The Puget Sound faults under the heavily populated Puget Sound region (Puget Lowland) of Washington state form a regional complex of interrelated seismogenic (earthquake-causing) geologic faults. These include (from north to south, see map) the: Devils Mountain Fault Strawberry Point and Utsalady Point faults Southern Whidbey Island Fault (SWIF) Rogers Belt (Mount Vernon Fault/Granite Falls Fault Zone) Cherry Creek Fault Zone Rattlesnake Mountain Fault Zone Seattle Fault Tacoma Fault Saddle Mountain Faults Olympia structure (suspected fault) Doty Fault Saint Helens Zone and Western Rainier Zone General background Earthquake sources and hazard The Puget Sound region (Puget Lowland) of western Washington contains the bulk of the population and economic assets of the state, and carries seven percent of the international trade of the United States. All this is at risk of earthquakes from three sources: A great subduction earthquake, such as the magnitude M 9 1700 Cascadia earthquake, caused by slippage of the entire Cascadia subduction zone, from approximately Cape Mendocino in northern California to Vancouver Island in British Columbia. Intraslab (Benioff zone) earthquakes, such as the M 6.7 2001 Nisqually earthquake, caused by slippage or fracturing on a small part of the subducting plate at a depth of around . Relatively shallow crustal earthquakes, generally less than deep, caused by stresses and faulting in the near-surface crustal structures. The energy released depends on the length of the fault; the faults here are believed capable of generating earthquakes as great as M 6 or 7. While the great subduction events release much energy (around magnitude 9), that energy is spread over a large area, and largely centered near the coast. The energy of the somewhat smaller Benioff earthquakes is likewise diluted over a relatively large area. The largest intra-crustal earthquakes have about the same total energy (which is about one-hundredth of a subduction event), but since they are closer to the surface they will cause more powerful shaking, and, therefore, more damage. One study of seismic vulnerability of bridges in the Seattle – Tacoma area estimated that an M 7 earthquake on the Seattle or Tacoma faults would cause nearly as much damage as a M 9 subduction earthquake. Because the Seattle and Tacoma faults run directly under the biggest concentration of population and development in the region, more damage would be expected, but all the faults reviewed here may be capable of causing severe damage locally, and disrupting the regional