Tectonic plate motions, crustal blocks, and shallow earthquakes in Cascadia

How do we recognize the coastal blocks

Cascadia's coastal blocks move northward

What happens in Puget Sound?

Tectonic plate motions, crustal blocks, and shallow earthquakes in Cascadia

What causes earthquakes in the Pacific Northwest? Click on any figure to enlarge. Motions of earth's tectonic plates cause earthquakes along plate boundaries Subduction, or underthrusting of the Juan de Fuca oceanic plate beneath the continent at about 36 mm/yr (~1.5 in/yr) produces three kinds of quakes: Great thrust earthquakes along the 1000 km-long plate boundary fault beneath the continental shelf (1700 AD Cascadia, M 9; and much smaller 1992 Cape Mendocino, M 7) Deep earthquakes in the downgoing slab, where it bends beneath the continent (1949 Olympia, M 7; 1965 Tacoma M 6.5; 1999 Satsop, M 5.9) Shallow earthquakes on crustal faults in western Oregon and Washington (900 AD Seattle, M 7; 1993 Scotts Mills, M 5.6; Klamath Falls, M 6). Geologic and historical evidence for the first two kinds of earthquakes is abundant. Shallow earthquakes and faulting in PNW are poorly known and are the focus of much current research.   Shallow earthquakes are part of a larger picture. Cascadia and its small plates are caught between N. America and the much larger Pacific plate, which is moving NW at 51 mm/yr (about 2 in/yr) As a result: The western U.S., including Cascadia is deforming over a broad area. The continental margin and the Juan de Fuca plate are breaking up into smaller crustal blocks that are being dragged northward by the motion of the Pacific plate. Migrating continental margin terranes breaking up into: Washington (W), Oregon Coastal (OC), and Sierra Nevada (SN) blocks. Vancouver Island (VI) and Canadian Coast Mountains represent a relatively fixed buttress against which coastal terranes are deformed.

How do we recognize the coastal blocks? Geology and geophysics help define block boundaries The earth's gravity is high over hard, dense, crustal blocks (red colors) and low over soft, less dense rocks (blue colors). Dense, accreted basalt forms much of the crust of the Coast Range. Offshore are softer, offscraped sedimentary rocks and basins filling the Cascadia trench. Coastal Oregon appears to be one large block, sometimes called a microplate; Washington appears to be several small, fault-bounded blocks. Faults, volcanoes, and earthquakes occur along block margins Block margins are zones of active faulting, seismicity, and volcanism. The interior of the large Oregon coastal block is relatively quiet. In contrast, Western Washington is seismically active, and earthquakes indicate N-S compression. Volcanoes are most abundant on the eastern edge of the rotating Oregon block, a zone of crustal extension. Magnetic directions frozen in ancient lava flows during cooling show rotation of coastal block with respect to North America North-seeking magnetic directions point eastward in coastal Oregon rocks, indicating clockwise rotation. (Tick marks show direction; pie shaped wedges show uncertainty. Gromme et al, 1986; Wells and Simpson, 2000) 70° rotation since 60 million years ago; 18° in last 15 m.y. have affected coastal Oregon. In Washington, the small coastal blocks have rotated less.

Cascadia's coastal blocks move northward Tectonic model for long-term motion of Cascadia coastal blocks: If we link clockwise rotation of the Oregon coastal block (OC, pink) to NW motion of the Sierra Nevada block (purple) by a "hinge" in Klamath Mountains (KM), we can predict the long-term velocity of coastal blocks (yellow arrows) with respect to stable North America. Cascade volcanic arc (orange) erupts along trailing edge of Oregon block in region of crustal extension(compare the greater abundance of volcanism south of Mt. Rainier (MR) to lesser volume in the north, where block motion compresses the volcanic arc). Northward motion of Oregon block squeezes western Washington (green) against Canadian Coast Mountains restraining bend, causing N-S compression, uplift, thrust faulting, and earthquakes. Global Positioning System measures present day velocity of coastal blocks Using satellite receivers (GPS), the present motions of coastal blocks with respect to stable North America can be measured (e.g., Thatcher et al, 1999; Savage et al., 2000; Khazeradze et al., 1999; McCaffrey et al., 2000). The Sierra Nevada of California is moving northwest, and coastal Oregon north with respect to North America at 10-12 mm/yr (about 1/2 in/yr). Part of the motion can be explained by the long-term tectonic model, above. In coastal OR and WA there is an additional eastward motion due to build up of elastic strain above the subducting Juan de Fuca plate. The eastward motion will probably be released in future great subduction earthquakes, but the northward component is likely to be accommodated by faulting and earthquakes in the upper plate.   What happens in Puget Sound? Some of the earthquakes and faults in Puget Sound are caused by the northward migration of coastal blocks East-west faults (white lines) are mostly thrust faults that absorb N-S compression and crustal shortening as the region is squeezed against the Canadian Coast Mountains buttress by northward-moving coastal blocks. The Seattle fault (1), the Kingston arch (2), and the Devils Mountain fault (3) are east-west fault zones that have a component of N-S thrust motion and related earthquakes (red dots) in Puget Sound. Intensive study of the Seattle fault and other crustal faults is underway to understand their contribution to the earthquake hazard On Bainbridge Island, a strand of the Seattle fault actually reaches the surface, where trenching confirms the strand is an active thrust fault, dipping to the north. Work is underway to determine the size and frequency of earthquakes on this and other faults in the Puget lowland.

The information provided on this page was originally presented as a poster at the Great Cascadia Earthquake Tricentennial Open House, January 26, 2000, Burke Museum, University of Washington, Seattle WA
Authors: 1) U.S. Geological Survey, 345 Middlefield Rd. MS-975, Menlo Park, CA 94025; rwells@usgs.gov, 2) USGS, MS-989, Menlo Park, CA 94025, 3) USGS at Univ. of Washington, Box 351650, Seattle, WA 98195