While magnitudes from shallow earthquakes are generally expected in the 5.7 to 6.3 Richter scale range, there is a potential in some areas for these shallow events to reach magnitude 7.5. Evidence supports that earthquakes of this intensity have occurred in the North Cascades in the late 1 800s and along the recently defined Seattle fault nearly 1000 years ago. The St. Helens Seismic Zone in the southern Washington Cascades is thought to be capable of producing a magnitude 7.0 earthquake. Eastern Washington, however, has not had an earthquake exceeding 5.8 in the last 150 years. The risk of earthquakes prevails throughout the state.
Some areas are at higher risk than earthquake risk others.
Site conditions and the types of soil or rock affect the amount of shaking and the potential for damage. Solid rock or bedrock does not increase the shaking. However, soft materials such as mud, artificial fill and layers of sand and clay will make the consequences of ground shaking much worse. This, because they increase or amplify the effects of an earthquake. Steep slopes may experience landslides.