Megathrust

Giant Megathrust Earthquakes

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. Megathrust earthquakes have never been observed in the short (~150 year) written history of the west coast of Canada, but there is compelling evidence that they did occur in prehistorical times. Some of this evidence includes:

• Buried tidal marsh or coastal forest soils pointing to sudden land subsidence of about 1 metre occurring 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 seismic shaking.
• Tsunami evidence from local sources - marine organisms swept into and preserved in the bottom muds of coastal lakes that are separated from the ocean by land elevations of some 5 miles high.
• Tsunami evidence from 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 pm!

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 and Vancouver. 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 leveling, changes in gravity, and long term tide gauge measurements.