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
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
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
Marine geology and geophysics are required to
further our understanding of earthquake hazards