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 indicates 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 devastate the region someday.
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 viewpoint 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
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