... like I'm 5 years old
Earthquakes are like when you shake a rug to get the dust off. The Earth has layers, just like a rug has threads. Sometimes, these layers (or 'plates') move against each other. This causes a lot of pressure to build up. When the pressure becomes too much, it is released as an earthquake. The shaking you feel during an earthquake is similar to the shaking of the rug. The shaking can sometimes cause buildings to fall, just like dust falling off a rug.
Imagine if you're playing tug of war and the rope suddenly snaps. The sudden release of tension causes both teams to stumble back. That's what happens during an earthquake. The Earth's plates are pulling and pushing against each other until the pressure becomes too much and is suddenly released, causing an earthquake.
... like I'm in College
The Earth's crust is made up of several large and small plates that float on the mantle, which is semi-fluid. These plates are always moving, albeit very slowly (about as fast as your fingernails grow). Most earthquakes happen at the boundaries where these plates meet. Sometimes the plates glide smoothly past each other, but other times they get stuck. When they get stuck, pressure builds up. When the pressure gets too much, the rock breaks or shifts and an earthquake occurs. The point where the rock breaks is the epicenter of the earthquake.
The energy released during an earthquake travels in waves throughout the Earth. There are two main types of waves: P-waves which are fast and shake the ground back and forth, and S-waves which are slower and shake the ground up and down and side-to-side.
Imagine you're building a tower with LEGO bricks. Each brick represents a section of the Earth's crust. You're not just stacking them neatly on top of one another, you're pushing and pulling them in different directions because the Earth's plates are always moving.
Sometimes, the LEGO bricks move past each other smoothly. But other times, they get stuck. You keep pushing and pulling, but they just won't move. The tension keeps building until finally, one brick slips. That sudden movement is an earthquake.
The shaking from the movement can cause the other bricks in your tower to fall, just like buildings during a real earthquake. The energy from the 'earthquake' travels through the tower, shaking other bricks as it goes. This is similar to how seismic waves travel during an earthquake, shaking the ground and anything on it.
... like I'm an expert
Seismic activity is primarily associated with tectonic forces and the resulting strain energy accumulation and release. The Earth's lithosphere, composed of tectonic plates, is in constant movement due to asthenospheric convection currents. Interactions at plate boundaries cause seismic activity.
At convergent boundaries, one plate subducts beneath another, leading to intense pressure accumulation. The sudden lithospheric rupture at the subduction zones, termed as megathrust earthquakes, can be extremely powerful, often generating tsunamis.
At transform boundaries, plates slide past each other, and pressure accrues at the asperities—the points where the plates are locked. The sudden release of this pressure is manifested as an earthquake.
Seismology, the study of earthquakes, uses seismic waves to measure the magnitude and intensity of earthquakes. The Richter scale measures the magnitude, while the Mercalli scale quantifies the observed effects and damage.