When an earthquake occurs, the shaking of the ground causes stress on the Earth's crust. This stress is transferred to the plates, causing them to move. The amount of movement depends on the size of the earthquake and the type of plate boundaries involved.
At transform boundaries, plates slide past each other horizontally. The amount of horizontal slip during an earthquake is usually much less than the vertical slip that occurs at other types of plate boundaries. However, because transform boundaries are usually found in highly populated areas (such as along California's San Andreas Fault), even small amounts of horizontal slip can cause damage.
At convergent boundaries, one plate moves under another and both plates sink into the mantle. The sinking motion is accompanied by an increase in heat and pressure, which melts some of the rock on both plates. This molten rock (magma) rises to the surface and can erupt from volcanoes. Earthquakes at convergent boundaries are often large and can cause extensive damage due to both the shaking of the ground and eruptions of magma.
At divergent boundaries, two plates move away from each other as new crust forms between them. The most well-known example of this is mid-ocean ridge spreading, where hot mantle rock rises up and pushes against cold oceanic crust. As this happens, earthquakes occur along faults that form between the two plates. These earthquakes are generally small and not very damaging because they occur in remote areas with little or no human activity