The Puget Sound faults under the heavily populated Puget Sound region of Washington state form a regional complex of interrelated seismogenic geologic faults. Intraslab earthquakes, such as the M6.72001 Nisqually earthquake, relatively shallow crustal earthquakes, generally less than 25 km 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 M6 or 7. While the great subduction events release much energy, that energy is spread over a large area, the energy of the somewhat smaller Benioff earthquakes is likewise diluted over a relatively large area. The largest intra-crustal earthquakes have about the total energy, but since they re closer to the surface they will cause more powerful shaking. The Puget Sound region is not just potentially seismic, it is actively seismic, the southern limit nearly matches the southern limit of the glaciation, possibly the seismicity reflects rebound of the upper crust after being stressed by the weight of the glacial ice. The first definite indications of most of these came from gravitational mapping in 1965. As of 1985 only the Saddle Mountain Faults had been shown to have Holocene activity, not until 1992 was the first of the lowland faults, the Seattle Fault, confirmed to be an actual fault with Holocene activity, and the barest minimum of its history established. Aeromagnetic surveys, seismic tomography, and other studies have contributed to locating and understanding these faults. Western Washington lies over the Cascadia subduction zone, where the Juan de Fuca Plate is subducting towards the east. This is being obliquely overridden by the North American plate coming out of the northeast and this bend has distorted the subducting slab into an arch that has lifted the Olympic Mountains and prevented them from subducting. For the past 50 million years or so these have been thrust by subduction up against the North Cascades, which sit on the North American Plate. This forms a pocket or trough – what one local geologist calls the big hole between the mountains – between the Cascades on the east and the Olympic Mountains and Willapa Hills on the west and these terranes were covered by the basalts of the Crescent Formation. Folding and faulting has exposed these basalts in some places, the basins have been filled by various sedimentary formations. Glacially deposited and shaped fill covers most of the elevations of Puget Sound. Some upper-crustal formations have been pushed onto the older basement of the North Cascades, there is a general north or northeast directed compression within the Lowland causing folds, which eventually break to become dip-slip thrust or reverse faults. Some strike-slip movement is expected along the peripheral faults, further complicating this is a feature of unknown structure and origin, the Olympic-Wallowa Lineament. This is a seemingly accidental alignment of features that runs roughly east-southeast from the north side of the Olympic Peninsula to the Wallowa Mountains in northeastern Oregon
Image: Centralia Chehalis District faults GM 34
In red: Saddle Mountain faults (west and east) extension to the southwest inferred from aeromagnetic and LIDAR evidence, Dow Mountain fault (offset by SM east), and Frigid Creek fault.
The Southern Washington Cascades Conductor (SWCC, yellow) located at depth approximately between Mount St. Helens (MSH), Mount Adams (MA), Goat Rocks (GR), Mount Rainier (MR), and Riffe Lake, with a lobe extending towards Tiger Mountain (TM). Also shown: Entiat Fault, Straight Creek Fault (inactive, southern continuation unknown), Southern Whidbey Island Fault, Rattlesnake Mountain Fault Zone, Olympic-Wallowa Lineament, White River/Naches River fault, Rimrock Lake Inlier (outlined in green), surface outcrops of the Crescent Formation (outlined in brown), Olympia Structure, Portland Hills fault zone.