Most are relatively small but the potential exists for major shallow earthquakes as well. It is estimated that the St. Helens seismic zone could produce a magnitude 6.2-6.8 earthquake. There are believed to be other similar faults elsewhere in the Puget Sound area but they have not been studied in detail. Generally, these earthquakes are expected to have magnitudes less than 8 and last between 20-60 seconds. The actual movement of the ground in an earthquake is seldom the direct cause of injury or death.
Most casualties result from falling materials.
The effects of an earthquake in King County are hard to define because of the many unpredictable variables involved. Normally, an earthquake occurring nearer the surface will cause more damage than one occurring at a greater depth. The time of the earthquake has a large impact on the potential for human casualties. The potential for casualties is greatest during hours of heavy traffic and when large numbers of people are concentrated in schools and business areas. Typically, the twelve hour period from six o'clock in the morning to six o'clock in the evening has the greatest potential for human casualties.