An earthquake is a sudden movement of the ground caused by the release of energy stored in Earth's crust. When this energy is released, it creates seismic waves that travel through the ground, causing the shaking we feel. The epicenter is the point on the surface directly above where the earthquake starts, while the hypocenter is the actual underground location where the earthquake begins.
Earth consists of four main layers that help us understand where earthquakes occur. The crust is the outermost solid layer, ranging from 5 to 70 kilometers thick, and this is where earthquakes primarily happen. Below is the mantle, a hot dense rock layer about 2,900 kilometers thick that drives plate movement. The outer core is liquid iron and nickel, creating Earth's magnetic field, while the inner core is solid iron and nickel under extreme pressure and temperature.
Tectonic plates are massive slabs of rock that make up Earth's crust. These plates move slowly over time due to convection currents in the mantle below. There are three main types of plate boundaries: convergent boundaries where plates collide creating mountains and trenches, divergent boundaries where plates separate forming new oceanic crust, and transform boundaries where plates slide past each other causing frequent earthquakes. The interactions between these moving plates create stress and pressure that eventually leads to earthquakes.
Faults are fractures in Earth's crust where rock movement occurs, creating the specific locations where earthquakes happen. There are three main fault types. Normal faults occur when the hanging wall moves down due to tensional stress, commonly found at divergent boundaries like the Basin and Range in the USA. Reverse faults form when the hanging wall moves up due to compressional stress, typical at convergent boundaries like the Himalayas. Strike-slip faults involve horizontal movement caused by shear stress, common at transform boundaries like the famous San Andreas Fault in California.
Seismic waves carry earthquake energy through Earth's layers in three main types. P-waves or primary waves are the fastest, traveling at 6 to 8 kilometers per second, and arrive first at seismic stations. They compress and expand material as they move and can travel through both solids and liquids. S-waves or secondary waves are slower at 3 to 4 kilometers per second, moving material up and down, but can only travel through solids. Surface waves are the slowest but most destructive, traveling along Earth's surface with complex rolling motions that cause most earthquake damage.