Earthquake is a violent shaking of the ground that occurs when tectonic plates move suddenly. Most earthquakes happen at plate boundaries, where slabs of Earth’s crust float on a semi-fluid layer beneath them called the asthenosphere. When these plates collide and stick together, they build up stress that eventually lets go in a big snap, releasing energy as vibrations we feel as earthquakes. Earthquakes can also occur where one plate dips down under another, causing the surface of the overlying layer to drop or be pushed up. This type of movement is what causes the sideways motion in California’s San Andreas Fault.
The strength of an earthquake is measured on the Modified Mercalli Intensity Scale. Intensity is determined by how long and how hard the shaking lasts, how much of the ground shakes, and what kind of geology there is at the site. Shaking from a fault site with soft sediments will last longer and be more intense than shaking at a stable bedrock site.
The point inside the Earth where an earthquake starts is called the focus and is located at a depth of several kilometers. Scientists can determine the location and depth of an earthquake by analyzing the records on seismic stations at different locations. They can also calculate the intensity of an earthquake by observing how much the P and S waves on a seismogram move over time. In general, depth determinations are more difficult to make than location determinations.