Acehtsunami

A megathrust earthquake is the border between a subducting and an intervening plate. A megathrust earthquake is produced by an unexpected slip along this fault. The world’s largest earthquakes are all refered as megathrust earthquakes.

While megathrust earthquakes have not been experiential in the short (~150 year) written history of the west coast of Canada, there is forceful evidence that they have happened in prehistorical periods. Some of this evidence includes:

• Covered tidal marsh or coastal forest soils point to an abrupt land subsidence of about 1 metre happening at the same time from Vancouver Island to Northern California.

• Alters in tree ring growth from coastal old-growth also suggest a sudden, extensive subsidence and drowning of roots.

• Sand layers on top of the covered coastal marshes, driven in from the offshore bars by the wave of the large tsunami that quick into the subsided coastal region.

• Silt landslide layers on the deep sea floor far off the coast from underwater mudslides, likely caused by strong seismic shaking.

• Tsunami evidence from:
local sources - marine organisms cleaned into and preserved in the bottom muds of coastal lakes that are separated from the ocean by land elevations of some 5 m high level.

distant sources - huge tsunami in Japan with no local Japanese earthquake. Representation this tsunami has exposed that the most recent earthquake for Cascadia was M ~9.0 and happened on January 26, 1700, at around 9 p.m.