Journey into Earthquakes

Journey into Earthquakes: Why They Happen, Where They Strike, and How We Measure Their Power

Cartoon of two blocks of Earth’s crust offset on one side of a fault.


An ordinary (dip-slip) fault is a tilted fracture where the rock mass above the fault plane moves downward (public domain). An earthquake occurs when two pieces of Earth suddenly slide past each other. The region where they slip is the fault or fault plane. The point beneath the Earth’s surface where an earthquake starts is called the hypocenter and the point directly above it on the surface is called the epicenter. 

Sometimes, smaller earthquakes called foreshocks occur. These small earthquakes happen in the same place as a larger earthquake that follows. Scientists cannot tell if an earthquake is a foreshock until a larger earthquake happens. The biggest earthquake is referred to as the mainshock. Mainshocks are always followed by aftershocks. Smaller earthquakes occur in the same general area as the mainshock. The duration of aftershocks might last for weeks, months, or even years, depending on the magnitude of the main shock!

Cartoon of Earth’s layers – crust, mantle, outer core, inner core.

Earth has four major layers: the inner core, outer core, mantle, and crust. Layers and mantle together make up Earth’s skin. But this skin isn’t in one piece – it’s made up of many pieces, like a puzzle. Not only that, but these puzzle pieces are slowly moving, sliding past and bumping into each other. We call these puzzle pieces tectonic plates, and the edges of the plates are called plate boundaries. Most earthquakes happen along these boundaries because the edges of plates are jagged and get stuck while the rest of the plate keeps moving. Eventually, when the plate has moved far enough, the edges unstick on one of the faults, and an earthquake happens.


Example of a seismic wave with P and S wave labels.

An earthquake’s size depends on the fault’s size and the amount of slip on the fault, but it’s not something that scientists can measure with just a measuring tape since faults can be many kilometers deep beneath the Earth’s surface. So how do they measure the size of an earthquake? They use recordings made by seismographs placed on the Earth’s surface. The recording is called a seismogram. The seismogram shows an earth tremor’s magnitude. The amplitude of the wiggles in the seismogram tells us how big the earthquake was. A small zigzag line that doesn’t wiggle much means a major earthquake is indicated by a small earthquake and a lengthy, wriggling zigzag line. The length of the wiggle called the period depends on the size of the fault and the amount of slip on the fault.

Scientists also talk about the intensity of shaking from an earthquake, which depends on where you are on Earth.

Journey into Earthquakes

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