Cascadia Earthquakes and Tsunami Hazard Studies

Recent geologic findings indicate that earthquakes generated within the Cascadia subduction zone pose a significant hazard to urban areas of the Pacific Northwest. Evidence gleaned from syntheses of global subduction-zone attributes as well as from local tsunami deposits have been interpreted to suggest that great earthquakes have rocked the Pacific Northwest perhaps as recently as 300 years ago.

Geodetic-leveling data indicate that a large patch of the interplate decollement (the main fault between the North American Plate and the subducting oceanic plate) off southern Oregon is locked. This and other evidence has led to proposals that an earthquake with a magnitude as large as 9 could someday devastate the region.

Alternative viewpoints, however, portray a reduced hazard from earthquakes for some parts of Cascadia. For example, the margin might be segmented, so that earthquakes rupture small areas. Furthermore, interpretation of heat-flow data from northern Cascadia implies negligible friction along the decollement, suggesting weak interplate coupling. Other thermal modeling suggests that regionally along the decollement, the transition in frictional properties from stick-slip to stable sliding occurs at shallower depth in Cascadia than in other subduction zones. This shallow depth limits the maximum magnitude of earthquakes.

Geologic relationships crucial to resolving these opposing viewpoints lie obscured offshore and at depth within the margin. The part of the interplate decollement that is potentially locked lies largely offshore, as shown by both temperature and dislocation modeling. This means that the most likely sites for earthquake nucleation are inaccessible to study by onshore geophysics techniques. Marine geology and geophysics are required to further our understanding of earthquake hazards in Cascadia

Studies of Western Coastal and Marine Geology in Cascadia

• Seismic Hazards Investigation in Puget Sound: In March, 1998, a U.S./Canadian Consortium collected seismic data in Puget Sound and the Straits of Juan de Fuca and Georgia to investigate the regional earthquake hazard.
• In 1995 and 1996, seismic experiments were conducted on land by the USGS and at sea with the German research group GEOMAR (Christian-Albrechts University, Kiel, Germany) who brought their ship F.S. Sonne to Cascadia. Detailed images (205 kb) of the subsurface faults that generate earthquakes were produced.
• Reconnaissance high resolution data were collected in the Columbia River between Astoria, Oreg., and Washougal, Wash. in areas where faults are inferred to cross or coincide with the Columbia River and tributary channels or rivers. Onshore, fault exposures are concealed beneath vegetation, urban sprawl, or young river deposits. Offshore, the seismic reflection profiles show evidence for faulting of recent river deposits over the Portland Hills and Firn Hill faults. In other areas, including the Frontal fault zone, the Clatskanie fault zone, and areas near Portland, relatively recent faulting is suggested, but not confirmed, by dipping and/or truncated reflectors. The data were released in U.S. Geological Survey Open-File Report 95-668.
• Paleo-tsunami deposits were identified in coastal lake and estuarine deposits at several sites in the Pacific Northwest, some of which contain evidence for sudden subsidence and tsunami deposition 300 years ago along the Cascadia coast. Analyses of a core from the north side of the Seattle fault identified rupture as occurring about 1100 years ago. These studies are part of larger, ongoing USGS and university investigations that are studying the 300 BP (years before present) and 1100 BP seismic events, and are important to delineating the seismic history of the Pacific Northwest.
• Computer simulations were made of tsunamis generated by potential earthquakes along known faults offshore of Washington and Oregon. The relationship between the parameters that define earthquake rupture and local tsunamis is complex. For tsunamis that have traveled far from the origin of the earthquake, the magnitude of the earthquake is a good measure of the size of the tsunami. For local tsunamis, however, more knowledge than the magnitude of the earthquake is needed to calculate the eventual runup of the tsunami. This study was designed to calculate how local tsunamis are affected by variations in earthquake source parameters specific to faulting along the Cascadia subduction zone.
• The Cascadia Megathrust and Tectonic Stress in the Pacific Northwest