These cross-sections show the configuration of key sedimentary basins in western Washington (within the grid). In this image, only the upper 10 km of the cross-sections are used. The Seattle basin is most prominant, with low velocity sedimentary rocks extending down to > 10 km depth. The Seattle fault zone (Johnson et al., 1994) forms the southern side of the Seattle Basin, and is dextrally offset several km just west of Seattle, as indicated by the strike-slip arrows. The Tacoma basin is indicated in low velocities NW of Olympia. The SWCC (white outline) is a sedimentary complex underneath volcanic rocks in the southern Cascades, roughtly between Mt. Rainier, Mt. St. Helens, and Mt. Adams. It is intruded by plutonic rocks between Mt. St. Helens and Mt. Adams and south of Mt. Rainier. This inferred sedimentary complex was discovered with magnetotelluric (deep electrical sounding) surveys.

Johnson, S.Y., Potter, C.J., and Armentrout, J.M., 1994, Origin and evolution of the Seattle basin and Seattle fault: Geology,
v. 22, p. 71-74 and insert.