While we tend to think of our planet as one cohesive sphere, the Earth’s surface actually consists of several, distinct “plates” that are constantly sliding past one another. According to the United States Geological Survey, an earthquake is what happens when two plates slip past one another. The area where the slip occurs is called the fault or fault plane. The exact location where the earthquake starts is called the hypocenter, and the location directly above the hypocenter is called the epicenter.

How Earthquakes Are Measured

The size, types, and frequency of earthquakes measured by geological scientists over a period of time in a certain area are collectively referred to as seismic activity. Seismic activity can include foreshocks (smaller earthquakes that happen shortly before the “main” earthquake, or mainshock) and aftershocks (smaller earthquakes taking place after the mainshock.

People in the St. Louis area have been asking for years not if, but when we are going to see the big one.

Aftershocks Are Almost Always Felt After An Earthquake

As you might guess, it is difficult to differentiate a foreshock, a mainshock, and an aftershock from one another until an entire earthquake episode has taken place and been measured. This can take time, as aftershocks can occur days and sometimes years after the mainshock. Aftershocks are caused by minor readjustments along a fault plane that slipped during the mainshock. Shallow earthquakes (those that occur in the top 30 km of the Earth’s crust) are more likely to produce aftershocks than deep earthquakes (those occurring 30 km or below the Earth’s surface).

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