1700/01/26

Offshore

~9

Subduction earthquake off the west coast of Vancouver Island. Coastal native village destroyed and native houses on Vancouver Island may have been damaged.

This great earthquake occurred about 9 pm on the night of January 26 in the year 1700. This was almost 100 years before the first European explorers made contact with native peoples on the west coast. The time and date of the earthquake are known because the large tsunami, or tidal wave, generated by the earthquake swept across the Pacific Ocean and did considerable damage at several places along the east coast of Japan. Japan has a written history dating to that time, and the size of the tsunami and its arrival time were well recorded. Using this, and other historical data, scientists were able to calculate the time of occurrence, location, and size of the generating earthquake.

This was one of the world's great earthquakes with a magnitude estimated at 9. Oral traditions of the native peoples of Vancouver Island suggest the tsunami destroyed a village at Pachena Bay on the west coast of Vancouver Island, and that the shaking damaged houses in the Cowichan Lake region of south central Vancouver Island.

This earthquake occurred on the Cascadia Fault, the boundary between the North American and Juan de Fuca plates. This type of large earthquake is now known as a megathrust earthquake.

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:

Burried 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 seismic shaking

Tsunami evidence from:

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 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 levelling, changes in gravity, and long term tide gauge measurements.