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A megathrust fault is the boundary between a subducting and an overriding
plate. A megathrust earthquake is produced by a sudden slip along this
fault. The world's largest earthquakes are all megathrust earthquakes.
While megathrust earthquakes have not been observed in the short (~150 year)
written history of the west coast of Canada, there is compelling
evidence that they have occurred in prehistorical times. Some of this evidence
Cascadia megathrust (click on image to link to large images - 228 KB)
- Buried tidal marsh or coastal forest soils point to sudden land
subsidence of about 1 metre occuring at the same time from Vancouver
Island to Northern California.
- Changes in tree ring growth from coastal old-growth also
suggest a sudden, widespread subsidence and drowning of roots
- Sand layers on top of the buried coastal marshes, driven
in from offshore bars by the wave of the large tsunami that rushed
into the subsided coastal region
- Silt turbidite (landslide) layers on the deep sea floor far
off the coast from underwater landslides, likely caused by strong
- local sources - marine organisms swept into and preserved
in the bottom muds of coastal lakes that are separted from the
ocean by land elevations of some 5 m high
- distant sources - large tsunami in Japan with no local
Japanese earthquake. Modelling this tsunami has revealed that
the most recent earthquake for Cascadia was M ~ 9.0 and occured
on January 26, 1700, at around 9 p.m
Tidal marsh studies (click on image to link to large images - 228 KB)
The last point agrees with some legends of first nation people. One legend describes
a severe ground shaking on a winter night accompanied by huge waves that destroyed a coastal village
. This legend is likely reporting the effect of the last megathrust earthquake.
Geothermal and seismic structure studies are being used to estimate the
downdip extent of the potential seismic rupture zone. The landward extent
of the rupture is an important factor for the shaking hazard
at the inland
cities of Victoria
The potential for the next megathrust earthquake for Cascadia is also being studied by monitoring the deformation of the crust
using very precise satellite technology (GPS), repeat levelling, changes in gravity, and long term tide gauge measurements.