces and compression generated during subduction can lead to powerful earthquakes. Additionally, the bending and deformation of the overriding plate can cause significant seismic activity.
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Transform Boundaries
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- Transform boundaries are characterized by plates sliding past each other horizontally. The motion of these plates is neither toward nor away from each other but rather in opposite directions. The grinding and scraping of plates along transform boundaries result in high levels of friction, which can suddenly release in the form of earthquakes. Transform boundaries are known for producing some of the most destructive earthquakes, as the release of energy is often rapid and intense.
- When the stress along a fault line exceeds the strength of the rocks holding it together, it causes the rocks to rupture. This sudden rupture generates seismic waves, which radiate outward from the epicenter—the point on the Earth’s surface directly above the focus, or the location of the initial rupture. The seismic waves travel through the Earth, causing the ground to shake.
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