An Earthquake is caused by the fracturing of rock. The resulting break is called a fault. As plate tectonics cause the crust of the earth to shift and bend, great stresses are built up underneath the ground. When the rock can no longer take the stress of being pushed and pulled, it breaks creating a fault along which earthquakes can occur.

The Seattle area is susceptible to three major fault systems. There is the Cascadia Subduction zone (shown in red), the deep Benioff zone (shown in pink), and the shallow crustal faults (shown in yellow).

Photo caption: The above photograph was taken after the Nisqually earthquake. This is not a fault, but an example of what is called ground failure.

Next >>>