Earthquakes are a sudden shaking or rumbling of the ground. This powerful natural event is a rapid release of energy that has been building up over a long period beneath the Earth’s surface. Understanding what causes this ground movement requires looking beneath our feet at the structure of our planet.
The Earth is a Giant Puzzle
The Earth’s hard outer shell, which includes the crust and the uppermost part of the mantle, is called the lithosphere. This layer is not a single, unbroken surface but is instead fractured into many large, rigid sections known as tectonic plates. These massive slabs of solid rock fit together imperfectly, much like the pieces of a giant puzzle.
The plates essentially float on a semi-molten, hot layer of rock called the asthenosphere, which is part of the mantle. This layer is not liquid but is hot enough to be ductile, allowing the plates above it to slowly move and shift. The movement rate is extremely slow, typically ranging from zero to about 10 centimeters per year. Earth’s internal heat, generated from radioactive decay, drives this motion through slow-moving currents within the mantle.
What Happens When the Puzzle Pieces Move?
As the plates move, they interact along their boundaries in three distinct ways, and these interactions are the primary source of nearly all earthquakes.
At a divergent boundary, two plates pull away from each other, which allows magma to rise and create new crust, typically resulting in smaller, shallow earthquakes.
When two plates collide, they form a convergent boundary, which is responsible for about 80% of the world’s earthquakes. If an oceanic plate meets a continental plate, the denser oceanic plate sinks beneath the continental one in a process called subduction. This intense compression and friction generate powerful earthquakes, often associated with deep ocean trenches and volcanic activity.
The third type is a transform boundary, where two plates slide horizontally past one another in opposite directions. These sliding movements produce shallow earthquakes. These boundaries cause the rocks to grind together, leading to a massive build-up of stress as the plates try to continue their motion.
Where Earthquakes Start
The actual shaking happens along a fault, which is a fracture or crack in the Earth’s crust where movement has occurred. Even though the tectonic plates are constantly trying to move, the irregular surfaces of the rocks along the fault line get stuck due to friction.
While the fault remains locked, the rest of the plate continues to move, causing tremendous strain to accumulate in the surrounding rock. This stored energy is like the tension in a stretched rubber band. When the stress finally overcomes the friction holding the rocks together, the fault suddenly slips. This rapid release of stored energy travels outward in the form of seismic waves, which is the shaking we feel during an earthquake.
How Scientists Measure the Shaking
Scientists use specialized instruments called seismographs, or seismometers, to detect and record these seismic waves. A seismograph operates on the principle of inertia, using a mass suspended by a spring that tends to remain stationary when the ground around it moves. The instrument records the difference between the movement of the ground and the stationary mass, producing a visual record called a seismogram.
The size, or magnitude, of an earthquake is reported using a numerical scale that reflects the total energy released. Today, seismologists primarily use the Moment Magnitude Scale (Mw), which provides a more accurate measure for all earthquake sizes than the older Richter Scale. The Moment Magnitude Scale calculates the seismic moment, which accounts for the area of the fault that ruptured and the amount of slip, ensuring a more reliable estimate of the earthquake’s power.